THE EFFECTS OF DISTURBANCE FROM HUMANS AND
PREOATORS ON l'HE BREEDlNO DEClSlONS
AND PRODUCTIVITY OF THE GREAT BLUE HERON
IN SOUTH-COASTAL BRITISH COLUMBIA
by Ross G, Vennesland B.Sc., Simon Fraser University, 1996
THESIS SUBMlTl'ED iN PARTIAL FClLFlLLMENT OF THE REQUIREMENTS FOR 'l'HE DEGREE OF MASTER OF SCIENCE in the Department of Biologicd Sciences
O Ross G. Vennesland 2000 SIMON FRASER UNIVERSITY December 2000
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The author retains ownership of the L'auteur conserve h propriété du copyright in this thesis. Neither the cimit d'auteur qui protège cette thése. îhesis nor substantial extmts hmit Ni la thése ni des -ts substantieis may be printd or 0th-e de celle-ci ne doivent être imprimds reproduced without the author's ou arrtrement reproduits sans son permission. autorisation. Abstract
The effects of disturbances from humans and predators were studied at 35 Great Blue Hem breeding colonies in south-coastai British Columbia during 1998 and 1999. Hemn breeding productivity was the lowest reported in North America, and has probably declined since 197 1 due to increased breeding abandonment. Levels of human and Bald Eagle activity in this region have increased over the past half-centwy. Productivity in the Fraser Valley was the highest of al1 regions, and might be offsetting the significantly lower productivity of the Sunshine Coast and Vancouver Island. Regional productivity varied due to breeding abandonment. In 1999, breeding abandonment occurred in 58.5% of 1247 breeding attempts, and 42% of 3 1 colonies were totally abandoned. Breeding abandonment accounted for 96% of the variation in productivity among colonies, and was due to eagle disturbance and, to a lesser degree, human disturbance. Colony productivity was negatively and significantly related to eagle disturbance and human pedestrian activity. Herons abandoned breeding due to real and perceived threats of disturbance from eagles and humans. Productivity increased significantly with colony size due to the higher frequency of breeding abandonment at smail colonies compared to large coionies, and productivity wu significantiy lower late in the breeding season than early in the season due to breeding abandonment. Herons significantly increased their nest defense against the approach of the investigator through the breeding season. My study is the first to show experimentaly that herons habituate to non-threatening human activity near breeding areas through the season. The level of response varied significantly among colonies, indicating different perceptions of risk, and varied significantly with the level of urbanization near colonies. Restrictive buffer zones around colony sites are recommended to reduce human disturbance of breeding herons. A calculated set-back distance of 165m should protect heron colonies from pedestrian disturbance. However, set-backs wili not reduce the impact of stronger disturbances, hmeither humans or eagles. 1hypothesize that increasing disturbance of breeding herons in south-coastal BC will resuit in a redistribution of hemns among colonies, such that fitness benefits become equalized among breeding sites. Acknowledgements
1 wish to thank my supervisory cornmittee, Rob Butler, Ron Ydenberg and Fred Cooke for their support and interest throughout my tenure at SFU. Financial support for my fieldwork was provided by the Habitat Conservation Trust Fund (BC Ministcy of Environment), Science Horizons (Environment Canada), and Rob Butler (Canadian Wildlife Service). Fred Cooke of the NSERCICWS Centre for Wildlife Ecology at SFU provided financial support for logistics, travel and administrative needs. 1 received assistance with my research fmm numerous people, 1extend great thanks to dl the kind landowners that provided access to their property - withiiut them my project would not have been possible. 1also thank my field assistants, Hillary Dunn, Karen Truman and Holly Butler for their effort and interest through long field seasons. Ian Mou1 of Fou1 Bay Ecological Ltd., Ann Eissinger of Nakeeta Northwest, John Elliott and Moira Lemon of the Canadian Wildlife Service, and Andrew Scott of Halfmoon Bay also helped with data collection, and provided me with much insight into the relations of herons, humans and eagles. 1 also thank Tom Ainsworth for his expertise at me climbing and other monkey related activities, and Bany Smith of the Canadian Wildlife Service for catastrophic discussions. 1have benefited greatly from interactions with al1 my graduate student compatriots at SFU and elsewhen, and give speciai thanks to al1 present and past members of the Ydenberg, DiIl and Cmke Labs. 1thank Dave Moore and Yolanda Morbey for statistical assistance, Steve Shisko and Falk Huettman for assistance with mapping and graphics software, and Ray HoUand for assistance with electronics. Barbara Sherman, Connie Smith, and the office staff in Biological Sciences provided much appreciated administrative support. For providing media exposure to my research I thank the Discovery Channel, Andrew Scott of Halfmoon Bay and the Georgia Stnit, hg Studios of Edmonton, and Howard Youth hmthe National Wiidlife Federation, USA. And iast but certainly not least, 1acknowledge the love and support received from ail Fast, present and future members of the Grant-Vennesiand Realm, including la Niai the crazy hast. Table of Contents .. Approval ...... il Abstract ...... al Acknowledgements ...... iv Table of Contents ...... v. List of Tables ...... VLI List of Figures ...... v~u...
CHAPTER 1. GENERAL INTRODUCTION ...... 1 Study Context ...... 1 Disturbance and Animal Behaviour ...... 2 Changes in Respnse to Perceived Threats...... 4 Group Dynzimics under lncreasing Disturbance ...... 5 The Impact of Human and Predator Disturbances ...... 5 Terminology ...... *...... 7 Snidy Area ...... 8 Generai Methods ...... 10 Study Species ...... Il The Great Blue Hem ...... 11 The Bald Eagle ...... 13 Objective and Outline ...... 14
CWPER2 . PRESENCE AND IMPACT OF DISTURBANCES FROM HUMANS AND PREDATORS AT GREAT BLUE HERON COLONIES IN SOUTH-COASTAL BRITISH COLUMBIA ...... 17
introduction ...... 17 Objective and Ouîiine ...... 19 Methods ...... 20 Study Area and Species...... 20 Study Sites ...... 20 Hem Bteeding and Disturbance Surveys ...... 21 Breeding Proâuctivity ...... 22 Breeding Phenology ...... 23 Disturbance Obse~ations...... 24 Data Analyses ...... 26 Resuits ...... 27 Breeding Productivity ...... 27 Abandonment. Productivity and Colony Size ...... 27 Breeding Phenology ...... 30 Antagonists and Disturbances ...... 31 Human Disturbance ...... 32 Eagle Distutbance ...... 34 Discussion ...... 35 Breeding Productivity ...... 35 Abandonment, Productivity and Colony Size ...... 37 B reeding Phenology ...... 40 The Significance of Disturbance ...... 42 The Impact of Disturbance...... 43 The Perceived Risk of Disturbance ...... 45 Disturbance and Heron Breeding Populations ...... 46 Su- ...... 50
CHAETER 3 . RESFONSES OF THE GREAT BLUE HERON TO REPEATED DISTURBANCE STIMULI THROUGH THE BREEDiNG SEASON ...... 60
Introduction ...... 60 Objective and Oudine ...... 65 Methods...... 65 Study Area and Species...... 65 Study Sites ...... +66 Investigator Approach ...... 66 Data Analyses ...... 68 Results ...... 70 Seasonal Changes in Hem Response ...... 70 Colony Differences in Heron Response ...... 70 Heron Responses Controllhg for Date ...... 71 Discussion ...... 72 Seasonai Changes in Keron Response ...... 72 Colony Differences in Heron Response ...... 73 Behavioural Flexiiility ...... 75 The Influence of Seasonal Timing ...... 78 Perceived Risk of Disturbance ...... 80 Set-back Distances ...... 8 1 Summary ...... 84
CHAPTER 4. GENERAL DISCUSSION: IMPLICATIONS OF INCREASING DISTURBANCE FOR BREEDiNG GREAT BLUE HERONS IN SOUTH-COASTAL BRITISH COLUMBIA ...... W
Appendices ...... 107 Appendix 1...... 107 List of Tables
Table 2.1. Breeding productivity by region for Great Blue Herons nesting in south-coastal British Columbia in 1999. Productivity values are presented for initiated and successful breeding attempts, and are averages based on counts of fledglings and breeding attempts from samples at colonies. The number of breeding attempts used in calculating the averages are provided. Superscripts denote averages that are statistically different (see results)...... 32
Table 2.2 The frequency of nest and colony iibandonments by region for Great Blue Herons breeding in south-coastd British Columbia in 1999. Al1 percentages are averages based on samples of breeding attempts at colonies within each region. The number of nests or colonies used in calculating the averages are provided. Superscripts denote averages that are statisticalty different (see results)...... 53
Table 2.3 Antagonists and disturbances observed in 1999 on 446 OSh surveys at 22 Great Blue Hem colonies in south-coastai British Columbia. Provided is the percent of surveys the antagonist was present within 250m of the cobny edge, the percent of these surveys resulting in disturbance of herons, and the percent of these disturbances resulting in incursions. Superscripts denote values that are statistically different (see results)...... 54
Table 2.4 Breeding productivity and the frequency of eagle incursions observed during total observation time at Colonies 16 and 23 in 1998 and 1999. The number of nests used to calculate mean productivity and the num ber of hours of disturbance observations are provided. For cluity, eagle activity has been rated as high or low. Subscripts refer to the statistical cornparisons listed at the base of the table ...... 55
Table 3.1 The responses of Great Blue Herons to the approach of the investigatot through the breeding season at 10 colonies in south- coastd British Columbia hm1998 to 1999. Included are univariate regressioas to illustrate changes in heron response through the season, mean responses, and the range of responses. Negative response distances occurred when herons allowed access past the colony edge. Study duration and the nurnber of observations at each colony are pmvided. AU significant sIopes were negative telationships. Astensks refer to the pmbability of a Type I statistical emir as foliows: * Figure 1.1 Locations of 35 Great Blue Hem colonies chronicled in the Strait of Georgia, Strait of Juan de Fuca and northem Puget Sound from 1998 to 1999. Shaded circles are proportional to colony size and numbers refer to the colonies listed in Table 1,l. Major urban centres are also shown ...... 16 Figure 2.1 Breeding productivity vesus the proportion of breeding pairs that abandoned at 3 1 Great Blue Heron colonies in south-coastal British Columbia in 1999. Productivity is presented for a) al1 initiated breeding attempts, and b) only successful breeding attempts. AU productivity observations by initiated bceeding attempts must fa11 within the unshaded region of the figure. For statistical tests, productivity data were treated with a In(x+l) transformation and abandonment data wete treated with an angular transformation...... 56 Figure 2.2 Breeding productivity versus colony size for 3 1 Great Blue Heron colonies in south-coastal British Columbia in 1999. The productivity of successful breeding attempts is represented by the open circles and illustrates the maximum potential colony productivity for 1999 (n=l8). The productivity of al1 initiated breeding attempts is represented by the closed circles and illustrates the actual productivity for 1999 (n=31). Ali observations for successful atternpts have a correspondhg observation for initiated attempts at that colony. For statistical tests, productivity and colony size data were treated with In(x+l) transformations...... 57 Figure 2.3 Frequency disuibutions of the nurnkr of nestlings fledged by Great Blue Hem pairs edyand late in the season at 11 colonies in south- coastal British Columbia in 1999. Early breeding pairs were pairs whose eggs hatched from 8 April to 22 May and the eggs of late breeding pairs hatched hm23 May to 6 July. The total number of pairs observed during eacb period is provided. The distribution of the number of young fiedged was significantiy different between early and late breeders (x2=17.1, P Figure 2.4 Breeding productivity per initiated breeding attempt versus the proportion of surveys with human pedesttian traff~cQ50m from the colony edge on 0.31meys at 22 Great Blue Heron colonies in south-coastal British Columbia in 1999. For statistical tests, productivity data were treated with a ln(x+l) transformation and abandonment daia were treated with an angular transformation...... 59 Figure 3.1 Predicted responses of the Great Blue Hem to repeated and non- threatening investigator disnubance through the breeding season. The top figwe reports the difference between experimental and control exposures if only seasonal phenornena influence hem responses through the season. The bottom figure shows the difference if herons adjust kir responses through the season based only on pnor experience with the stimulus...... ,...., ...... 86 Figure 3.2 Seasonal trends in the response distances of Great Blue Herons at al1 colonies where a response was observeci to the investigator approach from 1998 to 1999. Regression trendlines are provided with colony numbers. Statistics provided are for aii nine colonies and both years pooled. See Table 3.1 for detailed statistics. Ail respnse dates have been adjusted relative to the phenology of colonies using fint incubation at each colony as &y zero. The dashed iine represents the kginning of incubation at dlcolonies. Ail slopes, except at Colony 16, were signifiant at the 0.05 level...... 87 Figure 3.3 Mean response distance in metres versus the Level of urbanization at a11 colonies in the Strait of Georgia during 1998 and 1999 (n=10). Error bars represent one standard deviation. Sample sizes are provided. See Chapter 2 for details on this measure of human activity...... 88 Figure 3.4 Results frorn the date conuolled investigator approach experiment conducted in 1999 at, a) Colony 13, and b) Colony 23. Circutar data points refer to heron response distances on experimentd approach routes, and squares derto response distances on control approach routes. The interaction between experimental and control approach routes was significant at both colonies (Colony 13, P4.05; Colony 23, PcO.01). Regression. . statistics pmvided are based on univariate analyses of individual approaches...... 89 Chapter 1 General Introduction Study Context Shortly before the turn of the 21st century the human population of the earth surpassed six billion individuals. As the human population continues to expand, so does Our impact on the naturai systems around us. An understanding of our impact on this environment is crucial to enswing the detection and correction of problems before they becorne unmanageable (Tuyttens and MacDonald 20). Furthemore, a strong scientific underpinning is also needed for conservation decisions that must be made outside current ernpirical knowledge (Goss-Custard and Suthedand 1997). Because other animals commoniy perceive humans as potential predators, some authors have proposed that the responses of animds to human activity be considered similar to those of predator activity (Wdiher 1969, Gill et al. 1996, Sutherland 1996). This premise ailows the use of the extensive theoreticai knowledge surrounding predation nsk (reviewed by Lima and Dili 1989, Lima 1998) to investigate the impact of disturbance fmm humans. The Great Blue Heron, Ardea hrrodias, is a suitable species to study the impacts of both human and predator disturbance. Herons in south-coastal British Columbia breed in urban and curai areas (Butler 1997) (Figure l.l), and nesting sites are located close and fat from the breeding territories of Bald Eagles (Haliaeetus leucocephalus). This enables cornparisons arnong breeding populations that should experience different ievels of disturbance. As well, both human and eagie populations in this area are increasing (Moore 1990, McAllister et al. 1986, Vermeer et al, 1989, Elliott et al. 1998), implying 1 that disturbance might be increasing. Finally, the sub-species of the Great Blue Heron that occurs in this area, A. h. fannini, has been listed as 'vulnerable' by the Committee on the Status of Endangered Wildîife in Canada (COSEWIC) (Butler 1997). Concem for A. h. fannini hemns arose because of a 6% decline in hem counts obsewed on Breeding Bird Surveys in the Strait of Georgia between 1966 and 1994 (Downes and Collins 1996). This conservation concern provided the impetus for my study. A recent review of the status of the Great Blue Heron in the Suait of Georgia by Gebauer and Mou1 (2000) has provided fiirther concem. There bas been an increase in the proportion of breeding attempts ending in failure at item colonies from 197 1 to 1999, implying that breeding productivity might be compromised. In addition, the number of herons observed on Christmas Bird Counts between 1991 and 1999 declined on the Sunshine Coast and Vancouver Island (see Figure 1.1 for locations). Disturbance and Anima1 Behaviour The conceptuai vade off of resource availability and predation risk between foraging locations has been widely used to quantify the effects of predation cisk on the behaviour of animals (reviewed by Lima and Dill 1990, Lima 1998). When investigating the disturbance of breeding animals, the trade off is between the breeding productivity and the perceived risk of disturbance among potential breeding sites. This simple framework enables quantification of results, and can provide population level predictions of tht: impact of disturbance based on the number of displaced animals (Gil! et al. 1996, Gill and Sutherland 2000). Many studies have assumed that the species most susceptible to distucbance shouid be the ones that respond the strongest (Burger 198 1, Vos et al. 1985, Rodgers and Smith 1995). but Sutherland and Giii (2000) contend that this is not aiways the case, and the opposite may in fact be true - animals that respond strongly may do so because the perceived costs of moving to a new location are srnail (Gill and Sutherland 2000). For any potential life threatening disturbance stimulus from humans or predators, a breeding hem must decide whether to tolerate or respond to the stimulus. This decision is important because failure to properly gauge the threat posed could result in the los of a breeding attempt or the death of a breeding adult (Lima and Dill 1990). When breeding herons perceive a threat from the approach of an antagonist, breeding attempts are generally defended with a steady escaiation in aiarm. At fmt herons become alert and silent, but as the perceived threat continues to increase they begin to vocalize, fmt with repetitive 'cluck' cails, followed by loud screarns. If individual nests become targeted, such as during a predator incursion, herons defend their breeding attempts mostly by vocalizing, posturing, and staying at the nest site. Herons only occasionaily physically engage an antagonist (Burger 198 1, Forbes 1989). If herons are unable to repel the threat with the level of response chosen, breeding attempts are frequently abandoned. The abandonment of breeding is a decision made by individual parents, and can be temporary or permanent, depending on how an individuai perceives the nsk to itself, the benefits of continuing the breeding attempt, and the condition of the brood and mate upon return. If a parent returns to the nest and the young are dead or gone, the decision to abandon breeding may be an obvious one. The behavioural decision of interest is when abandonment occurs before the brood has either fledged or died. In a study of £ive ciconiifonn species in Florida, Frederick and Collopy (1989) found that 33% of 826 abandoned nests had been deserted before clutches or broods had been damaged or young had fledged. Abandonment before damage to nest contents or the fledging of young has been documnted in a diverse range of birds, including the Barrow's Goldeneye, Bucephala islandica (Eadie and Lyon 1998), the Greater Rhea, Rhea americana (Fernandez and Reboreda 2000). the Brown Pelican, Pelecanus occidentalis (Anderson 1988), and at test two Penguin species (Yorio and Boersma 1994, Olsson 1997). Changes in Response to Perceivd Threats Animals shouid respond to a potential antagonist only if a real threat is presented, because a response may have negative fitness costs as well as potential benefits (Ydenberg and Dill 1986). Animals should therefore habituate, or respond flexibly, to repeated exposures of a non-threatening disturbance stimulus. Habituation is a