Causes and Consequences of Sibling Aggression in Nestling Ospreys (Wdionhaliaetus)
CAUSES AND CONSEQUENCES OF SIBLING AGGRESSION IN NESTLING OSPREYS (WDIONHALIAETUS)
by Marlene Machmer B .Sc., Simon Fraser University, 1988
THESIS SUBMIlTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of Biological Sciences O Marlene Machmer 1992 SIMON FRASER UNIVERSITY October 1992
All rights reserved. This work may not be reproduced in whole or in part, by photocopy or other means, without the permission of the author. APPROVAL
Name: MARLENE MACHMER
Degree: Master of Science
Title of Thesis:
CAUSES AND CONSEQUENCES OF SIBLING AGGRESSION IN NESTLING OSPREYS (PANDION HALIAETUS).
Examining Committee: Chair: Dr. R. Mathewes, Professor
7 --. br. R.C -iate Professor, Senior Supervisw Department of Biological Sciences, SFU
Department of Biological Sciences, SFU
/ - a .J.N.M. Smith, Department of Zoology, dverSig of ~riishColumbia
of ~iolo~icalSciefi'ces, SFU I hereby grant to Simon. Fraser Unfvarsl$y the tlght to lend my ?thesis, proJsct or extended essay*(the ?It la of xhlch is shown below) to users of tha ShhFraser Unlverslty ~l br$ry, and to make partfnl or sfngla copies ant y for such users or f n response to a request f tam the l ibrary of any oihsr untverslty, or other $ducat tona l i nst f tut ion, on its own behalf or far one of its users. 1 further agrea that permission for-mulf.lp#s copylng of this work for scholariy purposes my be granted by me or tho Daan of Graduate Studios. it Is undersTood that,copyfng or pubflcatlon of ihls work for flnanclat galn, shall not be af towed without my wrltten permission.
fftje of Thssfs/ProJect/Extended Essay
Author: , .,- . a lslgnaturet
( name I Abstract Sibling aggression and brood reduction were studied in a population of Ospreys (Pandion haliaetus) breeding in the Kootenay region of British Columbia in 1989 and 1990. Almost all of the aggression observed occurred early in the nestling period when broods were less than 20 days of age. Senior siblings were more likely to use aggression and during the first 20 days of the nestling period, they received ~i~cantlygreater shares of food relative to their junior nestmates. After 20 days of age, aggression was rare and all brood members received equal shares of food. In cases of partial brood loss, over 90 percent of mortality involved junior chicks and all known mortality occurred when victims were less than 20 days old. Based on the pattern of food allocation and aggression within broods, and a general decline in aggression with nestling age, brood reduction appeared to be an outcome of aggression-mediated starvation. Brood reduction at successful nests occurred at rates of 12 and 31 percent in 1989 and 1990, respectively. Nestlings grew more slowly in the latter year, suggesting that mortality occurred in response to food limitation. There was tremendous variation in the level of sibling aggression observed between broods. Observations of broods under natural conditions provided little evidence that food amount, within-brood competitive asymmetry or brood size influenced this variation. Results of a controlled feeding experiment in 1989 and 1990 supported the food amount hypothesis for sibling aggression in nestling Ospreys. Broods were slightly but significantly more aggressive when they were hungry indicating that sibling aggression in this species is proximately influenced- - - by food shw-geta. Senior siblings took a greater share of food when they were hungry however this additional food did not come from the shares of younger nestmates. Instead, females compensated for the hunger of their broods by taking smaller shares for themselves and increasing the meal sizes fed to their broods. Experimental results also indicated that some of the inter-brood variation in aggression may be explained by differences in competitive asymmetries between nestmates. Aggression tended to decrease with an increase in brood age asynchrony and with an increase in brood weight asymmetry. The latter decline was significant. This effect is consistent with studies that have . .. manipulated brood hatching asynchrony or weight asymmetry and suggests that wuma&qp adjust their aggression levels based on the perceived level of competition within their broods. Acknowledgements I would like to express my thanks to the members of my committee for their help in improving this thesis. Ron Ydenberg, my senior supervisor, provided guidance and always found time for me during my impromptu visits. I thank him for his patience and understanding given the extended nature of my degree. Bernie Roitberg and Jamie Smith provided helpful criticism on earlier versions of this work. Thanks also to my collegues Scott Forbes and David Green both of whom worked through previous drafts of this thesis. Scott had useful suggestions during the planning of this project and lazy afternoon discussions with David helped me clarify my thinking. I owe special thanks to a number of enthusiastic assistants who I coaxed and who coaxed me up to Osprey nests. John