simple form of learning, and can be thought of as a type of phenotypic plasticity (Dukas 1998). If the costs of responding to a stimulus are higher than the costs of learning or of being naïve, anirnals should adjust their behaviour based on pnor experience with a stimulus (i.e., they s hould be behaviourally flexible). However, changes in the intensity of nest defense in birds should depend not only on the behavioural flexibility of breeding individuals, but also on the stage of the breeding season (reviewed by Montgomerie and Weatherhead 1988). Most studies of nest defense behaviour have failed to properly separate these influences (Knight and Temple 1986, Sidenus 1993). Group Dynamics under Increasing Disturbance Most herons in British Columbia breed in colonies of a few to many pairs (Butler 19921, and iherefore provide an opportunity to examine ideas about the frequency and productivity of different group sizes undet a scenario of increasing disturbance fmm both humans and predators. if subjected to increasing disturbance, herons have several options each season [witbin other conscraints such as food and habitat availizbility). They can return to the same colony, muve to another colony, or delay breeding until the next season. The exact advantages for individuals in different group sizes under the risk of predation anr unclear (Lima and Di11 199û), but as the risk of disturbance changes the benefits of a particular group size shouid also change {Ydenberg and Di11 1986, Butler and Vennesland in press). Smail colonies are cryptic, and are probably more difficult for predators to locate thm large colonies. Large cobnies are more obvious and provide a greater resource to predators, but confer the advantages of group defense and predator dilution (Brown et al. 1990, Roberts 19%). Heron colony size is related to available foraging habitat (Fasola and Barbieri 1978, Gibbs 1991, Butler 1992, Gibbs and Kinkel 1997). However, predation is aiso a strong selective force on animais (Brown et al. L990, Lima and Diii 199û), and may determine the size and dismiution of colonies in which herons choose to nest. The Impact of Human and Predator Dbturbances The consequences of human activity for bisand colonial breeding waterbirds have ken reviewed by Parnell et ai. (1988), Hockin et ai. (1992) and Rodgers and Smith (1995). Hockin et al. (1992) found that 36 of 40 studies reported ceduced breeding productivity due to human disturbance. The effects of disturbance depend on the timing, fiequency and magnitude of antagonistic acîivity (Roberts and RJph 1975, Ellison and Cleary 1978, Tremblay and Ellison 1979, HiIl et al. 1997), and the concentrated nature of colonial breeding birds may increase their susceptibility to disturbance (Vos et al. 1985, Parnell et al. 1988, Rodgers and Smith 1995). Human activity disiurbs b~edingGreat Blue Hemns (Werschkul et al. 1976, Simpson and Kelsall 1978, Vos et al. 1985). and has been linked to reduced breeding productivity (Forbes et al. 1985b, reviewed by Parnell et ai. 1988). Carlson and McLean ( 1996) found that the distance of hem colonies from human activity and the width or efficacy of the buffer zone around colonies were positively related to breeding productivity. Buffer zones included vegetation, water and fencing (Carlson and McLean 1996). Watts and Bradshaw (1994) reported herons nesting further from human development than would be expected by chance, and Parker (1980) observed that colony size increased with distance from roads. Several studies have indirectly linked the abandonments of Great Blue Heron colonies to human activity (Bjorklund 1975, Mark 1976, Werschkul et al. 1976, Simpson and Kelsall 1978, Forbes et al. 1985b), but direct observations of such disturbances have not been reported. Herons tolente some human activity near breeding areas (Mark 1976, Kushlan 1979, Webb and Forbes 1982, Butler 1997), and show more tolerance for repeated mechanicd disturbances than for pedestrian traffic (Vos et ai. 1985, Carlson and McLean 1996, Rodgers and Smith 1995). Predators are an important cause of mortality for breeding birds and theu young (Ricklefs 1969, Lima and Dili 1990), and therefore have the potential to greatly affect reproduction. Many studies have reported that predators disturb and reduce the breeding productivity of waterbirds (Dusi and Dusi 1968, Taylor and Michael197 1, Burger and Hahn 1977, Forbes et al. 1985b, Shields and Parnell 1986, Post 1990, Holt 1994). In British Columbia, Bald Eagles prey on hem nestlings, juveniles and adults (Simpson and Kelsall 1978, Forbes et al. 1985b, Forbes 1987, Forbes 1989, Simpson et ai. 1987, Norman et al. 1989, Butler et al. 1995, Butler 1997), and have Likely been responsible for some reduced breeding productivity at colonies (Nonnan et al. 1989). Repeated eagle predation is the suspected cause of many colony abmdonments (Forbes et ai. 1985b, Simpson et al. 1987, Butler 1991, Butler 1997), although herons also tolerate eagles nesting close to their colonies (Koonz 1980, Butler 1995). Mammals also prey on breeding herons, but in British Columbia disturbance hmnon-human mammals is rare (Butler 1997). Terminolosy A colony was defined as 21 breeding pairs of Great Blue Herons. A solitary pair is not normally considered a colony, but because single hem nests in this area are few they have been included. A heron nest was considered initiated if eggs, nestlings, or an adult heron was present on >1 consecutive visit, or if feces were observed on or under a nest late in the season. An initiated nest was considered a breeding atternpt. A successful nest was a breeding attempi that fledged 21 nestlings. Colony size was defined as the numbet of initiated breeding attempts at a colony, and is reported as the number of breeding pairs at each colony. Colonies with dObreeding pairs were considered small colonies, and colonies with 150 breeding pairs were considered large colonies. A rypical colony size was deterrnined for herons in the study area by considering the sin of colony that the average hem pair nested within. The colony site was considered as the geographical location of the colony, and the colony edge as the outer perimeter formed by al1 nest trees and their canopies. Antagonists were defined as any human or predator stimuli that herons responded to on 21 occasions over the study period, and a disturbance wsdefined as any adverse behavioural response from >1 heron when an antagonist was present (after Sutherland 1996). A heron response was considered adverse if it had the potentiai to reduce adult fitness through increased energy expenditun or damage to the nest contents or breeding adult. Disturbance of breeding birds may reduce parental fitness direct1y through predation on breeding adults or the brood, or indirectly through a reduction in the energy available for pwth and reproduction due to nest defense (Stems 1992, Lima 1998). or the costs associated with abandoning breeding. Thus, movement and vocalization were considered as tesponses, but aiertness was not. Disturbances from antagonists inside the colony edge were defined as incursions. incursions are therefore generdly synonyrnous with an attack from an antagonist. A heron colony was considered abandoned if aii breeding pairs at initiated nests departed €rom the colony site for the season without successfully raising any fledglings. An initiated nest was considered abandoned if both adult hemdeparted fmm the nest site for the cemainder of the season without successfully raising any fledglings. Study Am The study was conducted in south-coastal British Columbia (Figure 1.1). Heron colonies were lacated primady in the Strait of Georgia and the lower Fraser River Vaiiey, with a few sites in the Strait of Juan de Fuca and northem Puget Sound (Figure. 1.1). This area contains large and growing human populations in the cities of Vancouver and Victoria, BC and Seattle, Washington. The human population of the Greater Vancouver area grew at 9.1 percent between 198 1 and 1986 (Moore 1990). The current population is slightly more than two miUion people and is growing at about two percent per annum (GVRD 2000). This human dominated landscape also extends beyond the Greater Vancouver area. Ward et al. (1998) estimated that 92% of the land base on the east coast of southern Vancouver Island had been modiied by human activity. Based on geography and heron breeding densities, the study area falls into tnree regions: the Fraser River Vailey (and adjacent mainland areas), Vancouver Island, and the Sunshine Coast. The lower Fraser River Valley and nonhern Puget Sound (hereafter the Fraser Valley) support more colonies of >100 breeding pairs than Vancouver Island, and no colonies of > 100 pairs occur on the Sunshine Coast (Figure 1.1, Appendix 1). These differences in the occurrence of large colonies are probably due to the available foraging habitat for herons across the study area. Available foraging habitat is related to, and probably Limits, heron colony size (Butler 1992), and heron colonies are located close to good foraging areas (Butler 1995). The estuaries of the Fraser and Skagit Rivers, which drain large tracts of the continent, comprise much of the Fraser Valley region and provide extensive beaches for focaging herons. The rivers of Vancouver Island and the Sunshine Coast drain much smailer mas and provide fewer and smaller beaches. Colony size is consequently smaüer in these regions than in the Fraser Valley. General Methods The study was conducted hm2 1 March to 4 August, 1998 and from 17 Febmary to 5 September, 1999 at 35 Great Blue &con colonies in south-coastal British Columbia (Figure 1.1). Hem biology has been studied in this area for many years (reviewed by Butler 1997). Colony locations were obtained from an unpublished annual report prepared for the provincial government (Moul 1998). and local biologists and naturaiists. Throughout this thesis, colonies are referred to using the colony identification numbers provided in Appendix 1. To facilitate reference to historical records, al1 colonies are listed in Appendix 1 using the colony names provided by Gebauer and Moul (2000). Colony sites were visited from five to 40 times each season, following protocol described by Moul et al. (in press). At seven small colonies (SI0 breeding pairs), visits were made by local biologists and natwalists. Variation in the number of visits was due to reduced survey effort at colonies that abandoned breeding completely. After colony abandonment, sites were visited at a reduced frequency until at least 1 June, prirnarily to confirm any further activity by herorrs. In 1999, successful breeding was rare after 1 June (2 of 147 breeding pairs observed), and no colony sites were both completely abandoned and re-colonized by hemafter 6 April. Observations were made of hem breeding productivity, abandonment and phenology from a sample of nests at each site. Antagonists and disturbances were tracked by direct observations and a rneasure of the level of urbanization within 250m of the colony site. The behavioural respoases of herons to human disturbance were tested experimentaiiy to measure the flexibilityof heron tesponses. Detailed methods are described in subsequent chapters. Study Species The GmtBlue tleron The Great Blue Hem (.4dea herodias) is the largest and most widespread Ardeid in North America (Payne 1979, Hancock and Kushlan 1984, Butler 1992). Most northern populations migrate south during the winter, but on the northwestern Pacific Coast from Washington to Alaska, the Great Blue Hem is resident year round (Payne 1979, Hancock and Kushlan L984, Butler 1997). Taxonomists consider these coastai herons as a distinct sub-species (A. h. fannini) from the continental form (A. h. herodias), bsised on plumage and morphoIogica1 characteristics (Hancock and Elliot 1978, Payne 1979). The breeding population of A. h. fannini is estimated at less than 5000 pairs (Butler 1997). A. h. fannini breeds singly and in colonies of up to about 400 pairs (Butler et al. 1995). The number of breeding pairs ac heron colonies in 1999 mnged from one to 400, with a mem of 62 nests (SD=94, N=3 1), a median of 26 nests (Appendix 1). Weighting each breeding heron pair equally, the typical heron nested in a colony of 199 breeding pairs in 1999. Br~edingis concenirated in the Süait of Georgia and Puget Sound. Washington, due to the presence of severai colonies of >1ûû breeding pairs (Eissinger 1996, Butler 1997). It has ken estimated that about 80% of the A. IL fannini population breeds in this area (3utler 1997). Large colonies are associated with the extensive estuarine mudflats and eelgrass beds mund the Fraser and Skagit River deltas (Butler 1993, Eissinger 1996). Colony size has ken associated with avdlible foraging area for the Great Blue Hem and the ciosely relatai Grey Hem (Ardea ciriera) in Europe (Fasola and Batbieri 1978, Gibbs 1991, Butler 1992, Gibbs and Kinkel 1997). Colonies are located in both urban and rural areas, using relatively contiguous forest, fragmented forest and solitary trees (Butler 1997). in south-coastal British Columbia, breeding is initiated between February and April (Butler 1992). Males anive at the colony site and estabiish temtories, followed about a week later by the females (Butler 199 1). Courtship and nest repair / building take from several days to about a month (Butler 1991, pers. obs.). Monogamous pairs are established for the season (Simpson 1984), and an average of four eggs is laid at about two-day intervals (Vermeer 1969, Pratt 1970). Incubation begins soon after the first egg is laid and results in asynchronous hatching (Buder 1992). Hatching occurs after about 27 days of incubation (Butler 1992). Young are reared on the nest for about 60 days, fed mostly fish caught near the colony site (Krebs 1974, Simpson 1984). One breeding cycle requires about 100 days, and herons reproduce for about 200 days around the Strait of Georgia. Thus, herons cm potentially breed more than once if their first attempt fails. Heron breeding sites can be relocated rapidly because nests can be built in three days (Butler 1997) and eggs can be laid within about one week (Butler 1997). An average of 37.5% of eggs from successful heron nests across North America failed to result in fledged young (Butler 1992). Tnre nest mortality is actually much higher because herons cornmonly abandon breeding attempts (Forbes et al. 198Sa, Forbes 1989, Elliott et al. 1989). Forbes et al. (1985) estimated that 20% of breeding pairs in British Columbia abandoned theu breedhg attempt, but most studies throughout No& Arnerica have not accounted for this type of breeding failure in theù estimates of breeding productivity (Butler 1992). Nest predators in British Columbia include the Northwestem Crow (Cowus caurinus) and the Common Raven (Corvus corax) on eggs (Butler 1989), with the Baid Eagle and the Red-tailed Hawk (Bureo jamaicensis) preying on both eggs and nestlings (pers. obs., Simpson 1984, Simpson and Kelsall 1978, Foks et al. 1985b, Norman et ai. 1989). Bald Eagles also kill adult and juvenile herons in this am(Forbes 1987, Butler 1997). Cited threar to breeding populations of the Great Blue Hem were reviewed by Parnell et al. (1988), and include reduced reproductive productivity as a result of hurnan and eagle distucbance at hem colonies (Mark 1976, Simpson and Kelsall 1978, Forbes et ai. 1985b, Parnell et al. 1988, Norman et ai. 1989, Forbes 1989, Butler 1995, Butler et ai. 1995, Rodgers and Smith 1995, Butler 1997), shortages of food during the breeding season (Collazo 198 1, fratt and Winkler 1985), reduced hatching and fledging success as a result of exposure to contaminanis (Elliot et al. 1989, Speich et ai. 1992), habitat loss (Markhm and Brechtel 1978), and weather (Pratt 1970, Parneil et al. 1988)+ The Bald Eagle The Bald Eagle is a large raptor that is resident in southwestern BC (Hancock 1964, Blood and Anweiller 1994). Eagle nests commonly occur near heron colonies (Forbes et ai. 1985b, Butler 1995), sometimes within 25m of active hem nests (pers. obs.). Eagles may be present at their nest site for most of the year, moving north to forage on salmon mns between August and October (Blood and Anweiiier 1994). Eagles are thetefore a potential threat CO herons for most of the breeding season (hmNarch to July). Bald Eagles are temtorial, defending their breeding mato an average of 600m from the nest tree (Mahaffy and Frenzel1987). Breeding densities are higher on the coast than in the interior, with some of the highest densities occuning in the Strait of Georgia (Blood and Anweiller 1994). Bald Eagles nest in large trees and nests are used for an average of five years (Blood and Anweiller 1994). The nest is generally large, taking up to a year to build (R. Butler, pers. comm.). Eagles therefore exhibit lower nest site mobility than herons. Breeding eagles feed pirnarily on fish, but aquatic birds (including gulls and herons) and carrion are also important sources of food (Blood and Anweiller 1994). Boih adults and immature herons have been reported as prey to bald eagles in this ma (see previous section). The number of breeding eagles increased 30% in the Gulf Islands (Vermeer et ai. 1989)' and 34% in the Puget Sound (McAllister et ai. 1986) from the mid-1970's to the mid- 1980's. Eagle breeding productivity from 1992 to 1995 was higher in the Strait of Georgia than on the West coast of Vancouver Island or in Johnstone Strait, and was producing a 'considerable' surplus ofjuveniles (Elliott et ai. 1998). The reasons for ihis increase are unclear, but were probably due to increasing prey populations (e.g., gulls utilizing human refuse), declining contaminant levels in prey (Vermeer et al. 1989, Eliott et al. 1998), or both. Objective and Outline The objective of this thesis was to i) establish the relative importance and risk of human and eagle disturbance to Great Blue Hembreedhg productivity in south-coastd British Columbia, ii) experimentally identify and discuss the flexibility of hem behavioural responses to disturbance, and iii) discuss the potentid long term impacts of disturbance on hem breeding populations in this area. Chapter 2 examines the activity of antagonists and the impact of human and predator disturbances at heron colonies in south-coastal British Columbia. 1 investigated the relationships between hembreeding productivity, colony and breeding abandonment, breeding phenology, and human and predator disturbances. Chapter 3 outlines the temporal changes in heron response to disturbance through the breeding period, and experimentally addresses the question of whether these changes are due solely to the seasonai timing of the breeding attempt, andfor to the behaviourd flexibility of herons. My final chapter is a discussion of how increasing disturbance may alter the future breeding productivity and distribution of heron colonies in this region. Figure 1.1 Lautions of 35 Great Blue Hemcolonies cbmmcled in the Sûait of Georgia, Strait of Juan de Fuca and northern Puget Sound hm1998 to 1999. Shaded circles are proportional to colony size and numbm refer to the colonies iisted in Appendix 1. Major urban centres are also shown. Chapter 2 PRESENCE AND IMPACT OF DISTURBANCES FROM HUMANS AND PREDATORS AT GREAT BLUE: HERON COLûNIES IN SOUTFI-COASTAL BRITISH COLUMBIA Introduction Life history theory predicts that organisms with long reproductive lives should be more likely to trade off the current breeding attempt in favour of surviving to breed again than organisms with shorter lives (Clutton-Brock and Godfray 199 1, Stems 1992, Ghalambor and Martin 2000). It therefore follows that parents of long-lived birds, such as the Great Blue Heron (Ardea herodias) (Butler 1992). would be expected to abandon breeding when faced with a perceived risk that places future reproductive opportunities in jeopardy for the sake of current offspring. Human disturbance in particular should result primarily in breeding abandonment because a perceived nsk is often the only real threat pcesented by humans (Gill and Sutherland 2000). The negative influence of breeding abandonment due to human and predator disturbance on the productivity of breeding birds has been widely documented (reviewed by Parnell et al. 1988 and Hockin et al. 1992) (Chapter 1). Gill et al. (1996) provide a conceptual framework, outlined in Chapter 1, which examines the impact of disturbance on animais. The response of foraging birds to disturbance is best described by the expected costs of rnoving to an alternate location, and is demonstrated by the popuIation change in a patch (i-e. the strength of the response) resulting from exposure to a disturbance stimulus (Gill et ai. 1996). Essentiaiiy this is a made off between the perceived risk of disturbance and the resource avdabiliiy mong patches. Although based on foraging, this view of the perception of risk by animais is a 17 usehl approach to help design questions about the impact of disturbance on breeding animais and their populations. An implicit assumption with this view of perceived survival (after Gill and Sutherland 2000) is that animais adjust their behaviour based upon the density dependent costs or benefits associated with moving to another site to engage in the same activity, namely foraging. When considering breeding animais, the trade-off is between the risk of further disturbance and the expected reproductive fitness returns between patches. Breeding animals can therefore reduce disturbance by choosing among sites as with foraging mimals, or by delaying breeding until later in the same season or the next season. Delaying breeding may allow an individual to avoid cornpetitors and any density dependent costs associated with early breeding. The perceived surviva.1 rate of breeding animals may thetefore be influenced by many other factors in addition to density dependent survivai expectations. For breeding herons, tkxfactors might include the risk of direct mortaiity from predation (Forbes 1989, ButIer 1997), or expenence with a stimulus (Butler 1995, Chapter 3). Density dependence mlty be important to breeding herons, but Butler (1994) reviewed the liierature and found little evidence for this process in heron life history. Gill and Sutherland (2000) show that the traditional way of looking at the effects of disturbance might be misleading. It is commonly implied that the strength of the response received hmwading birds to antagonistic events is positively related to the perceived negative impacts of the event (Bwger 1981, Vos et al. 1985, Rodgers and Smith 1995). Gill and Sutheriand (2000) contend that this is not dways the case, and in fact the opposite may be tme due to the costs of abaudoning the current location. Animais 18 that sespond strongly may do so because the costs of moving to a new location, relative to staying at the current location, are small (GU and Sutherland 2000). It is therefore important to quantify the negative impacts of disturbance in a currency that relates directly to an animal's fitness, and also io gain a Fm understanding of the system king investigated. Changes in the timing of breeding due to disturbance may also have fitness consequences for breeding animals. Many species of birds, including herons, show a seasonal decline in breeding productivity (Lack 1954, Pemns 1970, Butler 1995, Femandez and Reboreda 2000), and breeding synchronization is important for colonid nesting species such as the Common Tern (Strenu hirundo), the Bank Swallow (Riparia iparia) and the Common Guillemot (Uria aalge) (Nisbet 1975, Emlen and Demong 1975, Hatchwell 1991). Timing may affect parental fitness in several ways. Late breeding birds may fledge fewer young from the nest as the season progresses due to increased predation (Nisbet 1975) or breeding abandonment (Butler 1995, Femandez and Reboreda 2000), poor foraging (Eden and Demong 1975), or inexperience (Hedgren 1980). Late breeding may also reduce juvenile survival by providing less time to lem to hunt and build energy reserves before the onset of winter (Butler 1991, Butler and Vennesland in press). Objecîive and Outline The objective of this chapter was to i) quanta the relative influences of human and predator disturbances on the breeding productivity of the Great Blue Heron in south- coastal British Columbia, ii) establish the mechanisrn of impact for any negative influence of disturbance at hecon colonies, and üi) quanti@ the effects of breeding phenology on hem productivity. in this chapter, 1report colony and regional differences in the breeding productivity of herons, and investigate the influence of colony size and breeding abandonment on productivity. 1examine the frequency and cause of nest reuse by herons, and the influence of hatching date on the breeding productivity of individual breeding attempts. 1 report the antagonists pnsent at heron colonies, the disturbances they elicit from breeding herons, and the relationship between the level of disturbance observed and heron breeding productivity at colonies. Findly, 1discuss how and why herons decide to abandon breeding. Methods Shrdy Area and Species The study was conducted in south-coastal British Columbia, Canada (Figure 1.1) between Febmary and September in 1998 and 1999. Generai information on this region and on the biology of the Great Blue Hecon and the Bald Eagle is outlined in Chapter 1. Study Sites Al1 35 heron colonies observed during the study are depicted in Figure 1.1 and are tisted in Appendix 1. in 1998, disturbance was documented at 16 colonies in the Fraser River Valley. The study was expanded in 1999 to inchde Vancouver Island and northern Washington State for a total of 3 1 sites in the second year. Twelve colonies were used by herons and surveyed in both years. Ali heron colonies known to be in use around south- 20 coastal British Columbia were included in the study in 1999. Colonies rangeâ in size hm1 to 400 pairs. The breeding season for herons in the 35 colonies spanned the period from 17 Febniary when herons fmt arrived at colonies in preparation to breed, to 5 September when the last fledgling departed its nest. Heron Breeding and Dkîurbance Surveys Hem colonies were surveyed through the breeding season to determine the productivity and phenology of heron breeding, and aiso to document the presence of antagonists and distucbances. Supplementary information on the productivity of breeding herons was aiso obtained from local biologists and naturiilists. The number of visits to a colony depended on the presence of herons. General methodology and descriptions of the terminology used in this chapter are described in Chapter 1. in 1998, each of 16 colonies was visited five to 20 times from 2 1 Masch to 4 August. in 1999, each of 3 1 colonies was visited five to 40 times (about every €ive days while herons were present) from 17 Febmary to 5 September, for a total of 488 visits in 1999. Standardized 0.31 disturbance surveys were perfomed fmm five to 18 rimes at 13 colonies in 1998, and from five to 38 thes at 22 colonies in 1999. A total of 446 0% disturbance surveys were perfomed in 1999, and total observer the at aü colonies was 582 hours. Visits to colonies were rotated through available daylight hours. Observations of breeding herons and potential disturbances were made through a 15-40x spotting scope, 8x binoculars and the naked eye. A summary of hem breeding biology for ali 35 colonies observed over the study period is provided in Appendix 1. Because the study was unevenly replicated through the 2 1 study period, only information from 1999 was used for inter-colony comparisons across the study area. Information from 1998 was utilized for reporting colony abandonment and disturbance events, intra-colony comparisons, and anecdotal information only. Breeding Productivity Breeding productivity was defined as the number of fledgiings pduced in hem nests, following the protocol described by Moul et al. (in press). Productivity was measured for samples of nests at colonies, and was considered both by the total number of initiated breeding attempts and by the totiil number of successful breeding attempts at colonies. Nestlings were considered to have fledged when seen on branches near the nest or when more than an estimated six weeks old (based on the hatching date - see next section), For productivity measurements, 1observed as many breeding attempts as possible, but sample sizes varied from one to 190 nests, depending on the size of colonies and the ease of viewing nests. The sarne samples were used to calculate the number of breeding attempts that were abandoned. Breeding productivity and abandonment were examined by region, by colony and for individual nests. Productivity arnong regions was compared using the average number of nestlings fledged per initiated or successful breeding attempt. This variable was calculated by dividing the totai number of tledglings produced in the nest samples within each region by the total number of breeding attempts observed within each region. This technique avoids bias from variation in colony size. Nest abandonment between regions was compared in the same manner as for productivity. Moul et ai. (in press) speculated that during years of relatively intensive survey 22 effort (such as during my study) there may be an increased likelihood of observing short- term faiied breeding attemps, compared to years with less intensive effort. However, of 208 breeding attempts observed through tbe 1999 breeding season, only eight were abandoned before incubation began, and oniy 14 were initiated for less than three colony visits. Any over-estimation of abandonment should therefore have minimd influence on my results. Additionally, to merensure a conservative estimate of breeding abandonment, failed breeding attempts were not considered as abandoned nests until al1 herons had ddeparted for the season (i.e., 1did nat consider nest Euse alone to indicate an abandonment event). Breeding Phenology The length of the breeding season was defined as the petid from the start of incubation in the earliest nest to the last fledging event at a colony. Breeding stage was determined by observing the behaviour of a sample of breeding herons at 13 colonies. Nest sample sizes ranged from three to 26 nests, A heron standing on a nest with no visible nestlings was considered to be initiating a breeding attempt or to have recently hatched nestlings. A heron lying flat on the nest was assumed to be incubating eggs or brooding recentiy hatched nestlings. Hatching was assumed if eggshells were observed beneiith a nest or if nestlings were seen in the nest. Herons eject eggshells from the nest swn after hatching, and the vocal nestlings can be nadily heard minutes after hatching (Butler 1997). If hatching date was not obvious, other cues such as date of first incubation or fledging date were used to project hatching date by assumirtg a nestling period of 60 days and an incubation period of 27 days. The reuse of nests was detemiined in several ways for the samples of nests described above. Brooding posture is similar to that of incubation (Butler 1992), so nests were not considered to be reused untii a minimum of 27 days plus >l colony visit, a gap of about 15 &y, had elapsed since the beginning of incubation at that nest. Additionally, reuse was assumed if bouts of incubation were broken by non-incubating posture on >1 visit, or if eggshelis were collected more than once from under the same nest. Hatch dates were deterrnined for breeding pairs at 11 colonies in 1999, and ranged fmm 8 April to 6 July. To compare the bceeding productivity of breeding pairs nesting early in the season with those nesting late, I used the median observation frorn the range of hatch dates (May 22) to separate early hmlate breeding. Disturbance Observatiuns I quiedy approached the edge of colonies where observations of disturbance could be made of severai nests. All disturbance surveys were initiated five minutes after 1 arrived at colony sites to allow herons to adjust to my presence. Observations were made at locations away from the colony edge - staying ouiside the radius at which >1 heron responded by fleeing nests early in the season (about 10 to 100m). A location with û good view of the colony was selected, and the colony was then observed from a motioniess position for the duration of the survey. Tbe same Iocation at each colony was used for all disturbance surveys. Humans and predators witbin 250m of the colony edge (horizontal and vertical) were considered as potentiai disnubances. PotentiaI disturbances to which herons responded on Il occasioa over the study period were considered to be antugunisrs. A 24 disturbance was defined as any adverse behavioural response from >1 breeding heron when an antagonist was present (see Chapter 1 for rationaie). Disturbances from antagonists inside the colony edge were defined as incursions. Because humans rarely entered colonies, incursions were basically synonymous with attempted depredation events. Successful depredation eveats were directiy observed and assumed if stationary screams were heard from a breeding hem for more than 10s. Activities of antagonisis further than 250m from the colony edge rarely caused a disturbance and were frequently difficult to observe. Exceptions were toud and unusual events, such as logging machinery or rock concerts, which were considered antagonistic regardless of their distance. The duration in seconds and estimated distance in metres of al1 potentiaily antagonistic events were recorded. 1recorded the frequency, distance and behaviour of pedestrians, vehicles, airplanes, trains and ail other machinery, Bald Eagles (Haliawtus leucocephalus), Red- tailed Hawks (Buteo jamaicensis) and ail other raptors, Cornmon Ravens (Corvus corai.) and Northwestern Crows (Corvus caurinus) during my disturbance surveys at colonies. The presence of antagonists and disturbances was calculated as the proportion of surveys that an antagonist or disturbance was present. To examine the impact of eagle disturbance, 1compared the nurnber of incursions per unit time to the breeding productivity of herons at two colonies - those with data hmboth years and where either zero or at least five incursions were obserued. Eagle disturbances did not Vary significantly with the tirne of day of surveys (n=22, r =0.14, t =0.50, P>0.50), or the date of surveys (n=22, r d.22, t =0.98, PM.90). Observations outside of standardized surveys and anecdotai evidence from landowners were also used to track intense and uncornmon events. Reports firom 25 landowners and local biologists were used to document colony abandonrnents at five colonies and colony wide disturbances at thecolonies. Eggsheii collections fmm under nests were us& to confirm predation at six heron colonies. Avian predators typically open eggs perpendicular to their axes, whereas hatched eggshells are perforated on the equator. At 20 heron colonies in 1999,i recorded the presence of and distance to active Bald Eagle nests to investigate how the proxirnity of breeding eagles affects heron breeding produc tivity. Each colony was ranked on a three-point scaie, based on the primary human development wiihin 250m of the colony edge. information from colony visits and 1:50,000 scaie topographical maps was used to classi@ the masunounding colonies as: 1) rural development (>50% undeveloped or agricultural), 2) residential development * (>S0% huusing and small roadways), or 3) urban development (>50% heavily developed with large buildings ancilor highways). Data Analyses Statistical analyses were conducted using Minitab version 1 1 for windows and SAS version 6.12 for Widows. Al1 variables were tested for nonriiility using the Shapiro- Wilks method, and any significantly non-normal distributions (P<0.05)were transformed. Ail proportions were treated with an arcsin square root transformation, while productivity and colony size were In (x+l) bmsfonned. Ail transfomations resulted in normal distributions (PS.05). Results Breeding Productiviry Means of heron breeding productivity for 3 1 colonies in south-coastal British Columbia during 1999 ranged from zero to four fledglings per initiated breeding attempt, and average productivity across the study mawas 0.82 fledglings per initiated breeding attempt (Table 2.1, Appendix 1). Means of colony breeding productivity per successful breeding attempt for 18 colonies ranged from 1.43 to 4.00, with an average of 1.98 fledglings per successful breeding attempt (Table 2.1, Appendix 1). Regionaily in 1999, average productivity was 0.92 fledglings per initiated breeding attempt in the Fraser Valley (n=16 colonies), and 0.49 fledglings per initiated breeding attempt on Vancouver Island (n=12 colonies) (Table 2.1, Appendix 1). Ail colonies on the Sunshine Coast failed to raise any fledglings (n=3 colonies) (Table 2.1, Appendix 1). Average productivity per initiated breeding attempt in the Fraser Valley was higher than on Vancouver island (x2= 19.4, P4.001, df=l), and productivity on Vancouver Island was higher than on the Sunshine Coast (x2=5.3. P<0.05,df=l). Productivity per successful breeding attempt did not differ significantiy between the Fraser Valîey and Vancouver island (XG2.4P9.10, hl). 1conclude that heron breeding productivity vacied significantly among regions of south-coastal British Columbia, being highest in the Fraser Valley and lowest on the Sunshine Coast, Abandonment, Productivüy and Colony Size individual nests were abandoned at 28 out of 3 1 colonies in 1999 (Appendix 1). 27 Moreover, an average of 58.5% of di initiated breeding attempts was abandoned by breeding herons (Table 2.2). Colony abandonment occuned at 38% of heron colonies in 1998 (n=16), and at 42% of colonies in 1999 (n=3 1) (Table 2.2). Three colonies abandoned in both years for a total of 19 colony abandonment events in 47 colony-level breeding attempts over the smdy period (Appendix 1). Nest and colony abandonment by region for 1999 is summarized in Table 2.2. Average nest abandonment was 73% on Vancouver Island (n=247 initiated breeding attempts), 55% in the Fraser Valley (n=989 nests), and 100% on the Sunshine Coast (n=l 1 nests). Nest abandonment was significantly more frequent on Vancouver Island than in the Fraser Valley (~'d.5,W.01, df=l), but nest abandonment was not significantiy more frequent on the Sunshine Coast, compared to the Fraser Valley (~'2.0,P>O. 15, df=l). The proportion of colonies that abandoned in the Fraser Valley was 19% (n=16), on Vancouver island 58% (n=12), and on the Sunshine Coast 100% (n=3). The proportion of colonies that abandoned in the Fraser Valley was not significantly different from either Vancouver Island 0('=2.2, Pd.10, df=l), or the Sunshine Coast (2=2.9, P>0.05,df=l). Mean productivity per initiated breeding atternpt was significantiy and negatively correlated with the proportion of breeding attempts that were abandoned at colonies in 1999 (Figure 2. la). Abandonment accounted for 96% of the variation in productivity among hem colonies (Figure Lia), Abandonment accounted for 88% of the variation in productivity when 13 colony abandonments were excluded (100% failure by definition). The residuals from rhis relationship were plotted against the mean productivity of successful breeding attempts at 18 colonies th& fiedged Young, and produced a 28 significant and positive relationship (n=18, M.56, ~~2.7,P4.05). This demonstrates that the remaining variation in the relationsbip between productivity per initiated breeding attempt and abandonment was explained by the productivity ciifferences of successful breeding attempts among colonies. The productivity of successhl breeding attempts was not significandy related to bmding abandonment at 18 colonies in 1999 (d.0, td.25, Pfl.80) (Figure 2. lb). This relationship was still insignificant excluding the two unusud observations of four fledglings per successfui attempt (n=16, PM.20). Mean productivity per successful breeding attempt was not significantly correlstted with colony size at 18 hem colonies in 1999 (Figure 2.2). The productivity of successful breeding attempts represents the maximum potential production of fledglings at colonies. Mean productivity per initiated breeding attempt was significantly and positively related to colony size at 3 1 colonies in 1999 (Figure 2.2). The productivity of initiated breeding attempts represents the actual production of fledglings at colonies. This positive relzitionship occurred because 65% of 20 small colonies (40breeding pairs) totally abandoned breeding in 1999 (Appendix 1). Small colonies contained 15% of al1 initiated breeding attempts (n=1928), and nest abandoment at srnail colonies accounted for 9% of tbe estimated breeding failure across the study area in 1999 (58.5% of an estimated 1928 breeàing attempts). Consequently, 9 1% of the observed bdingabandonment in 1999 occurred at large colonies (L50 breeding pairs). Abandonment is thus important for herons in all colonies (Figure 2.2), incIuding those in the typical colony size of 199 breeding pairs. The strong infiuence of breeding abandonment on the pductivity of large colonies is üiustrated in Figure 2.2. At Colony 34 in 1999,399 of 400 initiated breeding atternpts were abandoned. Nesting failure of this magnitude in a colony this large is 29 unusual, and has not been previously reported in the study area. Furthemore, at three of the remaining 10 large colonies, 225% of al1 initiated breeding attempts were abandoned in 1999 (Appendix 1). 