Black and Ann Bussell persevered with me and the mosquitos through an extended field season in 1989. Their sympathetic pregnancies gave us all a good chuckle. My thanks also go to Kyla Super who volunteered her help during the fledgling study. David Green was tremendous help in 1990. His enthusiasm kept me going through a virtually sleepless field season. Over time, I discovered and came to share the secret to David's energy: yogwt icecream and chocolate waffle cones. Thanks to the staff at the Creston Valley Wildlife Management Area for their cooperation during this project. On behalf of the Ospreys, compliments to the Fish & Wildlife Branch in Nelson for the many frozen fish they supplied and thanks to Guy Woods in particular for his continued support of this project. Roger and Charlotte Perquy gave us a home in 1989 and made us feel part of their family. Roger's mirror and Charlotte's cinnamon buns were valuable additions to the study. Travis who made his presence felt at the beginning of the first field season, provided me with lucid insight on the theory and practice of parent-offspring conflict. I thank him for being a good-natured adventuresome baby and for teaching me the meaning of compromise. I hope he suffers no permanent damage as a resdt of the wrath inflicted on him by the senior sibling at nest N78. Last of all, I want to save special thanks for Chris Steeger. Not only was he "super dad" for a summer, but he also found time to collect data, struggle through numerous drafts of this thesis and put up with my endless thesis babble. I could not have done it without him. My research was supported by an NSERC Postgraduate Scholarship and a Graduate Fellowship Award from SN. The World Wildlife Fund and an NSERC operating grant to Ron Ydenberg provided additional financial support. TABLE OF CONTENTS
Approval ...... ii ... Abstract ...... u1 Acknowledgements ...... v List of Tables ...... ix List of Figures ...... x I . GENERAL INTRODUCTION ...... 1 I1. SIBLING AGGRESSION AND BROOD REDUCTION IN THE KOOTENAY OSPREY POPULATION Introduction ...... 4 Methods ...... 7 Results ...... 14 Extent. Timing and Variation in Sibling Aggression ...... 14 Aggression and Food Allocation ...... 19 Aggression and Nestling Growth ...... 24 Nestling Growth and Mortality ...... 24 Proximate Influences on Sibling Aggression ...... 29 Discussion ...... 34 Conclusions ...... 42 I11 . A TEST OF THE FOOD AMOUNT HYPOTHESIS FOR SIBLING AGGRESSION IN NESTLING OSPREYS Introduction ...... 43 Methods ...... 46 Results ...... 51 Hunger. Food Allocation and Aggression ...... : ...... 51
vii Aggression and Relative Nutritional Condition ...... Aggression and Competitve Asymmetries Between Nestmates ...... Aggression and Brood Size ...... Discussion ...... Conclusions ...... IV. GENERAL CONCLUSIONS ...... LITERATURE CITED ...... APPENDIX 1 ...... LIST OF TABLES Table Page Number and proportion of aggressive acts performed by Osprey nestlings during observations of unmanipulated broods ...... 15 Summary statistics by year for Osprey breeding variables ...... 27 Sources of Osprey egg and chick mortality by year ...... 28 Summary of weighted mean growth rates of ranked Osprey chicks during 1989 and 1990 ...... 30 Results of correlations of brood aggression rate per feed with hunger indices .. 32 Results of correlations of mean daily aggression rate per brood during feeding periods with provisioning indices ...... 33 Aggression rates of Osprey broods in feeds following feed and mock feed treatments ...... 52 LIST OF FIGURES Figure Page Mean aggression rate per brood during (a) feeding periods, (b) non-feeding periods and (c) feeding periods with observation session two on brood N56 omitted ...... 17 The mean proportion of aggressive acts performed by and directed at ranked chicks in two and three-chick broods ...... 18 The mean proportion of bites received by chicks in two and threechick broods ...... 20 The mean proportion of bites received by chicks in different age categories for two and three-chick broods ...... 21 The mean proportion of bites received by chicks in feeds with and without aggression for two and three-chick broods ...... 23 Correlations of aggression rate per feed with the proportion of bites received by A, B and C-chicks ...... 25 Correlations of mean brood aggression rate with brood and A-chick weight- versus-age residuals ...... 26 Mean bite rate per brood versus brood age range ...... 3 1 Regressions of brood age asynchrony and brood weight asymmetry with mean aggression rate per brood during feeding periods ...... 35 Mean aggression rate per brood during feeding periods in two and threechick broods ...... 36 Comparison of mean aggression rate per brood in feeds following experimen- tal treatments versus natural control feeds ...... 53 Frequency distributions of the proportions of bites received by A, B and C-chicks and females in feeds following feed and mock feed treatments ...... 54 Mean proportion of bites received by C-chicks in feeds following feed and mock feed treatments ...... 56 Mean proportion of bites taken by four viewing nest females in feeds following feed and mock feed treatments ...... 57 Frequency distribution of meal sizes in feeds following feed and mock feed treatments ...... 59 Correlations of mean aggression rate per brood with brood and Achick weight- versus-age residuals ...... 60 3.7 Regressions of brood age asynchrony with mean aggression rate per brood in feeds following feed and mock feed treatments ...... 61 3.8 Regressions of brood weight asymmetry with mean aggression rate per brood in feeds following feed and mock feed treatments ...... 62 3.9 Mean aggression rate per brood of two and three-chick broods in feeds following feed and mock feed treatments ...... 64 ckuumu GENERAL INTRODUCTION
In many species of birds, nestlings are killed by their nestmates as a direct or indirect outcome of sibling aggression (siblicide sensu Mock 1984a). Identifying the effects of siblicide on the inclusive fitness of the perpetrator(s), the victim and the parents and understanding the ecological conditions under which this behaviour is favored by natural selection has presented a longstanding challenge to behavioural ecologists. Early accounts of aggressive nestling interactions have been provided for several raptor species (Rowe 1947, Meyburg 1974, Gargett 1978). More recent investigations have identified two general forms of siblicide (Mock 1984a): obligate (where nestling mortality virtually always occurs) and facultative siblicide (where mortality is conditional and depends to some extent on prevailing food supplies). Retention of a second egg in obligate species is thought to function primarily as insurance against loss or injury of the first egg or chick (Cash & Evans 1986, Anderson 1990). Facultative siblicide is usually considered a form of brood reduction (Lack 1947, 1954, Ricklefs 1965, Temme & Charnov 1987) or bet-hedging (Kozlowski & Stearns 1989) in response to unpredictable resource variability. Parents establish competitive asymmetries between their nestlings through incubation and laying patterns and when food is scarce, older more dominant nestlings restrict their younger siblings' access to food, or kill them directly. In years of plenty, all brood members survive (Magrath 1990). Lack (1947, 1954) suggested that food supply during the nestling period sets an upper limit to brood size. He emphasized the importance of hatching asynchrony and associated inequalities in size and competitive ability among nestmates in facilitating brood reduction. Many tests of the brood reduction hypothesis have been attempted and several other hypotheses have been proposed for the evolution of hatching asynchrony (review in Magrath 1990). Regardless of why hatching asynchrony originally evolved, it is expected to have a profound effect on sibling interactions and the probability of brood reduction (Fujioka 1985b, Mock & Ploger 1987). Several theoretical models of avian siblicide have been proposed (O'Connor 1978, Stinson 1979, Parker et al. 1989, Godfray & Harper 1990, Forbes 1992, Forbes & Ydenberg 1992). Most have attempted to identify the thresholds of food limitation at which it pays one nestling to behave aggressively or to kill a sibling, within the framework of inclusive fitness theory (Hamilton 1964). Assuming siblicidal aggression is directed at full siblings, it is expected to evolve only if the benefit to the perpetrator is greater than twice the cost to the victim. Trivers (1974) and O'Connor (1978) expanded this line of reasoning, suggesting that the genetic interests of parents and offspring differ, potentially resulting in parent-offspring conflict over resource allocation and brood size. Quantitative field studies on facultatively siblicidal birds have been conducted only recently and due to sample size limitations, studies have been confined to a few highly colonial groups. Existing studies have attempted to (i) document the extent of sibling aggression and identify the costs and benefits of this behaviour to affected parties (Braun & Hunt 1983, Mock 1985, Fujioka 1985a, Ploger & Mock 1986, Drummond et al. 1986), (ii) search for evidence of parent-offspring conflict (Drummond et al 1986, Mock 1987, Drummond 1989) and (iii) identify proximate factors that potentially influence the occurrence or intensity of sibling aggression (Proctor 1975, Mock 1984b 1985, Fujioka 1985b, Mock & Ploger 1987, Mock et al. 1987%Drumrnond & Garcia Chevelas 1989, Mock & Lamey 1991). Factors promoting the observed variation in sibling aggression both between and within species remain unclear. Although aggression has been shown experimentally to be influenced by several proximate cues (e.g. the size of food items, brood size, the competitive asymmetry between nestmates, incoming food amount), the cues used differ between species. A clearer understanding of the types of factors that influence nestling aggression policies would provide insight into when siblicide is expected to occur and what form (obligate, facultative or no siblicide) it should take in