1conclude that heron breeding productivity varies across south-coastai British Columbia, and is detennined primarily by the absuidonment of breeding by individuai pairs of herons. Abandonment occurs in colonies of al1 sizes, and explains most of the variation in productivity arnong colonies. Overall population productivity is most greatly affected by abandonment events in large colonies. Breeding Phenology The length of the breeding season ai heron colonies in 1999 varied from 88 to 167 days, with a mean of 127 days (SDr23, n=12). individual nests were reused at 77% of colonies in 1999 (N= 13). The proportion of nests that were used more than once in 1999 ranged from O to 100%, with a~ average of 25% (n=208). Within season, colony-widc reuse occurred at 11% of colonies over the study period (N=35). The distribution of the number of fldglings pduced per initiated breeding attempt differed significantly between breeding pairs nesting early in the season and those nesting late in the season (Figure 2.3). This result is due to the higher frequency of breeding failure late in the season because there was no significant difference between productivity frequencies early and late in the season when failed nests were excluded hmthe anaiysis (x2=2.6, M.45, df=3). 1conclude that some colonies and individual nests that were abandoned were reused within the season, and the fitaess cost of nesting later in the season is lower 30 breeding productivity due primarily to increased breeding abandonment. Antagonists and Disturbances Breeding herons responded to the presence of humans, Northwestern Crows, BaId Eagles, Red-tailed Hawks and Common Ravens in 1999 (Table 2.3). An antagonist was present within 250m of colonies on 86% of surveys in 1999 (n-46 surveys, n=22 colonies). Humans were the most frequent antagonists at heron colonies (54% of surveys) (Table 2.3). The proportion of surveys with crows present (5 1%) was not significantly different from that of humans (x2=0.2, PS.60, df4). The presence of humans was significantly higher than that of eagles (23%) (x2=34.7, P4i.0001, df=l) and hawks (16%) (x2=l 1.2, Pcû.01, df=l). The presence of eagies was significantly higher than that of hawks (x2=7.4, Pc0.01, df=l), and the presence of hawks was not significantly different from that of ravens (1 1%) (x2=3.6, P9.05, df=l). Herons tesponded to the presence of eagles more frequently than to al1 other antagonists (Table 2.3). Of the 0.5h surveys with an antagonist present, a response was detected most often for eagles (65%), followed by h2wks (9%), humans (4%), ravens (4%) and crows (1%). Eagies disturbed breedig herons significantly more often than al1 other antagonists (humans, X'=85.1, P4.W1, df=l; hawks, ~25.5,Pcû.000 1, df=l). There was no significant difference among the frequency of disturbances from humans, hawks and ravens (x2=2.0, Pfl.30, df=2), and crows dishubed herons less frequently han humans (x2=4.8, Pa,df=2). Not ail buon responses wece foiiowed by incursions by antagonists. hcmions by eagles occurred on 34% of disturbance surveys where a response was observed hmhemns to the presence of an antagonist, and incursions by 31 hawks occurred on 17% swveys where a response was observed (Table 2.3). 1conclude that herons in the study area responded significantly more 0thto eagles than to al1 other antagonists at hem colonies, even though humans and crows were present more often. Human Disturbance There was no significant relationship between total human activity within 250m of 22 heron colonies and mean heron breeding productivity in 1999 (P>O.iS) (including pedestrians, cars, planes and land clearing equipment). including only pedestrian activity within 250m of colonies produced a significant and negative relationship with mean heron colony breeding productivity (Figure 2.4)- Four of five colonies with hurnan pedestrians present on more than 50% of surveys failed to raise any young to fledging. Eagles were heavily disturbing two of these colonies in the week pnor to abandonment, humans were heavily disturbing one, and in one case no information was available. The only successful colony of the five was also heavily disnirbed by eagles and most breeding pairs abandoned at this site (85%, n=48 breeding pairs). One colony site was abandoned as a cesult of direct human disturbance (Colony 10, Appendix 1). The colony was disturbed by chah sawing and lawn mowing on 3 1 March, 6 April and 27 May, 1999, and breeding herons abandoned the site for the cemainder of the season when heavy land-clearing machinery was operated within 5Om of the colony edge on 30 lune. Novel human distuhance was indirectly linked to the abandonment of one colony in 1998 (Colony 33, Appendix 1) and one colony in 1999 (Colony 4, Appendix 1). A golf course was built within lOOm of Colony 33 in 1996 and 32 1997, and this event was foiiowed by colony abandonments in 1997 and 1998 (directly tinked to eagles in 1998). At Colony 4 in 1999, the cutting of mes occurred within 5h of the colony edge in the week prior to the abandanment of the colony, although this event was not directly observed, and eagles attacked the colony closer to the date of abandonment. Two other novel disturbances were documen ted, but the original response of the herons to the disturbance was not witnessed. Propane powered bird scare devices were set up within lûOm of Colony 14 in 1999, and dike repairs were conducted within lûûm of Colony 27 in 1998. ln both cases the herons apparently habituated to these repeated and mechanical disturbances because they continued to breed after these events. Apart fmm Colony 10, no nest abandonment due directly to human disturbance was documented. merhuman disturbances that had no obvious impact, beyond provoking a response from herons, included gunshots (n=3), a rack concert, and low flying planes (n=2). In 1999, 10 heron colonies were identified as having pnmarily rutiil development within 250m of the colony edge, 13 had prirnarily residential development, and eight pnmarily urban development (N=3 1). The kvel of urbanization within 250m of the colony edge had no significant relationship with mean productivity per initiated breeding attempt at 31 hem colonies in 1999 (F=û.26, b0.75). Abandonments occurred at three urban colony sites, nine residential colony sites, and one mal colony site. There was no signifcant difference among these colony abandonment ftequencies (x2=3.0,P>0.20. df=2). 1conclude that human activity negatively influenced hem breeding productivity in the study area because of direct disturbance and pedestrian activity near colonies. The 33 level of urbanization within 250m of hem colonies was not significantly related to breeding productivity. Eagle Disîurbance Bald Eagles were responsible for 55 incursions on 0.5h suweys. Eagle incursions were observed at 55% of colonies in 1999 (n=22), although 73% of incursions were observed at the three largest colonies in the study area (Colonies 13,23 and 34, Appendix 1). Eagles depredated heron eggs, nestlings or fledged young during 9.8% of al1 observed incursions (239 incursions in 549 hours of observation). No attacks were directed at adult herons. During nest defense herons physically engaged eagles on only two occasions. in both cases, a single adult hemn incubating eggs remained on and defended a nest using its bill and loud vocalizations for about 10s before abandoning the nest site. Eagles depredated both nests &ter they were abandoned. Due to the propensity of herons to abandon breeding when threatened, eagles had very little dificulty in depredating heron broods. Eagles were directly involved in eight (57%) of the 14 heron colony abandonments over the study period for which the cause of breeding failure was thought to be known. Eggshell collections shortiy &ter colonies were abandoned impticated predators in a fuaher five events. Five colonies abandoned due to unknown causes and humans caused the abandonment of one colony. Colony abandonment due to eagle disturbance was significantly more frequent tban abandonment due to human disturbance (x2=4.8, Pcû.05, df=l). Additiooaüy, all but one of400 bceeding pain at Colony 34 abandoned in 1999 under frequent eagle incursions. 34 Mean heron breeding productivity per initiated breeding attempt was not significantiy related to the presence of Baid Eagles within 250m of 22 colonies in 1999 (b0.40). Fourteen of 20 hem colonies occurred within 500m of an active eagle nest, but colonies close to active eagle nests were not iess productive than those far from active eagle nests (n=20, F=1.4, P>0.25, df=l). Herons raised significantly more young to fledging age in years when eagle disturbance was low than when it was high (Table 2.4). At 18 colonies with pl0 breeding pairs in 1999, eagles frequentiy disturbed the €ive colonies with the lowest mean productivity (mean=0.52, SD=0.52, n=5). In contrast, eagles disturbed two of five colonies with the highest mean productivity per initiated breeding attempt (mean=1.94, SD=O. 18). The five colonies with the lowest productivity had significantly lower mean productivity than the five with the highest (t =6.8, P4.001,df=5). 1conclude that Bald Eagles were involved in more colony abandonments than other antagonists, and heron colonies that were frequently disturbed by eagles raised significantiy fewer offspcing than less disturbed cobnies. Discussion Breeding Roducîivity The breeding productivity of the Gceat Blue Heron in south-coastal Bcitish Columbia duing 1999 (Table 2.1) was the lowest yet reported for this species in North Arnenca. Historicaily, the productivity of hemns across North America ranged fiom 1.3 to 2.7 fledglings per initiated breeding attempt and fiam 2.0 to 3.0 fledglings pr successful breeding attempt (reviewed by Butler 1997, see also Pratt 1970, Vos et ai. 1985). The 35 productivity of herons in my study was well below the continental range for initiated breeding attempts (0.82 fledglings per attempt, Table 2. l), and was in the lower part of the continental range for successful breeding atternpts (1.98 fledglings per attempt, Table 2.1). This implies that breeding abandonment is more common in this area than elsewhere in North Arnerica. The proportion of hem pairs successfully breeding each year in south-coastal BC decreased between 197 1 and 1999 (Gebauer and Moul 2000). Heron numbers on Breeding Bird Suweys in the Strait of Georgia declined 6% between 1966 and 1994 (Downes and Collins 1996). Christmas Bird Count data from 197 1 to 1999 show a decline in numbers on Vancouver Island and the Sunshine Coast, but not in the Fraser Valley (Gebauer and Moul 2000). The regional differences in hemnumbers and breeding productivity observed in my study (Table 2.1) support previous concerns about herons nesting outside the Fraser Valley, and especiaily on the Sunshine Coast. The number of breeding pairs on the Sunshine Coast hafallen from 97 in 1978 (Forbes et ai. 1985b) to Il in 1999 (Appendix 1), despite increased seacch effort. Breeding productivity in this region was zero in both 1998 and 1999. In addition, productivity on Vancouver Island was lower than in the Fraser Valley. It is unlikely thai there are sizable colonies on the Sunshine Coast that cesearchers have missed over the years. No largc+bc&raging hemns have been seen on kaches, and steep slopes dominate the region, providing fewer potentiai nesting locations than other regions (Moul et al. in press). Moreover, few herons have been found in Pender Harbour where Simpson (1984) studied a colony of about 40 breeding pairs. 36 Breeding herons have therefore abandoned the region, died off, or are nesting in srnaller groups that are more difficult to Iocate (Moui et al. in press). Herons nesting solitarily would be difficult to locate. Since both Christmas Bird Counts and Breeding Bird Surveys take place away from breeding areas, the data would appear to support declines in the number of herons due to abandonment of the region or death of breeding adults. Less confidence can be placed in the historical surveys conducted on Vancouver Island and in the Fraser Valiey due to the discovery of several colonies of over 50 breeding pairs that went unnoticed for several yeacs (Mou1 et al. in press). Abandonment, Produclivi@ and Cuiony Size Breeding abandonment is clearly an important decision for herons breeding in south- coastal British Columbia. Abandonment was common, widespread, and explained most of the variation in breeding productivity for initiated breeding attempts. More than half of al1 initiated breeding attempts and nearly haif of al1 colonies were abandoned before any nestlings were fledged in 1999 (Table 2.2). Breeding abandonment occurred at al1 but three colonies, and at a third of al1 successful colonies at lest half of the initiated breeding attempts were abandoned (Appendix 1). Nest abandonment was significantly correlated with mean colony productivity per initiated breeding attempt, and explained 96% of the variation arnong colonies ia the breeding productivity for initiated attempts (Figure 2. la). This retationship must be positive (see shaded region of Figure 2.la), but the residuals hmthe relationship explained a significant pomon of the variation in productivity per successful breeding attempt. This implies that the remaining variation hmthe relationship of pductivity per initiated attempt and abandonment was 37 explained prirnarily by the productivity of successful breeding attempts. However, the productivity of successful nests was not significantly related to the frequency of breeding abandonment arnong colonies in 1999 (Figure 2, lb). Tt is therefore likely that breeding abandoment represents the smngest determinant of actual heron breeding productivity at colonies in south-coastal BC. The productivity of successful breeding attempts among colonies was not significandy related to colony sizc in 1999 (Figure 2.2). Thus, the maximum ptential productivity of heron colonies had no relationship to the number of breeding herons at colonies. However, the productivity of ail initiated breeding attempts among colonies was significancly and positively related to colony size in 1999 (Figure 2.2). The productivity of initiated breeding attempts represents the rictual production of fledglings at colonies, and this measure of pductivity wm related to the number of breeding herons at colonies. The productivity of initiated breeding attempts arnong colonies was significantly and positively retated to colony size due to the high kquency of total colony breeding abandonment at small colonies (~50breeding pairs) (Figure 2.2). Most small colonies abandoned and al1 colony abandonments occurred at smd colonies in 1999. Small colonies thus bave a higher risk of complete breeding failure îhan large colonies (2% breeding pairs), However, abandoned colonies accounted for only a smdl proportion of al[ initiated breeding attempts across the study main 1999. Ody 15% of al1 breeding pairs nested in smali colonies, so the impact of this abandonment on the hem population of the study ma should be small, àepenàing on the exact function of sdcolonies in the population as a whole. As an example, srnaii colonies may be important alternate breeding sites for young and inexperienced birds (Brown et al 199û), and so couid serve 38 an important function for the population aside hmtheir contribution towatds the overall production of fledglings. Large colonies had significantly higher breeding productivity than small colonies in 1999 Figure 2.2), and no large colonies completely abandoned breeding in 1999 (Figure 2.2) (Appendix 1). However, most of the observed breeding abandonment in 1999 occurred at large colonies. As with small colonies, the actual productivity of large colonies was affected by abandonment, and was lower than the maximum potential productivity of these sites (demonstrated by productivity per successful breeding attempt) (Figure 2.2). Heron colony productivity is strongly influenced by breeding abandonment at large colonies, as well as at small colonies. At Colony 34 in 1999, 399 of 400 nests abandoned breeding without fledging any nestlings. This was an unusual event, and is the Iargest abandonment event that has been documented in this area to date. Historicaily, the largest abandonment was at a colony of about 100 breeding pairs on Sidney Island (Butler 1995). However, excluding Colony 34, breeding abandonment was still common at large colonies in 1999. Breeding abandonment at the remaining 10 large colonies accounted for 56% of aU abandoned breeding attempts obse~edin 1999 (58.5% of an estimated 1928 breeding attempts), and 49% of ail initiated breeding attempts at large colonies were abandoned (n= 1276 breeding attempts, excluding Colony 34). Breeding abandonment probably explains the historical declines in the productivity and number of breeding herons in south-coastal BC (Downes and Collins 1996, Gebauer and Mou1 2000). As with regionai differences in productivity (Table 24, nest abandonment is more common on Vancouver Island than in the Fraser Valley (Table 2.2). In addition, abandonment in the Fraser Valley should have been much lower, 39 because 67% of the initiated breeding attempts that were abandoned by herons in this region were at the largest colony in the study area (Colony 34). The prevalence of colony and nest abandonment in this area is an indication that a high level of within-season colony interchange likely occurs. Colony abandonments occumd mostly early in the season before eggs had hatched and while there was still a high potential for renesting. Of 19 colony abandonments over the study period, 17 occurred when >50% of ail breeding pairs were initiating breeding or incubating eggs (x2=8.5, PcO.01, df=l), venus the remaining two events that occumd when >50% of breeding pairs when rearing nesthgs. Colonies are dynamic, and individuals as well as colonies may change locations to avoid disturbance (Butler 1992). Heron movements within south-coastal BC are not well documented, but herons probably exhibit low colony, nest site, and mate fidelity. At a colony on the Sunshine Coast, Simpson et al. (1987) found that 40% of the breeding herons in 1978 did not teturn in 1979, and the majocity of breeding herons were on different nests and with different mates in 1979. Breeding Phenology Breeding herons in south-coastal British Columbia were not highly synchronized in their nesting habits, probably because of delays caused by breeding abandonment. Herons require about three montbs to fledge Young, but colonies in 1999 took an average of about four months to complete nesting. The longest breeding period at a colony was about 6 months. One quarter of the nests that were abandoned by breeding herons was reused by the same or another pair. There are severai indications that many of these cases were second nesting attempts by breeding pairs. Nests wece frequeatly reused immediately 40 after abandonment, severai coIonies were completely reused, and herons fiequently retumed to their nest and cleared eggshells after predation events (pers. obs.). Second nesting attempts may also include late breeders, such as young or inexperienced herons (Fernandez-Cruz and Campos 1993), but abandonment likely delays breeding for many herons. An estimate of within-season colony interchange across the study area would be useful for quantiQing how often pairs renest at the same or different colonies. Although many breeding herons probably renest at the same site, some will also decide to delay breeding or move to other sites to breed again. if within-season colony interchange is cornmon, then counts of breeding pairs in this area may be over-estimated. This may explain why counts of breeding pairs in south-coastal BC have been of limited use for assessing population size (Gebauer and Mou1 2000). An estimate of within-season colony interchange across the study area would ailow the calculation of a correction for any double counting of breeders. Delays in the timing of breeding likely have negative consequences for heron breeding productivity, because the productivity of heron nests was higher early in the season than late in the season (Figure 2.3). As with the productivity differences discussed earlier, this relationship is primarily due to breeding abandonment. Butler (1995) also obsewed reduced productivity through the season due to breeding abandonment. The loss of the current brood is the obvious cost of the decision to abandon a breeding attempt for a heron, but the fitness of further breeding attempts in the same season may also be depressed due to a seasonal increase in îhe frequency of abandonment. The fitness of late breeding attempts may be depressed fwther still because of a reduction in the tirne for fledged young to lem to hunt and build reserves for the winter (Butler 1994). 