different species and under different ecological circumstances. In this thesis, I examine sibling aggression and brood reduction in nestling Ospreys (Pandion haliaetus). In Chapter 11, I present evidence related to the extent, timing and inter- and intra-brood variation in aggression in Ospreys. I investigate the relationships between aggression, food allocation and nestling growth, and how the latter factors affect the pattern and timing of mortality within broods. I also consider the role of several proximate cues potentially influencing aggression. In Chapter 111, I present the results of an experiment designed to test whether aggression is influenced by incoming food amount and mediated by hunger in this species. I studied Ospreys in the Kootenay region of British Columbia. Ospreys are large, piscivorous birds of prey with conspicuous nesting, hunting and feeding behaviour. Ospreys exhibit facultative brood reduction and sibling aggression has occasionally been observed in the Kootenay population (Forbes 1991). Approximately 100 pairs of Ospreys breed in the Kootenay region each summer, some of which nest on man-made structures. These factors, in addition to the tolerance Ospreys show to periodic human disturbance, make this species highly suitable for experimental manipulation. Two nesting concentrations are evident within the study area: along the Kootenay River in the Creston Valley and along the West Arm of Kootenay Lake, near Nelson. Details of the study area and the breeding chronology of Kootenay Ospreys are described in Steeger (1989), Steeger et al. (1992) and Forbes (1989). i2HEEuI SIBLING AGGRESSION AND BROOD REDUCTION IN THE KOOTENAY OSPREY POPULATION
INTRODUCTION Fatal sibling aggression (siblicide, sensu Mock 1984a) has been described for a variety of avian taxa (see O'Connor 1978, Stinson 1979, Mock 1984a for reviews). Based on the probability of nestling mortality, Mock (1984a) distinguished two general forms of siblicide, obligate and facultative. In obligate species, death of one brood member almost always occurs, and mortality appears to be independent of food supply at the time of the victim's demise. In facultative species, mortality is conditional and depends to some extent on prevailing food levels. Obligate brood reduction is well known in a number of eagle, booby, pelican and penguin species that typically lay two eggs, but rarely fledge more than one chick (Meyburg 1974, Gargett 1978, Brown et al. 1977, Stinson 1979, Edwards & Collopy 1983, Cash & Evans 1986, Evans & McMahon 1987, Drummond 1987, Simmons 1988, Anderson 1989, 1990; Lamey 1990). The retention of a second egg in these species is thought to serve as insurance against the loss or inviability of the first egg or chick (Dorward 1962, Warharn 1975, Meyburg 1974, Cash & Evans 1986, Anderson 1990). This insurance hypothesis is supported by recent experimental and theoretical studies (Cash & Evans 1986, Anderson 1990, Forbes 1990b). The brood reduction hypothesis (Lack 1947, 1954,1968) is the most cited explanation for siblicide in facultative species. It proposes that in unpredictable environments, selection favors females that lay an optimistic clutch size and that create offspring of unequal competitive ability. The latter is achieved through (i) incubation before egg-laying is complete, leading to hatching asynchrony and nestlings that are graded in size and competitve ability, (ii) differences in egg size; or both. When food is insufficient, competitive disparities between nestmates are expected to result in the sequential elimination of chick(s), beginning with the youngest, until brood size matches prevailing resource levels. In siblicidal species, aggression is thought to further exaggerate the competitive asymmetries established through asynchronous hatching, thereby facilitating secondary adjustment of brood size while minimizing the effects on surviving young (Lack 1968, Mock & Ploger 1987, Magrath 1990). Unpredictable resource variability (primarily food) is implicit in Lack's hypothesis and it is thought to be a key factor in the evolution of brood reduction strategies (O'Comor 1978, Mock 1984a, Magrath 1990). Its importance has been emphasized in theoretical treatments (Temrne & Charnov 1987, Forbes & Ydenberg 1992, review Pijanowski, in press), and has been demonstrated in recent experimental studies (Magrath 1989). Mock et al. (1987a) have proposed that within-season variability in food supply determines which pattern of facultative siblicide is likely to evolve in a particular system. When food varies predictably from day to day, a proximate link between hunger and aggression (conditional facultative siblicide, sensu Mock 1984a) may be favored, because chick hunger levels will accurately predict future food shortfalls. Therefore aggression will only be triggered when shortages are imminent. If food levels fluctuate unpredictably, then the best strategy for a senior sibling may be to fight irrespective of current food levels (probabilistic facultative siblicide, sensu Mock 