41 The Signwcance oJLXrturbance There are many potential factors in south-coastal British Columbia tbat codd cause declines in beron breeding productivity (Chapter l), but distucbance ai hem colonies is the most likely exphnation for the low productivity and common breeding abandonment reported in my study. Breeding abandonment is the primary mechanism through which heron productivity varies across colonies and regions, and nest abandonment has ben cited as a consequence of human disturbance for rnany species (reviewed by Parnell et al. 1988, Hockin et al. 1992). Increased nest abandonment bas been related to increased human disturbance in at least one species, the Brown Pelican, Pelecanus occidenralis (Anderson 1988). Factors other than disturbance do not appear to have a strong influence on heron breeding abandonment. Habitat shortages are a potential problem in highly developed areas (Parnell et al. 1988, Butler 1997), but would likely cause a reduction in the number of nesting herons rather than the widespread breeding abandonment observed in my study (Tabb 2.2). Furihemore, regional declines in hem numbers have been documented (hwnes and Collins 1996, ûebauer and Moui 2000), but they have occurred in the least developed regions of the study area (e.g., the Sunshine Coast). Contaminants have been declining over the past decade and currently are not seen as a widespread problem (Elliott et al. in prep.), although contaminants were likely involved in the abandonment of one colciny near Vancouver Island in the late 1980s (Elliott et aL 1989). Reductions ui heron breeding productivity due to adverse weather conditions have not been reprted in this area. Heron colony size is related tc food availability near breeding areas (Fasola and 42 Barbieri 1978, Gibbs 1991, Butler 1992, Gibbs and Kinkel 1997). and food availability probably bitsthe number of young raiseci by successful pairs (Butler 1995). but food shortages are not a likely explanation for the widespread breeding abandonment observed. A deficiency of food would probably have resulted in breeding abandonment, but it should have aiso caused reductions in the number of fledglings fmm successful breeding attempts. The productivity of successful breeding attempts is within the range of this rneasure of productivity for herons in North America, and was not related to the level of breeding abandonment at colonies (Figure 2.1 b). In addition, there were no seasonal changes or regional differences in the productivity of successful breeding attempts during my study. As well, feeding trips were frequentiy observed at colonies, fish were observed regularly under nests, and little starvation or unexplaineci nestiing monality was obse~ed during the course of my study. The Impact of Disturbance Human activity directly caused the abandonment of one colony in 1999, and was implicated in the abandonment of two other colonies. However, Bald Eagles exerted a much sttonger influence on heron breeding productivity in swth-costal BC, Eagles were present about half as often as humans or crows in 1999, but disturbed nesting herons more than six times as frequently as any other antagonist (Table 2.3). Eagles were the only antagonists observed smashing eggs and taking nestlings and fledged young. The breeding productivity of herons was significantly and negatively comlated with the fiequency of eagle incursions at two colonies in 1998 and 1999 (Table 2.4). and eagle disturbance was observed immediately before eight of 14 colony abandonments. Eagles 13 were iinked to more colony abandonrnents than humans were. in addition, because eagles were the only significant predators of heron broods, and unknown predators were involved in five other abandonments, it is likely that eagles were actually involved in ail but one colony abandonments (human disturbance caused the remaining event). Eagle disturbance is the most likely explanation for the cornmon and widespcead breeding abandonment observed in south-coastal BC (Table 2.2). Eagle populations in south-coastal BC have increased over the past half-century (McAilister et al. 1986, Forbes 1989, Vermeer et ai. 1989, Elliott et al. 19981, and eagle incursions were significantly more frequent in 1999 than in 1988 QL=17.6, Pc0.0001, df=l). Nomet ai. (1989) documented 56 incursions at eight colonies in 1988, with a total observation time of 578 hours. One incursion was observed on average every 10.3 hours. 1 observed 55 eagle incursions during 223 hours of observation at 22 heron coIonies in 1999. One incursion was observed on average every 4.1 hours. Increased eagle disturbance may be responsible for the increase in abandonment and consequent decline in heron productivity reported by Gebauer and Mou1 (2000). As suspected by previous researchers (Norman et ai. 1989, Butler et al. 1995), it is likely that eagle activity has substantiaiiy altered the productivity of breeding herons in south-coastai BC. Heron breeding productivity was significantly and negatively predicted by the frequency of pedestrian iraffic within 250m of colonies (Figure 2.4). However, the low productivity documented at colonies with high levels of pedestrian activity was more directiy due to eagle disturbance. Hence, there might be an interaction between human and eagle disturbances at heron colonies. Total human activity (including pedestrians, cars, planes and land clearing equipment) had no relationship to hembreeding 44 productivity, likely because herons perceive less risk from mechanical stimuli than from pdestcian activity (Vos et al. 1985, Carlson and McLean 1996, Rodgers and Smith 1995). The Perceived Risk of Disîurbance if the breeding attempt of a heron is damaged or destroyed, such as during a successful eagle incursion, abandonment would seem to be an obvious decision. However, successful predation events were rare and could not account for the frequent and widespread breeding abandonment that 1observed (Table 2.2). Herons abandoned breeding due to direct threats and also perceived dangers. Humans never directly targeted breeding herons and were not responsible for any incursions in 1999. Human activity therefore represented only a perceived risk of disturbance. Eagles were frequently responsible for incursions, but abandonment due to ii perception of risk from eagle activity aIso occurred. Al1 but one of 400 breeding pairs at Colony 34 abandoned their breeding attempts in 1999, and eagle incursions were frequent at this colony prior to this large and unusual abandonment event. Because this colony was located within an active eagle breeding territory (less than lûûm from an active eagle nest), it is likely that the eagIe disturbance at this colony was from only a few resident eagles. It would take many eagles to directly thteaten every nest at a colony this large, so clearly many herons at this colony abandoned breeding likely due to a perceived risk. Heron respooses to antagonists, or fack theceof, also show that bteeding herons considered threats based on a perceived risk of distucbance during my study. Successful eagle incursions were rare, yet herons ftequently responded to the presence of eagles 45 (Table 2.3). Herons did not aiways respond to the presence of humans or eagles (Table 2.3), and reduced their response to non-threatening human activity through the season (Chapter 3). Herons aiso responded to eagies and hawks on many occasions when the predators were flying near to, but not directly towards, colonies. Finally, breeding herons temporarily abandoned colonies with no apparent stimuli on several occasions early in the season. The toleration of human activity near some breeding colonies but not others also demonstrates that the perception of cisk is important for herons. Herons nest in urban areas and apparently habituated to repeated mechanicd disturbances at two colonies (see Chapter 3 for an experirnental test of habituation in herons). Other studies have dso reported tolerance of human activity by herons (Kushlan 1979, Webb and Forbes 1982, Vos et ai. 1985, Butler 1997). Breeding herons aiso appeated to habituate to eagle activity. In generai, herons only responded to eagles that were attacking or surveying a colony, and tolerated eagle activity if no threat was posed (Koonz 1980, Butler 1995). Individuals are predicted to respond to a disturbance that poses only a perceived threat (Gill and Sutherland 2000), and abadonment due to a perceived threat has been docurnented for ciconiiforms in at lest one study. Fredetick and Collopy (1989) followed the fate of 1609 heron breeding attempts in FIori& and found that 3 1% of abandoned nests were deserted More nest contents were damaged or young had fledged. Disiurbance and Heron Breeding Populaiions The common and widespread breeding abandonment that occurs in this region may imply that the density dependent costs for herons of movùig to aitemate breeding sites are low, 46 relative to the costs of staying, so herons therefore show a propensity to abandon breeding (Gill and Sutherland 2000). A situation with low density dependent costs concurs with current knowledge of hem population regulation. In a review of the factors regulating populations of ciconiiforms, Butler (1993) found little evidence for population regulation through density dependent factors. However, most evidence was equivocal so it is difficult to conclude that the density of cornpetitors has negligible costs for herons. Hem colony size and location are influenced by available foraging area (Fasola and Barbieri 1978, Gibbs 1991, Butler 1992, Butler 1995, Gibbs and Kinkel 1997). irnplying that food supply rnight establish the number of breeders at colony sites (Butler 1994). Rensity dependence may be increasing for breeding herons in south-coastai BC through group size effects, such as dilution of predation risk (Roberts 1996) (Chapter 3, Chapter 4). Smaller colonies abmdoned more frequently than larges colonies during my study (Figure 2. la). This has not been reported previously in this area, and could be a new trend due to increasing disturbance (Chapter 4). However, at present smail colonies account for only a srnail proportion of the initiated breeding atternpts in the study area (Appendix 1), so any density dependent influence will currently have a limited impact. This could change if heron breeding disperses from its' current distribution, or if the risk of disturbance increases at large colonies (Chapter 4). Habitat availability could also lead to density dependent population regulation if some herons could not find a suitable breeding site, but also does not currentiy appear to have a strong ifluence on heron breeding in this area (Bder 1992, Butler 1994, pers. obs.). In summary, density dependent costs to breeding herons appear to have a modest influence on the breeding productivity and distr%ution of herons in south-coastal BC, but 47 this may change if the costs of disturbance at colonies continue to increase in a density dependent manner. Density independent factors, such as the availability of food due to water levels or the survival of young during winter, may also be important for regulating herons numben (Butler 1994, Butler and Vennesland in press). No information is available to examine these hypotheses in BC, but other density independent factors cm be addressed. The renesting potentiai of herons is likely high early in the season (Chapter 3). and this may influence the costs of moving to a new location to breed again, relative to the costs of staying at the same location. The physiological cost of producing additional clutches of eggs for large birds, such as herons, is aiso low (Butler 1994). The length of the breeding season at colonies in 1999 was much longer than required, implying that herons frequently renest, and colony abandonments occurred mostly during colony initiation and incubation (i.e., before June). Estimates of within season colony interchange and the number of clutches per breeding pair would assist in assessing the costs for breeding herons of moving to a new location to breed. The costs of abandoning breeding sites may aiso be low for reasons other than the relative costs of moving to another breeding area. Gill and Sutherland (2000) only considered foraging in their model, and more choices are available to breeding individuals other tban deciding among breeding sites, because they cm also be inactive away hmbreeding areas (Goss-Custard and Sutherland 1997). This situation could be interpreted in two ways (Gill and Sutherland 2000). Fit, breeding abandonment couid have been common due to a relatively low cost of delaying breeding until the next season, relative to continuing the current breeding attempt, This is possible due to the longevity 48 of herons (Butler 1992). but seems unlikely given the high rate of breeding abandonment documented (Table 2.2), and its' relationship to the low and potentially declining productivity of herons in this ma(Table 2.1). If herons commonly decide to delay breeding, significant lifetime fitness costs should result. Second, breeding abandonment might have been common due to the relatively high costs of rnaintaining the current breeding attempt, or the high costs of moving to another breeding location. Both of these latter situations are possible and seem more likely given the results of my study. Eagles pose a direct threat to breeding herons because they prey on adu1ts as well as nestiings (Forbes 1987, Norman et ai. 1989), and cm also pose an important perceiveci threat, as demonstrated by the large abandonment event at Colony 34 and the responses of herons to non-threatening human activity. Eagle activity is widespread near heron colonies, and could produce an appreciable cost for herons at both current and altemate locations, If herons are frequently unable to breed due to eagle disturbance, as seems likely given the low productivity across the study area (Table 2.1), heron populations may decline. The impact of human disturbance appears modest compared to eagle disturbance, but also could have consequences if increases in human activity are seen (Chapter 4). A breeding heron leaving a large colony (250 pairs) for another site has few suitable choices available. It can settle in another large colony of which the= are few, or it can settie in one of many smail colonies (c50 pairs) (Appendix 1). Breeding productivity in smali colonies is low relative to larger colonies (Figure 2.2), so the risk of cbanging locations is very real. In Chapter 4.1 discuss the implications of increasing disturbance for the distribution of breeding herons in south-coastal BC. The number of herons displaced by disturbance can be reasonably infemd hm 49 my study, but we must also have diable information on the costs of al1 options open to herons to properly predict the population level consequences of any disturbance. Reliable estirnates of within-season colony interchange and the number of clutches per breeding pair are crucial because herons can choose to bceed again, or can delay breeding until the next season. It is important to idenci@ whether herons commonly abandon breeding in south-coastal BC due to the relatively low costs of delaying breeding or of breeding again at the same site, or due to the relatively high costs of continuing the current breeding atternpt. If the relative costs of breeding again are low, then disturbance may have a limited impact on the breeding productivity and population size of herons in this area. The same would apply if the relative costs associated with delaying breeding are low. However, if breeding abandonment is comrnon due to the relatively high costs of breeding again at the same location, as seems likely, then productivity could be cornpromised and the breeding population may decline. Given the high level of breeding abandonment that occurred in this area and its' relationship to the low and possibly declinhg breeding productivity of herons, reproduction is likely cornpromised at many locations. This negative influence is likely due to the activity of eagles. Seasonal changes in the perceived risk of herons and the influence of experience with a disturbance stimuius rnay also have an influence on how we view the choices available to herons, and this is the subject of Chapter 3. Summaty Great Blue Heron breeding productivity is iow in south-coastal BC and has probably declined since 197 1 (Gebauer and Mou1 2000), likely due to increased levels of breeding 50 abandonment. The number of breeding hemns has declined on the Sunshine Coast, and possibly acmss the study area (Gebauer and Mou1 2000). Productivity is highest in the Fraser Valley, and may be offsetting the low productivity on the Sunshine Coast and Vancouver Island. Breeding productivity increased with the nwnber of breeding pairs at colonies because of a higher frequency of breeding abandonment at small colonies than at large colonies. Breeding abandonment was common and widespread, and was the primary mechanism through which heron breeding productivity varied across both colonies and regions. Nest abandonment had a much stronger influence on breeding productivity than colony abandonment because dl abandoned colonies were srnall. Eagles were the primary predator observed at hecon colonies, and disturbance from eagles was likely responsible for most of the breeding abandonment observed across the study area. Human disturbance was limited, but pedestrian activity was linked to reduced productivity through eagle disturbance. Herons abandoned breeding due to direct threats of mortality, as well as perceived threats of disturbance. Delays in breeding likely resuIted €rom disturbance, and breeding productivity was higher early in the season than late in the season due to breeding abandonment. Table 2.1. Breeding productivity by region for Great Blue Herons nesting in south- coastal British Columbia in 1999. Productivity values are presented for initiated and successful breeding attempts, and are averages based on counts of fledglings and breeding attempts from sarnples at colonies. The nurnber of breeding attempts used in caiculating the averages are provided. Superscripts denote averages that are statistically different (see results). Productivity for Productivity for initiated breeding successful breeding Fledglings Nurnber of Fledglings Nurnber of per attempt attempts per attempt atternpts ------Vancouver Island 0.49~ 247 1 .78A 68 Fraser Valley 0.92~ 989 2.01 A 450 Sunshine Coast oc 11 - O Study Area Total 0.82 1247 1.98 518 Table 2.2. The frequency of nest and colony abandonments by region for Great Blue Herons breeding in soutb-coastal British Columbia in 1999. Al1 percentages are averages based on samples of breeding attempts at colonies within each region. The number of nests or colonies used in calculating the averages are provided. Superscripts denote averages that are statistically different (sec results). Region Nest abandonment Colony abandonment % n % n Vancouver Island 73A 247 ~8~ 12 Fraser Valley ~5~ 989 1gA t 6 Sunshine Coast 1 00~~~11 1O@ 3 - - Study Area Total 58.5 1247 42 31 Table 2.3. Antagonists and disturbances observed in 1999 on 446 0.5h surveys at 22 Great Blue Heron colonies in south-coastal British Columbia. Ptovided is the gercent of surveys the antagonist was present within 250m of the colony edge, the percent of these surveys resdting in disturbance of herons, and the percent of these disturbances resulting in incursions. Superscripts denote vaiues that are statisticiilly different (see results). Frequency of Antagonist Antagonists Disturbances Incursions Human 54* 4B O Northwestem Crow 51A 1C O Bald Eagle 23' 65* 34 Red-tailed Hawk 16' gB 17 Common Raven 1 lc se O Table 2.4. Breeding productivity and the fiequency of eagie incursions observed during total observation tirne at Colonies 16 and 23 in 1998 and 1999. The number of nests used to calculate mean productivity and the number of hours of disturbance observations are provided. For clarity, eagle activity has been rated as high or low. Subscripts refer to the statistical comparisons listed at the base of the table. ~- - Productivity Colony Year per initiated Eagle disturbance breeding attempt rating incursions No. of Mean SD N Der hout hours 1999 1.ea 1 .O 27 high 0.42a 36 16 1998 2.Za 0.9 54 low Oa 15 1998 0.6~ 1.1 165 high 2.ôb 47 23 1999 1.4~ 1.3 170 low 0.57~ 69 Proportion of breeding pairs that abandoned Figure 2.1 Breeding productivity vernis the proportion of breeding pairs that abandoned at 3 1 Great Blue Hexon colonies in south-coastai British Columbia in 1999. Productivity is presented for a) ail initiated breeding attempts, and b) only successfd breeding attempts. Al1 praductivity obmatims by initiated breeding attempts must fail within the uushaded region of the figure. For statistical tests, productivity data were treated with a in(x+l) transformatioa and abandonment data were treated with an aaguîar transformation. ô SuccessM breeding hitiated breeding O O 1O0 200 300 400 Colony size Figure 2.2. Breeding productivity versus colony size for 3 L Great Blue Hem colonies in south-coastai British Columbia in 1999. The productivity of successful breeding attempts is represented by the open circles and illustrates the maximum potential colony productivity for 1999 (nd8). The productivity of al1 initiated breeding attempts is represented by the closed circles and iiiustrates the actuai productivity for 1999 (n=3 1). Ail observations for successfui attempts have a corresponding observation for initiated attempts at that colony. For statistical tests, productivity and colony size data were treated with in(x+l) transformations. EARLY n (Nt114) Number of nestlinas fledaed per nest Figure 23. Frequency distributions of the number of nestlings fledged by Great Blue Heron pairs early and late in the season at 11 colonies in south-coastal British Columbia in 1999. Early breeding pairs were pairs whose eggs hatched from 8 April to 22 May and the eggs of late breeding pairs hatched hm23 May to 6 July. The total number of pairs observed during each pend is provided. The distriiution of the number of young fledged was signifcantly different between early and late breedea (z2=l7.1, P8.01, df4). Proportion of zruweys with humans Q50m Figure 2.4. Breeding productivity pet initinied breeding attempt versus the proportion of surveys with human pedestrian mc QSOm hmthe colony edge on 0.31surveys at 22 Great Blue Hem colonies in south-coastal British Columbia in 1999. For statistical tests, productivity data were treated with a ln(x+l) transformation and abandonment data were treated with an angular transformation. Chapter 3 RESPONSES OF GREAT BLUE HERONS TO REPEATED DISTURBANCE STXMüLI THROUGH THE BREEDING SEASON Introduction The defense of offspnng at the nest is an important component of parental care in birds due to the strong influence predators exert on breeding productivity (Rickiefs 1969, Lima and Dili 1990, Chapter 2). Breeding success has been positively correlated with the intensity of nest defense in birds (Andersson et al. 1980, Greig-Smith 1980, Blancher and Robertson 1982, Knight and Temple 1986, Breitwisch 1988), and many studies have shown that parent birds risk injury or death when mobbing or displying to predators (reviewed by Montgomerie and Weatherhead 1988). Nest defense is defined as any parental behaviour ti'iat increases the survival probability of the offspring, while simultaneously decreasing the probability that the parent will survive to breed again (Montgomerie and Weatherhead 1988). Parental fitness cm be ceduced directly through predation of breeding adults or the brood, or indirectly through a reduction in the energy available for growth or further reproduction due to the use of defense responses (Magnhagen 1991, Stearns 1992, Lima 1998). Assuming that defense of the nest deters predators from successfully attacking. nest defense shouid increase the probability that both the parent and the offspnng survive through the bteeding season (Andersson et ai. 1980, Montgomerie and Weatherhead 1988). However, parents of iteroparous organisms, that produce few young per year but have long reproductive lives, should be more likely to uade off the current seasons' reproduction for increasing the probability of survïving to breed again (Clutton-Bmk and 60 Godfray 1991, Steams 1992, Ghaiambor and Martin 2000). Thus, for the Great Blue Heron (Ardea herodias), a breeding attempt should be abandoned at some level of risk to the parent and either attempted again at the same or a different location, or delayed until the next breeding season (Chapter 2). Active nest defense, such as mobbing predators, is uncornmon for herons (Chapter 2), likely because they are too large to effectively harass srnaller and more maneuverabte predators (Forbes 1989, Burger 198 1). Herons thecefore defend nests mostly by vocaliWng and posturing near the nest site (Chapter 1). In south-coastal British Columbia, adult herons are at risk of mortality from humans and predatocs, Humans have been responsible for the rnortality of adult herons both accidentnlly and intentionally (Forbes et al. 1985b, Butler 1997), and the Bald Eagle (Haliaeetus leucocephalus) has killed adult herons (Forbes 1987, Butler 1997). Nest defense is generdly predicted to increase through the breeding season (reviewed by Montgomerie and Weatherhead 1988, sce aiso Breitwisch 1988, Weatherhead 1989). Parents might increase their intensity of nest defense through the season because of a reduced potentiai for re-nesting, a decrease in the difference betwmn the survival probabilities of the parent and the offspring, increasing aest conspicuousness, an increase in the value of the brood to the predator, or parental experiencehehavioural flexibility (reviewed by Montgomerie and Weatherhead 1988). Adjustments in behaviour due to experience are variously referred to in the literature ris acclimation or habituation. 1 wili refer to parental experience as the mechaaism for any changes, and behavioural flexibility as the variability in response to a standard stimulus. Behaviourai flexibility is the oaly factor that should not change as the breeding season progresses, although frtctors 61 dependent on the stage of the breeding season may overwhelm the flexibility of breeding birds (Breitwisch 1988). Nest defense puts the defending parent at higher risk of predation or accidents, increases energetic costs and may also identib the location of the nest to predators (Buitron 1983, Montgomerie and Weatherhead 1988). Because nest defense entails costs, parents should thecefore adjust their level of defense to match the seciousness of the threat (Ydenberg and Di11 1986, Lima and Dill 1990). Behavioural flexibility can be thought of as a simple form of learning, or as a type of phenotypic plasticity (Dukas 1998). if the costs of responding are higher than the costs of learning or of king naïve, animals should adjust their nest defense behaviour based on prior experience with the stimulus (Le., they should be behaviourally flexible). Studies have used investigators, mode1 predators, or both, to test the nest defense behaviour of birds (reviewed by Montgomerie and Weatherhead l988), but few studies have considered the influence of @or experience with a stimulus (Knight and Temple 1986, Montgomerie and Weathehead 1988). Gramza (1967) and Bacash (1975) warned of the potential confounding effect of parental experience on studies of changes in nest defense behaviour. Knight and Temple (1986) controlled for the effect of ce-visitation in a study of the American Robin (Turdus migratorius) and Red-winged Blackbird (Agelaius phoeniceus). They achieved this by varying the date and number of exposures of nests to both an investigator and a mode1 predator. They found tbat parents increased their defense response with repeated exposures to both humans and the mode1 predator, while single exposures always resulted in the same response, regardless of the date. Knight and Temple (1986) interpreted this cbange as behavioural flexibility - the parents 62 grew bolder each time they drove off the predator. However, the methodology used by Knight and Temple (1986) has been questioned (Weatherhead 1989), and Coleman (1987) argued that tbis would be a non-optimal response, because energetic costs to the parent would rise with no conesponding gain in the benefits obtained. Nevertheless, an increase in defense could theoretically occur if the perceived cost of not defending the n8st is high (Montgomerie and Weatherhead 1988). Breitwisch (1988) found some evidence for a re-visitation effect with the Northern Mockingbird (Mimus polyglottos), but didn't properly control for this outcome when designing the study. A snidy by Siderius (1993) is the only cleariy documented case of a behavioural change in birds due to prior experience with a stimulus. in her study, Eastern Kingbirds (Tyrannus tyrannus) were exposed to a model predator one ro three times through the breeding season. Controlling for date, kingbirds reduced their response with each successive exposure to the model predator. Researchers have dso found that parental experience had no influence on nest defense behaviour. Redondo and Carranza (1989) and Weatherhead (1989) both controlled for date in studies of the Magpie (Picu pieu) and the Song Sparrow (Melospizu melodia) respectively, and found that parental experience from re- visitation of the investigator had no effect on responses. Changes in the responses of animals to repeated disîurbance stimuli have not been considered in recent attempts to predict the consequences of disturbance on the survival and distribution of animais (Gill and Sutherland 2000). The model proposed by Gill and Sutherland (2000) predicts the impact of disturbance on animai populations using the strength of an animal's response to a perceived threat and the density dependent mortaiity costs expected at aiternate locations (Chapter 2). Many antagoaistic activities that induce 63 a perceived threat fiom anirnals are repeated (e.g., predators foraging in a patch or human activity in urban areas), and thus experience with a stimulus should affect the response obtained fiom animals. In contrast to most studies of nest defense behaviour, it is often asswned that the Great Blue Heron habituates to repeated non-threatening human disturbances (Mark 1976, Kushlan 1979, Webb and Forbes 1982, Vos et ai. 1985, Carlson and McLean 1996, Butler 1997). However, the effect of seasond factors has generaiiy been ignored in these studies. Vos et d. (1985) concluded that increasing 'investment' was likely the reason for increased nest attendance through the season, but investment was not defined in terms of expected future opportunities, and no controlled test was used to differentiate between changes due to the seasonal timing of nests and those due to parental flexibility . Behavioural flexibility may allow herons to breed close to non-threatening human and predator activity, but herons may perceive differing levels of risk through the breeding season due to seasonal factors. Understanding the interplay between the influence of seasond effects and those of flexibilitywill help to better understand the effect of disturbance on the productivity and distribution of herons in south-coastd British Columbia. The response of herons to the standardized approach of a pedestrian cm be used to establish estimates of the set-back distances required at colonies to preclude disturbance from human intruders. Set-back distances, or buffer zones, are defined as the minimum distance of non-intrusion by humans fiom the edge of a colony that will prevent disturbance to breeding birds (Rodgers and Smith 1995). Recommendations of the distance required to protect breeding herons range from LOO to 250m (Vos et al. 1985, 64 Erwin 1989, Rodgers and Smith 1995). Objecîive and Outline The objective of this chapter was to detennine whether Great Blue Herons, nesting in urban and nual areas of south-coastôl British Columbia, alter their nest defense bebaviour to the repeated and standardized approach of an investigator within and between seasons. In this chapter, 1report on changes in the cesponse of herons to the repeated and standardized approach of the investigator through the breeding season. By controllhg for the effect of breeding stage on the response of herons to disturbance, 1examine the influence of parental experience on how herons decide to respond. 1also investigate the relationship of heron response to coiony size and the level of urbanization near colonies. Findly, 1discuss the buffer zone, or set-back distance, required to adequately isolate heron colonies from human pedestrian disturbance. Methods Sîudy Area and Species The study was conducted in southçoastal British Columbia, Canada (Figure 1.1) between March and September during 1998 and 1999. General information on the study ma, the biology of the Great Blue Heron, and batsto breeding bird populations in genenl, and herons in particular, are provided in Chapter 1. The response of herons to antagonistic events varies greatly (Chapter 2), but tends to progressively increase as the perceived threat of an event increases (Chapter 1). Herons initialiy becorne vigilant, raising their heads in silence. As the perceived danger increases, herons vocalize with a cepetitive 55 'cluck' call, and sometimes stand up, hop off theii nest, or fly from the nest. Under heavy threat, herons vocaiize with a loud scream, and may flush from the nest me and the colony. Study Sites The study was conducted at 10 Great Blue Heron breeding colonies in south-coastal British Columbia (Table 3.1) (Appendix 1). Colonies were included in this chapter if they were active through the breeding season (i.e., fledged nestlings), and if there was pedestrian access to the colony edge. Observations were made at six colonies during 1998 and 1999, at one colony during ody 1998, and at three colonies ducing only 1999. Colony sizes ranged from 34 to 322 breeding pairs. Although both forested and open, ail sites except Colony 30 had a visibility range from the investigator to the colony edge of at lest 100 m. Colony 30 had a steep access route and a visibility range of about 50 m. The date of the first incubation of eggs by h 1 heron at colonies was assumed to be a better indicator of the seasonal timing of breeding attempts than the date of colony initiation (Chapter 2). Incubation was identified by the behaviour of herons on the nest, as described in Chapter 2. The date of fmt incubation ranged from March 12 to April3 (median March 24, n=10). Investigator Appmch The approach of the investigator was used to identiîj changes in heron response to potentially antagonistic events thugh the breeding season. The approach was standardized as foiiows. The investigator wore a dark hat and yeiiow min coat on each 66 visit. At heavily vegetated sites access trails were cut dunng the winter to ensure that the speed and noise dunng the approach were similar among colonies. Al1 habitat alterations were minimal and occurred oniy at sites with no public iiccess. At sites with sparse ground vegetation, bushes were rustled on approach. As with Erwin (1989) and Rodgers and Smith (1999, the investigator appcoached colonies at a speed of about one step per second. Investigator approaches were starîed between February 23 and Aprii 13. Relative to incubation, approach initiation at coionies ranged hm28 days before first incubation to 34 days after first incubation (mean=16 diiys after incubation, Sb20days, n=9). Variation in the initiation of approaches at colonies was due to variation in the date of coiony initiation by herons, access problems, and my own concerns with respect IO the sensitivity of the herons at particular sites dyin the season. The duration of observations within a breeding season mged from 52 to 146 days (mean=l05 days, Sb32days, n=9) (Table 3.1). Variation in the duration of observations was due to differences in colony phenology, and access problems €rom fencing and flooding. Rodgers and Smith (1995) defined a response as the movement of a breeding bicd away hma nest site. 1defined a hemresponse as the distance hmthe cobny edge at which >1 hem vocalized or moved due to the approach of ttie investigator. Silence and alertness were not considerai when determinhg the response distance (Chapter 2). The difference between these response definitions stiould be slight because movement generaliy foiiows shortly after vocalizations, If herons allowed the investigator to pass the coIony edge before responding, the responst distance was reported as a negative distance, measured inwards from the colony edge. This method was used because the respoose of 67 herons generaily increased as the investigator moved towards the centre of the colony. I used control groups of hemns at two large colonies (>lm breeding pairs) to examine if changes in hem response through the season were due to the seasonai timing of breeding attempts or the behavioural flexibility of herons. initiation of the control approach routes was delayed until two weeks after the first hatching event at each colony to ensure that most herons were rearing chicks and were no longer incubating. Hatching was determined as per the methods describecl in Chapter 2. The breeding stage of responding herons was similar in both expecimental and control groups because the control approaches wea at the same colony as the experimental approaches. The predictions of this controlled experiment are outlined in Figure 3.1. if herons adjust their response simply due to the stage of the breeding season, the responses received from the control groups of herons should closely follow those received from the expecimental groups. Conversely, if herons adjust their responses based on parental expenence alone, responses from the control groups should be higher than, aithough parallel to, those from the expecimentd pups (Figure 3.1). Data Analyses Statisticai anaiyses were conducted using Minitab version II for Windows and SAS version 6.12 for Windows. Because breeding phenology at colonies varied across south- coastai BC (Chapter 2), al1 response dates for inter-colony compatisons were adjusted with tespect to the date of first incubation at each colony so that first incubation was defined as day zero for each colony. An anaiysis of covariance was used to identify clifferences in heron response 68 distance among colonies and between breeding seasons. Differences in response distance among colonies and between seasons were assumed to be present if statistical interactions identified a signiFicant lack of covariance (P>0.05). Significant interactions were not identified among colonies or between seasons (P>0.10), so an analysis of variance with responses from nine colonies pooled was performed to determine if herons changed their response to the approach of the investigator through the season, and to test for changes in heron response through both seasons at individuai colonies. Colony 1 was excluded as no response was ever observed from hemns at ibis site. The mean response distances at individual colonies were tested for differences using the Ryan-Einot-Gabriel-Welsch Multiple Range test. Colony size (the number of breeding pairs) and the level of urbsuiization within 250m of the colony edge (see Chapter 2) were compiued to the mean response distance of breeding herons at colonies. No response was obsemed at Colony 1, but this colony was included in these analyses assuming a mean response distance of zero (a conservative estimate). To investigate whether changes through the season in hem response at Colonies 13 and 23 were due to the stage of the breeding season or the behavioural flexibility of herons, an analysis of covariance was performed to ideniif) interactions. Significant interactions were identifed at both colonies (Pd.05), so four univariate regressions were performed on the responses of herons on each approach route at each colony. This analysis was performed using data from 1999 only. Resulis Seasonal Changes in Eerun Response Changes in the response of herons to the approach of the investigator througb the breeding season at 10 south-coastal British Columbia heron colonies are summarized in Table 3.1. Regcessions of heron responses through the seiison, at colonies with observed responses (n=9), are depicted in Figure 3.2. With observations from al1 nine responding colonies pooled, herons significantly reduced their response distance through the breeding season by an average of O.48m per day (Figure 3.2). There were no significant interactions in the pattern of responses among colonies (PH.10).or in the responses obsewed between years at individual colonies (PM.10). At Colonies 1 1 and 19, negative distances were recorded late in the season when the herons would allow access beyond the cotony edge before responding (Table 3.1, Figure 3.2). Hemns at Colony 1 never responded to any human disturbance, presumably due to the continuous human presence below and around the colony. 