1984a). (See chapter III for a review of species that practice conditional and probabilistic siblicide). In addition to different patterns of facultative siblicide, the intensity of aggression is also variable between and within species. In some cases, aggression results in nestling mortality directly, through injury, physical abuse, or nest eviction and subsequent exposure (South Polar Skua Catharacta macconnicki, Proctor 1975; Black-legged Kittiwake Rissa tridactyla, Braun & Hunt 1983; Great Egret Casmerodius albus, Mock 1984a; Cattle Egret Bubulcus ibis, Fujioka 1985b; Blue-footed Booby Sula nebouxii, Drummond et al. 1986; Blue-throated Bee-eater Merops viridis, Bryant & Tatner 1990). In other cases, aggression is used to establish and maintain within-brood dominance hierarchies. Dominant siblings control access to parental feedings and when food is limited, junior siblings are effectively denied access, resulting in selective starvation (South Polar Skua Catharacta maccormicki, Proctor 1975; Little Blue Heron Florida caerulea, Wetschkul 1979; Western Grebe Aechmophorus occidentalis, Nuechterlein 1981; Laughing Gull Larus atricilla, Hahn 1981; Great Blue Heron Ardea herodias, Mock 1984b Great Egret Casmerodius albus, Mock 1984a; Ploger & Mock 1986; Little Egret Egretta garzetta, Inoue 1985; Cattle Egret Bubulcus ibis, Fujioka 1985b; Blue- footed Booby Sula nebouxii, Drummond et al. 1986). Though aggression is more subtle in the latter case, its outcome is similarly fatal. Investigations of the reproductive strategies of Ospreys (Pandion haliaetus) suggest that brood reduction is an integral part of the reproductive ecology of this species (Poole 1982, 1984; Hagan 1986, McLean & Byrd 1991, Steidl & Griffin 1991). Ospreys typically lay three eggs, although four egg clutches are not uncommon in some temperate regions (Poole 1989). Consistent incubation usually begins with the second egg (Poole 1984). Nestlings hatching last are at a competitive disadvantage because of a significant decline in egg size with hatching order (Poole 1984; but see Steeger 1989) and, more importantly, because third and fourth eggs hatch later and result in smaller nestlings (Poole 1984, Steeger 1989). The incidence of brood reduction is highly variable both between years (Poole 1989, Stinson 1977, McLean & Byrd 1991) and between geographic locations (Poole 1982,1984,1989). The extent of sibling aggression is also- highly variable, being entirely absent from some populations in some years (Green 1974, Stinson 1977, Poole 1982) and occurring to a variable extent in other populations (Judge 1980, Poole 1982, Jarnieson et al. 1983, Hagan 1986, McLean & Byrd 1991, Steidl & Griffin 1991). Aggression also varies with respect to its timing during the nestling period. Jamieson et al. (1983) observed aggression primarily in 5-8 week old chicks whereas Poole (1982) noted aggression in chicks ranging from 1-4 weeks of age. Where aggression does occur, it is most common in larger broods (Jarnieson et al. 1983, Hagan 1986, Steidl & Griffin 1991). As in other facultatively siblicidal species, aggression in Ospreys has been reported to result in intimidation of junior siblings and may allow senior siblings to obtain a disproportionate share of the available food. Poole (1982) and Forbes (1989) found that in feeds with aggression, older siblings were fed first and food allocation was skewed in their favor. However, Stinson (1977) found no dominance within multiple chick broods. Nestlings fed at random and relative within-brood nestling weights were reported to fluctuate from week to week, suggesting minimal size differences. Similarly, no dominance hierarchy among nestlings was noted by Ames (1%4) and Green (1974). In this study, I examine sibling aggression and brood reduction in a population of Ospreys nesting in the Kootenay region of southeastern British Columbia. Based on a two-year (1987- 1988) study of seasonal breeding trends in this population, chick mortality occurred in approximately 10 percent of nests (Steeger 1989). However, some deaths were of singletons and the actual rate of brood reduction was thus lower. During weekly nest surveys, Steeger (1989) found several chicks dead in the nest, extremely emaciated or with wounds on the head or neck. Forbes (1991) observed sibling aggression directly in unmanipulated three-chick broods. However, his observations were limited to a few nests and the timing and extent of this phenomenon in the population remains unclear. My objective was to examine sibling aggression in the Kootenay Osprey population. I sought evidence on the extent of aggression, the timing of aggression, the inter- and intra-brood variation in aggression and the relationships between aggression, food allocation and nestling growth. Furthermore, I investigated the interplay of sibling aggression, nestling feeding behaviour and nestling growth with the pattern and timing of mortality within broods. The role of food amount and other proximate cues potentially influencing aggression were also examined.