1conclude that herons significantly changed their response to the repeated approach of the investigator through the breeding season, but responses did not vary significantly between yem. Colony Digerences in Heron Response The pattern of change in heron response through the season varied among colonies in south-coastal British Columbia (TabIe 3.1, Figure 3.2). Colony 1 never responded to the investigator, or any other human activity. The regression slopes of heron responses at the remairhg nine colonies vacied in signifîcance from Pfl.50 at Colony 16 to P 70 Colonies 23.26 and 14 (Table 3.1). AU slopes were negative and significant at the ~~0.05level except for Colony 16. The mean response distance from 1998 to 1999 with ail responding colonies pooled was 25.7m (SD=25.1, n=158, n=9 colonies). Mean colony response distances fell into five overlapping groups, as shown in Table 3.1. Colony 13 had the lowest observed mean response distance (7m), and this mean was significantly different from the mean responses at al1 colonies except Colonies 23 (16m)and 25 (23m). The highest mean response distance occurred at Colony 16 (am), and the mean response at this colony was not significantly different from the mean response at Colony 29 (53m) or at Colony 27 (44m). The maximum response distance observed at heron colonies varied from 35 to 1ûOm (mean=74.2, SDd5.0, n=9) (Table 3.1). Minimum response distance ranged from -35 to 15m (mean= -2.2, SD=18.2, n=9) (Table 3.1). Negative response distances represent hem responses after the colony edge was passed. There was no significant relationship between the mean response distance and the number of breeding pairs at heron colonies (n=lO, P>0.45), although the two largest colonies aiso had the lowest mean response distances (Table 3.1). Mem heron response distance varied significantiyand negatively with the level of urbanization within 250m of the colony edge (Figure 3.3). 1conclude that the intensity of response varied significantiy among colonies, and was negatively and significantlyrelated to the level of urbanization near colonies. Eeron Responses Controüingfor hie Heron responses to the approach of the investigator were tested controllhg for the stage of the breeding season at Colonies 13 and 23 (Figure 3.4). Tbe interaction between date 7 1 and treatment group was significant for heron responses at both colonies (Colony 13, Pd.05; Colony 23, Pc0.01). The slopes of heron responses through the season therefore differed significantly between expecimental and control approach routes at both colonies. Univariate regressions of heron response distance on date at both colonies declined significantly through the season for both the expecimental and control approach routes (Figure 3.4). 1conclude that herons breeding at Colonies 13 and 23 significantly adjusted their responses to the investigator approach, based on experience with the stimulus and the stage of the breeding season. Discussion My study is the first to show that herons habituate to non-threatening antagonistic activity near breeding areas. 1demonstrated that herons breeding in south-coastal British Columbia adjusted their response through the season due to experience with non- threatening human activity (Table 3.1, Figure 3.2). I experimentally demonstrated that herons presented with novel antagonistic stimuli from humans part way through the breeding season, adjusted their responses through the remainder of the season to eventually converge with an independent group of herons at the same colony expose&to the same activity since the stm of the breeding season. Seasonal Changes in Heron Response A significant decrease in the response distance of herons through both 1998 and 1999 was observed at eight of nine colouies where responses to the investigator approach were 72 observed (Table 3.1). Further, 1 showed in Chapter 2 that most colony abandonments occurred early in the season before young had hatched, suggesting a bigher perceived nsk of disturbance early in the breeding season. Because herons rarely engage in the mobbing of predators (Burger 1981, Forbes I989), nest defense is best described by the disincf nation of herons to abandon a breeding attempt. Under this assumption, herons increased their nest defense through the breeding seasan, because a stmnger stimulus was needed to cause them to respond, or flee, as the season progressed (Table 3.1, Figure 3.2). For example, Bald Eagles regularly attack breeding herons in south-coastai BC, preying mostly on eggs and nestlings (Norman et al. 1989, Chapter 2), and rarely on aduIts (one report in Butler 1997). Adults are rarely depredated because hemns frequently abandon nests following little or no active defense (Chapter 2). This response leaves the nest vulnerabk and the adult safe, hence the eagle genedly preys on the nest contents instead of the adult. Eagles were never observed to target n fleeing adult during eagle incursions over the study period (Chapter 2). It is thus likely that responding to and staying with the nest represents nest defense for herons in south-coastal BC. Naturalists have long observed increues in nest defense by breeding birds through the season (e.g., Simmons 1955), and much effort has gone into understanding the adaptive significance of these changes in behaviour (reviewed by Montgomerie and Weathehead 1988). Colony Dueremes in Eemn Response Mean tesponse distances varied ~i~cantiyacross colonies in south-coastal British Columbia, and feu into four overlapping groups (Table 3.1). This implies that herons at 73 different colonies perçeived different levels of risk fcom the standardized investigator approach, and adjusted their response bas& on the perceived risk of distucbance (Chapter 2) (see further for experitnental evidence). A heron's view of the approaching intmder was similar for herons at al1 but one colony (Colony 25), so it is reasonable to assume that herons at colonies with low response distances had frequently detected the intnider, but bad decided that a response was not warranted. This distinction between detection and response makes economic sense energetically (Ydenberg and DiIl 1986), and it has been well documented that animals dter their reproductive behaviour due to predation risk within their lifetimes (Lima and Di11 1990, Lima 1998). Although 1showed that the differences in mean colony response were not significantiy correlated with the number of breeding pairs at colonies, large colonies may have been under-represented in this anaiysis. Colony size likely has an influence on the response of breeding herons because the two least sensitive colonies (Colonies 13 and 23) were the largest in tems of numbers of breeding individuals. This implies that colony size decreased, or diluted, the risk of distubance perceived by herons at these sites. Additionally, dilution of the impact of eagle depredation may account for the signifiant increase in breeding productivity with colony size observed in 1999 (Chapter 2). Heavy human activity wouid be an altemate explanation for the low sensitivity to human activity observed at these colonies (assuming that herons adjust their response to human activity based upnthe threat posed). However, both these colonies were more isolated fiom human activity than colonies with higher mean response distances (Table 3.1, Figure 3.2, Appendùc 1). Vigilance increases with group size, and the nsk of predation is Likely reduced in large groups (see reviews by Lima and Di 1990, Roberts 1996, and Lima 1998). Ydenberg and Di11 (1986) predicted that the distance from a predator at which an animal takes flight should decline with group size if the presence of conspecifics dilutes individual predation risk. In contrast, response distance should increase if interference from competitors occws, and if animais anange themselves so that net benefits are equai across group sizes then no relationship should be observed (Ydenberg and Dill 1986). Dilution of perceived risk is therefore the most iikely reason for a decreased response distance in larger group sizes, and likely occurred with breeding herons in this study. The mean response of herons over the study penod was significantly and negatively ~latedto the level of urbanization within 250m of the colony edge (Figure 3.3). Mean response distance was shorter at hem colonies with higher levels of urbanization than at colonies with lower levels of urbanization. This implies that herons are flexible in their response to human activity. Herons tolerated high levels of human activity at eight of 35 colonies, and appacently habituated to novel human disturbances at two colonies in 1999 (Chapter 2). Other studies have also commented on the apparent propensity of Great Blue Herons to adapt to non-threatening human disturbance, both acmss North Arnerica (Kushlan 1979, Vos et ai. 1985), and in British Columbia (Mark 1976, Webb and Forbes 1982, Forbes et al. 1985b, Butler 1997). Behaviouml FlmiiI&y Herons in south-coastal British Columbia adjusted their response through the bceeding season to the threat they perceived from low-level human activity. However, seasonal 75 phenornena, such as the potential for renesting or the value of the brood to a predator, would also predict an increase in nest defense through the season (Montgomerie and Weatherhead 1988). The effect of date must therefore be controlled to properly infer the influence of behavioural flexibility. Few studies have properly controlled for date or parental experience when investigating nest defense behaviour. Redondo and Cluranza (i989)and Weatherhead (1989) controlled for the effect of date, but found no flexibility in the defense behaviour of birds. Only Siderius (1993) has conclusively identified flexibility in the response of breeding birds to disturbance. The results of my expriment controlling for the stage of the breeding season in the response of herons to the approach of the investigator clearly indicate that herons respond flexibly to an intruder, but aiso respond based on the stage of the breeding season (Figure 3.4). Heron responses on the control approaches at both colonies began at a similac level to those on the experimental approaches, but then converged with the expenmentai approaches near the end of the season (Figure 3.4). The decrease in nsponse was significant on experimental and control approach routes at both colonies, and responses on both of the control approaches decreased at a significantly faster rate than on the expecimental approaches (Figure 3.4). If heron responses were dependent only on the stage of the breeding season, the responses on the control approaches should have closely foliowed those on the expecimentai approaches (Figure 3.1). if the seasonai condition was unimportant for herons, but behaviounl flexibility had influence, then the responses on the coatrol approaches should have been parallel to, but higher than, the cesponses on the experimentai approaches (Figure 3.1). Because the responses on the conml approaches at 76 both colonies began near the same level as on the experimentai approaches, pnor experience with the stimulus had a sign5cant influence on how hemns perceive the risk assocriated with a disturbance. Herons exhibited flexibüity towards repeated non- threatening human inasions into breeding colonies, but also responded based on the stage of the breeding season - especially for novel stimuli. Herons did not respond differently at breeding colonies in 1999 compared to 1998, suggesting that between only two seasons the influence of seasonai timing may overwhelm the behavioural flexibility of herons (Breitwisch 1988). Assuming that the approach of a single pedestrian well represents the influence of human activity in generai, the behavioural flexibility shown by breeding herons in south- coastal British Columbia could bode well for their ability to adapt to increasing human disturbance in this region (Chapter 4). However in Chapter 2,I reported that pedestrian traffic (an example of low-level human activity) had a negative infiuence on heron breeding productivity (Figure 2.4). Reduced productivity at colonies with high levels of pedestnan activity was directly due COeagle disturbance, although the nature of this potential interaction, and therefore the exact impact of human pedestrian activity, is not clear (Chapter 2). Why would an animai habituate to a stimulus (i.e., human activity) that is correlated with negative fitness benefits (i.e.,eagle disturbance)?This could occur if the indirect negative influence was novei or uncommon. Eagle populations are increasing in south-coastal BC (Chapter 11, and eagie disturbance probably is as well (Chapter 2). Furthetmore, if human activity is not dways associated with eagle disturbance, herons may not Iink the events (Dukas L998). The Influence of Seasonal Timing Irrespective of the influence of behavioural flexibility, the presence of a significant influence from the stage of the breeding season means that caution must be exercised with regard to disturbing hem colonies, especially early in the season when experience with a stimulus is likely to be limited. The decision of how to respond to a potentially antagonistic stimulus is to some degree inflexible because it depends on factors determined by the stage of the breeding season, and such seasonal influences may overwhelm the influence of parental experience (Breitwisch 1988). In this chapter, 1concluded that herons increase their intensity of nest defense through the season, as occurs for many species of birds (reviewed by Montgomerie and Weatherhead 1988, see also Breitwisch 1988, Weatherhead 1989). An increase in defense is expected through the season because of increasing nest conspicuousness, an increase in the value of the brood to the predator, a declining potentiai for re-nesting, or a decreasing difference between the survival probabilities of the parent and the offspnng. Increasing nest conspicuousness through the breeding season is unlikely because heron nests are usuaily located at the top of trees, and colonies are very obvious (Butler 1997, pers. obs.). Hem nestlings grow from about 50g to about 2kg in about 60 days (Butler 1992), and thus the value of the nest contents to predators should increase through the season. However, this factor also probably has a limited influence because eagles commonly preyed on eggs as weli as nestlings and there was no difference in the frequency of eagie incursions at heron colonies hughthe breeding season in 1999 (Chapter 2). The potential for renesting Likely had a notable infiuence on hem response 78 through the breeding season. Herons iikely have a higher potential for renesting early in the season compared t~ late in the season. The approximately 170 day breeding season for herons in south-costal BC is much longer than the 100 days they require, providing ample time to attempt breeding more than once if started before June (Chapter 2). Breeding abandonment is common early in the season, and colony abandonments occurred mostly during colony initiation and incubation (i.e., before June) (Chapter 2). Furthemore, eggs represent a modest cost to large birds such as herons (Butler 1994). Perhaps, the survival probability of nestlings also influenced heron response through the season. The relative difference between the expected future survivd of the offspring and that of the parent declines with increasing offspnng age (Andersson et al. 1980). Thus, the relative importance of the expected reproductive productivity of the offspring for the Fitness of the parent should increase. This should have an influence on al1 animais that exhibit parental care, and has been documented across a diverse assemblage of birds (reviewed by Montgomerie and Weatherhead 1988). Siderius (1993) reported no change in nest defense due to the stage of the breeding season because kingbird parents did not defend young more than eggs. Siderius (1993) surmised that, for kingbuds, replacing eggs might have been as cosdy as replacing young. However, the lowest level of defense observed in her study was from the group of nests disturbed before eggs were laid. This implies that parents were willing to risk more defending a nest with eggs or young than a nest with no brood. Assuming that the building or occupation of a nest site has a fitness cost, this would demonstrate an increase in nest defense based on seasonal factors. Prior to the laying of eggs the potentid for renesting would be higher than after laying (Montgomerie and Weatherhead 1988). 79 Perceived Ri& of Disturbance The flexible response of breeding herons to human distwbance has consequences for how we view the perceived survival concept introduced by Gill et ai. (1996) and Gill and Sutherland (2000), oudined in Chapiers 1 and 2. Their hypothesis posits that animals should trade-off the risk of disiurbance in a foraging patch with the perceived costs of moving to aiternate locations (Gill et al. 19%). in Chapter 2,I argued that this frarnework could be usehl for better understanding the influence of breeding abandonment for herons nesting in south-coastd British Columbia. Breeding abandonment is a comrnon response to disturbance (Anderson 1988, Gill and Sutherland 2000), but the impact it has on the productivity and distribution of breeding animais will depend on the costs of breeding again or of delaying breeding. These costs will determine the strength of response received from breeding animais (Gill and Sutherland 2000) (Chapter 2). It is clear from the results presented in this chapter that the strength of the response from herons changes due to prior experience with a potentially antagonistic activity. Herons probably alter their perception of risk over time with repeated non- threatening stimuli. This process should be apparent with foraging animais as well as with breeding animals, because many disturbances are rcrpeated. Predators may concentrate on specific patches of food over time (Lima and DiH 1990, Lima 1998), and human activity such as pedestrian and vehicular trafic is frequently of a repeated nature. The disturbance risk perceived by animais wiii thus likely depend not just on the costs associated with responding to distucbance, as hypothesized by Gi and Sutherland (2000). but also on the perceived costs of not responding. Ifdetecting but not responding has a low perceived 80 cost then herons may change their response over time. Repeated and non-repeated disturbances likely represent very different situations. Set-Buck Distances Much attention has been paid to the butTer zones, or set-back distances, required to protect breeding colonies of wading birds fiom human disturbance (Parker 1980, Vos et al. 1985, Carlson and McLean 1986, Erwin 1989, Rodgers and Smith 1995). The response distances observed for investigator approaches in my study enable discussion of the set-back distances required to protect breeding herons in south-coastal British Columbia from pedestnan disturbance. Rodgers and Smith (1995) documented response distances for colonial waterbirds breeding in Florida, and caiculated that Great Blue Hecons require a set-back distance of lûûm to be properly isolated from human activity. Envin (1989) also recommended a distance of IOOm, and Vos et al. (1985) recommended a distance of Zorn. Rodgers ruid Smith (1995) pcovided a formula to calculate set-back distances for wading birds using the rnean and standard deviation of heron response distances Gom a log-normal distribution [set-back(m) = (exp(mean + 1.6495 * SD))+ 401. To avoid autocorrelation between observations from heron experience with the distucbance stimulus, Rodgers and Smith (1995) visited colonies a maximum of once or twice per year. 1 used the formula provided by Rodgers and Smith (1995) to determine the set-back distance appropriate to isolate herons hmhuman activity in south-coastal BC. To avoid confounding effects hmexperience with the stimulus, 1used the first response to the investigator approach ceceived at each colony over the course of my study. This caiculation provided a set-back distance for herons in south-coastal BC of 165m. Tùe caiculated set-back distance for herons in this area is much larger than that calculated for herons in Florida. The perceived risk of disturbance from investigator approaches may be different in BC than in Flonda. but methodological differences also exist between my study and that of Rodgers and Smith (1995). Rodgers and Smith (1995) accounted for parental experience when designing their study, but they did not account for seasonai changes, and did not explain exactly when they conducted observations through the breeding season. Heron response in my study decreased through the serison regadless of experience with the stimulus (Figure 3.4). and this may imply that seasonal changes may have confounded the results of Rodgers and Smith (1995). Due to seasonal changes, measurements should be made at a consistent point through the season for ail breeding locations. isolating heron colonies in south-costal BC with set-back distances would be of benefit to the conservation of breeding herons in this area. Pedestrian activity near colonies was negatively and significantlycorrelated with colony breeding productivity in 1999 (Figure 2.4), and human activity bOm from breeding herons directly caused the abandonment of one colony in 1999 (Chapter 2). Most colonies were located away from roadways, so the dominant form of human disturbance at hem colonies was therefore of a pedestrian nature. The one exception was the abandonment at Colony 10 in 1999 caused by land clearing machinery (Chapter 2). However, a set-back at this colony may have lessened the impact of this disturbance by providing a larger distance between the disturbance stimulus and the breeding herons. 82 Set-backs are a useful conservation tml, but it is important to stress that the results presented here represent the influence of relatively low-level human distucbance. Human disturbance aiso results from higher intensity events such as logging or explosives (Parnell et al. 1988). hdclearing on a residentiai property Additionally, Bald Eagles disturb breeding herons more than humans, and have a stronger negative influence on heron breeding productivity (Chapter 2). Set-backs will not protect breeding herons from eagle disturbance, apact from any eagle impact that may be linked to human activity (Chapter 2). More research is needed to lem why eagle activity is increasing in this area before a proper solution can be determined (Chapter 4). In summary, a set-back distance of 165m would be a useful tool for helping to protect hem colonies from low-level human disturbance such as pedestrian activity in south-coastal BC, but herons are also at risk hmsttonger human disturbance stimuli such as land clearing and eagle disturbance. An effective and cornprehensive heron conservation plan should addcess not only pedestrian activity, but al1 forms of potencial disturbance. Summary Most Great Blue Herons breeding in south-coastai British Columbia hm1998 to 1999 were les inclined to abandon their nests through the breeding season when exposed to low-level human activity, indicating a general increase in nest defense through the season. The level of defense varied among colonies, indicating different perceptions of disturbance risk. This variation in response was not significantly related to coiony size, although the two largest colonies exhibited the Iowest responses. Heron response was significantiy related to the level of urbanization near colonies. Variation in the response of breeding herons through the season was due to both the stage of the breeding season and the behaviourai flexibility of herons. Caution should be exercised with novel disturbances, especiaily early in the breeding season. A set-back distance of 165m should be sufficient to isolate hem colonies in south-coastal BC hmlow-level hurnan disturbance such as pedestrian activity. To act conservaiively in management intervention, a set-back distance in excess of 165m might be more appropriate to protect heron colonies in the region from stronger human disturbances. However, set-backs will not protect breeding herons hmeagle distucbance. Table 3.1. The responses of Great Blue Herons to the approach of the investigator through the breeding season at 10 colonies in south- coastal British Columbia from 1998 to 1999. Included are univariate regressions to illustrate changes in heron response through the season, mean responses, and the range of responses. Negative response distances occurred when herons allowed access past the colony edge, Study duration and the number of observations at each colony are provided. Al1 significant slopes were negative relationships. Asterisks refer to the probability of a Type 1 statistical error as follows: *&.OS, **4.01, ***<0.001. Superscripts on mean responses refer to significantly different groups of means provided by the Ryan-Einot-Babriel-Welsch multiple range test. Colony Study duration Number of Heron response distance (m) number (days) Seamal changes Level of responses Range of responses f F Pr.>F Mean SD Max. Min. -Expsrimental approach - - - Control approach 8 Bm Nest initiation Fledging Figure 3.1. Predicted responses of the Great Blue Heron to repeated and non-threatening investigator disturbance through the breeding season. The top figure reports the difference between expetimental and controI expures if only seasonal phenomena idluence hem responses through the season. The bottorn figure shows the difference if hemns adjust their responses thtough the season based only on prior expexience with the stimulus. (al1 rerponses pooled) * 191 \ 1 -35 O 35 70 105 140 Julian date (relative to incubation) Figure 3.2. Seasonal trends in the response distances of Great Blue Herons at al1 colonies where a response wsis observed to the investigator approach hm1998 to 1999. Regression trendlines are pmvided with colony nurnben. Statistics provided are for ail aine colonies and both yem pooled. See Table 3.1 for detailed statistics. AU response dates have been adjusted relative to the phenology of colonies using fmt incubation at each colony as day zero. The dashed iine represents the beginning of incubation at al1 colonies. AU dopes, except at Colony 16, were si@icant at the 0.05 level. Anova F r 5.8 Urbankation class Figure 33. Mean response distance in metres versus the level of urbanization at aiî colonies in the Strait of Georgia during 1998 and 1999 @=IO). Error bars represent one standard deviation. Sample sizes are provided. See Chapter 2 for details on this measure of human activity. rb.52 0 P-" Figure 3.4. Results hmthe date controlled investigator approach expriment conducted in 1999 at, a) Colony 13, and b) Colony 23. Circular data points refer to heron response distances on experimental approach routes, and squares refer to response distances on control approach routes. The interaction between experimental and control approach routes was significant at both colonies (Colony 13, Pe0.05; Colony 23, Pc0.01). Regression statistics provided are based on univariate anatyses of individual approaches. Chapter 4 General Discussion: IMPLICATIONS OF WCREASING DISTURBANCE FOR BREEDING GREAT BLUE HERONS IN SOUTH-COASTALBRITISH COLUMBIA This chapter discusses how increasing disturbance from humans and eagles might aiter the productivity and distribution of breeding Great Blue Herons, Ardea herodias, in south-coastal British Columbia. It is important to understand what effects incceaing disturbance will have on heron breeding populations, so that management prescriptions for the conservation of this species cm be effectively applied. The breeding productivity of Great Blue Herons was low in south-coastd BC during 1999 (Chapter 2), and has declined since 197 1 (Gebauer and Mou1 2000) because of an increase in the frequency of breeding abandonment (Chapter 2). The Fraser Valley bas the primary concentration of breeding herons in this region, with higher productivity and larger colony sizes than on Vancouver Island or the Sunshine Coast (Chapter 2). If hem productivity becomes compromised in the Fraser Valley, there may not be other population sources or potential breeding sites to offset such a decline. Both buman and eagie populations, the primary antagonists of breeding herons (Chapter 2), are also concentrated in the Fraser Valley (Eiiiott et al. 1998, Moore 1990). The activity of both humaas and eagles has grown substantiaIly in south-coastal SC over the past severd decades (McAUister et al. 1986, Vermeer et al. 1989, Moore 1990, Elliott et ai. 1998, GVRD 2000). Midway through the twentieth century, the human population of the Vancouver area was about 40% of the current population of two million (GVRD 2000), and the eagle population of the Coast was significantiy lower due to persecution and contamination hmhumans (Fotbes 1989, Butler and Vennesland in press). In the relative absence of disturbance over the recent past, food was probably the best predictor of the productivity and distribution of breeding herons in south-coastal BC. Food availability Iimits the number of young raised by successful heron pairs (Butler 1995), and has ken positively related to colony size for breeding herons (Fasola and Barbieri 1978, Gibbs 199 1, Butler 1992, Gibbs and Kinkel 1997). Near the largest heron colonies in south-coastal BC, on the extensive foraging grounds of the Fraser River (Figure 1.1), Krebs (1974) reponed increased food intake for herons with increasing group size, and this was viewed as a strategy to take advantage of ephemeral but productive food sources. In contrast, Simpson et ai. (1987) reported that breeding hemns at a single smaller colony on the Sunshine Coast used a smaller foraging amand did not gain from information exchange because food was predictable in time and space. This difference in available foraging near colonies apparently resulted in different foraging strategies among colonies of different sizes, but productivity was equivalent across ail colony sizes (Butier et al. 1995). Brown et al. (1990) predicted that no colony sizes should be favoured when fitness retums are equai arnong different sized colonies, and conjectured that this situation might represent a stable equilibrium resulting Eiom an exchange of breeders between patches as individuds attempt to maximize fitness payoffs. If breeding herons apportion themselves among available colony sites to maximize individuai fitness rems, and if increasing disturbance is applied disproportionately among different colony sizes, ihen the distribution of herons should change to reflect these aiterations in the costs and benefits of different colony sizes. Humdisturbance appears to have a modest negative influence on hem breeding productivity, directly through distucbance and indirectly through eagle disturbance at colonies with high levels of pedestrian activity (Figure 2.4). A set-back distance for heron colonies of 165m should adequately pcotect breeding herons from pedestrian activity in this region. Habitat availability is currentiy not a concem for herons in this area (Butler 1997), but if habitat becomes further diminished or degraded due to development, as seems likely given the growth of the human population (Moore 1990, GVRD 2000), heron productivity may decline regardless of disturbance (Markhm and Brechtel 1978). Suitable habitat might also be declining if fngmentation and habitat loss provide easier access for predators into heron colonies. The abandonment of one colony in 1998 due to eiigle distubance after the surrounding habitat was altered for a golf course may illusuate this situation, although these two events could also be coincidentai. Currently it is possible that the costs of moving to another breeding location are low for herons, relative to staying at the current site (Chapter 2), but these costs could increase if nest sites become more limited. As conjectured by previous authors, eagle activity has probably had a substantial impact on the productivity of breeding herons in south-coastal BC (Norman et al. 1989, Butler et al. 1995). Increased eagle disturbance provides the most likely explanation for the increase in abandonment and consequent dechne in hem breeding productivity observed by Gebauer and Mou1 (2000) (Cbapter 2). Increasing eagle activity at hem colonies could alter the apparent equilibrium in heron breeding productivity across colony sizes reported by Butler et ai. (1995). in contrast to a decade ago (Butler et ai. 19951, heron breeding productivity duting 1999 in south-coastal BC was significantly and positively related to colony size because of breeding abandonment (Chapter 2). This new trend is most likely related to an increased frequency of eagle disturbances at small colonies (40 breeding pairs). Dunng the 1999 breeding season, 13 of 20 small colonies were completely abandoned, and eagle disturbance was Linked to al1 but one of these abandonments (Chapter 2). Additionally, there were more eagle incursions during 1999 than during 1988 at heron colonies in south-coastal BC (Chapter 2). Small colonies are more cryptic than large colonies, and in the recent past with fewer eagles, small colonies might have better avoided detection. hcreasing numbers of eagles might cumntly be detecting small colonies more frequentiy. Herons breeding in small colonies might decide to change breeding strategies, and could further disperse their breeding, move to larger colonies, or abandon the region completely. However, because these colonies currently comprise a small proportion of the heron breeding population, this situation shodd at present have limited consequences (Chapter 2). The eagle population of south-coastal BC has been rebounding from depressed numbers due to humm influences (Forbes 1989, Vermeer et al. 1989). so it has probably been increasing without a correspondhg increase in available prey. Many eagles might therefore be attracted to large hemn colonies (250 breeding pairs) because these sites can provide a relatively plentifùl food murce (Brown et al. 1990). Eagle disturbance was concentrated at the largest heron colonies in the study area during 1999 (Chapter 2), and this may indicate an increase in eagle activity at these sites. This could resdt in more frequent and larger abandonment events, as observed at one colony of 4OO breeding pairs in 1999 (Chapter 2), and could become further exasperated if human infiuences have a negative influence on more traditional eagle prey, such as fish. If the productivity of herons in large colonies is compromised, herons may redistribute their breeding - either with larger coIonies increasing their breeding dispersion or by increasing colony size further to dilute the nsk of increasing distwbance (Chapter 3). However, increasing the size of colonies may not be an option for breeding herons due to limitations on colony size imposed by food availabiiity during the breeding season. Several studies have found that breeding herons distribute themselves across the landscape in proportion to available foraging area (Gibbs 199 1, Butler 1992, Gibbs and Kinkel 1997), and this implies that food availiibility might set an upper iimit on colony size for hemns (Butler 1994). It therefore seems iikely that herons in large colonies experiencing increased disturbance will further disperse their breeding, or may move to new and quieter locations outside of this region. The distribution of heron colonies might also be influenced by the distribution of eagle breeding territories (Forbes 1989). Herons nesting dose to breeding eagles rnay suffer increased eagle disturbance if breeding eagies use hemns as a food resource to mise their Young. in contrast, breeding eagies could also provide benefits to breeding herons if they protect colonies hmother predators (Forbes 1989). The proximity of eagle nests had no signi ficant relationship to heron colony productivity in my study or in that of Butler (i995), implying that neither, or both, of these processes could be occuning. Breeding eagles are much less mobile than breeding herons because of the extended tirne and effort required to build their nests (Chapter 1). This ciifference in mobility codd be a result of interactions between these species over evolutionary time because eagles and herons have probably coexisted in this region since the 1st ice age. Herons have little effective physical defenses against attacking eagles (Chapter 2, Burger 198 1, Forbes 1989) and commonly abandon breeding when threatened by eagles (Chapter 2). Breeding abandonment may be an evolved response of herons against disturbance from large and dangerous predators such as eagles, as they attempt to loçate a quieter breeding site. When under threat from an attacking eagle, the best response for a hem might be to protect itself and abandon the breeding attempt. Herons likely have a high potential for renesting early in the season, and interchange between colonies appears to be high (Chapter 2, Chapter 3). This could explain why large birds such as herons build smail, relatively flimsy nest structures. Thece are two potential explanations for an increase in eagle numbers and disturbance at heron colonies in south-coastal BC (Chapter 2). Over the past several decades herons might have moved into new habitats that became suitable when humans depressed the eagle population, or alternatively, the rebounding eagle population might not have sufficient traditional food resources to support their population, and might be looking for altemate resources. if the eagle population is simply reniming to historical levels and this is the eagle population level we desire, then management intervention would not be a prudent option. However, if the increase in the eagle population is due to human activity, such as the availability of refuse or the degradation of traditional eagle food resources, management intervention might be effective. More research is needed on the relative costs and benefits of breeding abandonment for herons in south-coastai BC. Estimates of coIony interchange and renesting frequencies are needed for berons in different sized colonies so that the movements of individual herons witbin and between seasons can be properly determined for the study area as a whole (Chapter 2). This information will be usehl for modelling the impact of increasing disturbance on the productivity and distribution of herons breeding in this region. In addition, human activity may increase eagle disturbance at heron colonies, and the choice of nesting location for herons might depend on where eagles are breeding (Chapter 2). These potentiai interactions need to be further investigated for a clear undetstanding of the influence of disturbance on breeding herons. The establishment of set-back distances of 165111 at hem colonies should adequately protect breeding herons from human pedesvian activity (Chapter 3), and these buffer zones should be implemented. 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Surnmacy of basic biological and geographical information collected at colonies of Great Blue Herons in south-coastal British Columbia in 1998 and 1999. Region 1 is Vancouver Island, Region 2 is the Fraser Valley and Region 3 is the Sunshine Coast. See Gebauer and Mou1 (2000) for a complete listing of colony information. Nicomekl 14 2 13 37 2.5 2 3 Serpentine 15 2 3 8 O 2 100 CFB Chilliwack 16 2 8 83 2.2 1 1 Vedder Canal 17 2 6 2 2.5 3 O Tilbury Island t8 2 3 8 2.8 3 O Halfmoon Bay 19 3 4 3 O 2 100 Acon Street 20 3 4 2 O 1 100 Sutton Islets 21 3 4 5 O 2 IO0 Pacific Spirit Park 23 2 18 190 0.5 2 63 Stanley Park Aquarium 24 2 O 3 O 3 100 Cliff Street 25 2 12 50 2.8 2 4 Mary Hill 26 2 12 108 1.9 3 22 Alouette River 27 2 9 86 2.6 1 O Mission - Silvermere 28 2 8 60 2.0 1 7 Hatzic Lake 29 2 3 65 2.6 1 O Harrison River 33 2 4 15 O 2 1O0 NO. of F'ed@ings "&a- Neab Colony name Co'ony Region No' Of breeding initiated no. surveys per ization abandoned ctass (%) pairs atlemM Beacon Hill 1 1 24 48 0.2 3 85 McFadden Creek 2 1 22 131 1.O 1 50 View Royal 3 1 O 1O O 2 100 Wiffin Spit 4 1 9 16 O 2 100 Mill Bay 5 1 20 9 O 2 100 Drinkwater Road 6 1 5 71 1.4 1 21 Holden Lake 7 1 25 26 0.9 1 60 Beachcomber 8 1 7 3 O 2 100 Tralee Point 9 1 18 50 0.5 2 74 Royston 10 1 17 27 O 3 1O0 Kiîty-colernan Creek 11 1 5 1 O 2 100 Heriot Bay 12 1 O 5 O 2 1O0 Point Roberts 13 2 31 322 1.7 2 4 Nicomekl 14 2 30 34 1.2 2 50 CF0 Chilliwack 16 2 27 114 1.6 1 19 Vedder Canal 17 2 9 3 O 3 100 Tilbury Island 18 2 23 12 1.9 3 20 Halfmoon Bay 19 3 2 3 O 2 100 Acorn Street 20 3 2 5 O t 100 Tuwanek 22 3 1 3 O 2 1 O0 Pacific Spirit Park 23 2 38 174 1.3 2 39 Stanley Park Aquarium 24 2 O 3 O 3 1O0 Mary Hill 26 2 26 107 2.2 3 O Alouette River 27 2 27 120 1.2 1 23 Mission - Silverrnere 28 2 25 39 1.4 1 25 Hatzic Lake 29 2 25 67 2.0 1 16 Horseshoe Bay 30 2 1 2 4.0 2 O Snug Cove A 31 2 1 1 O 3 100 Snug Cove B 32 2 1 2 2.0 3 O Birch Bay 34 2 9 400 0.0 t 1 0.99 Samish Island 35 2 24 120 1.9 2 17