METHODS At the beginning of each breeding season, the Creston and Nelson areas were surveyed for occupied Osprey nests following Postupalsky's methods (1977). The subset of nests for this study were chosen based on accessibility. Clutch sizes were determined for all nests where females were clearly incubating (30 in 1989; 34 in 1990), either by climbing the nest structures or with the aid of binoculars and an adjustable mirror attached to a long pole. In cases where it was uncertain whether a clutch was complete (i.e. five clutches), nests were re-visited within three to four days. Beginning in the last week of May, all nests were visited every four to five days to determine hatching success. A brood was considered complete once the last chick had hatched, or once remaining eggs showed no hatching cracks within two visits following the hatching of the youngest nestling. I established brood size at hatching for 24 nests in 1989 and 32 nests in 1990. In the remaining nests (6 in 1989; 2 in 1990), it was not possible to determine whether eggs were depredated, or whether chicks had hatched and died before I was able to visit their nests. These nests were excluded from the calculation of brood reduction rates. Nestling ages could occasionally be determined by direct observation of hatching. In most cases, nestling ages were estimated from a regression of nestling culmen length (x) on nestling age (y) ( y = -13.6 + 1.5~.r2 = 0.96, n = 40 nestlings; Steeger 1989), established using chicks of known age (cf. Poole 1984). During the first nest visit following hatching, each nestling was weighed to the nearest 1 g or 5 g using 100 g or 1000 g Pesola spring balances, respectively. Culmen lengths were measured to within 0. lmm with Vernier calipers and nestling crop contents were estimated to the nearest 118 by palpitation. Nestling weights were later adjusted, based on crop content mass estimates for chicks of different age classes. Nestlings were marked with water-based paint on their head, neck and tarsus. Ranks (A,B,C) within a brood were assigned according to mass, and proceding from heaviest to lightest chicks, nestlings received red, green and yellow markings, respectively. Culrnen lengths were used to rank nestlings when masses were indistinguishable. Markings made it possible to distinguish individuals from week to week and during behavioural observations. The most accessible nests (12 in 1989; 13 in 1990) were visited approximately weekly throughout the nestling period to determine nestling growth rates and the timing of brood reduction. During these visits, markings were reapplied and measurements of nestling mass, culmen length, and crop content were repeated for all nestlings (30 in 1989; 34 in 1990). Less accessible nests were visited opportunistically during the nestling period to determine if brood reduction had occurred In late July and early August, all nests were re-visited to determine fledging success. Brood reduction was assumed to have occurred in all successful nests where the number of fledglings was less than the conhmed brood size at hatching. In 1990, behavioural observations were conducted on a subset of nests. Nests were selected based on their visibility and only nests offering an unobstructed view of the nest interior from a vantage point 30-150m away were used (n = 12). A shortage of accessible broods made it necessary to use both two and three-chick broods (six 2-chick broods; six 3- chick broods). Once a brood was complete, 8-10 how observation sessions were conducted on each brood at weekly intervals. These were continued throughout the nestling period or until brood reduction occmd. In two of the 12 broods, brood reduction took place before observations could be conducted. Two sessions were nevertheless completed on each of the latter broods. Observations were terminated at one nest because of excessive human disturbance. Based upon preliminary observations, aggressive acts were placed into one of five categories: (1) Bite: A chick seized anothex with its bill, while pulling and twisting the skin. (2) Hit: A chick leaned back and then lunged forward with its bill and delivered a hard blow to the neck, head or back of a sibling. (3) Peck: A chick struck another with its bill (without leaning back and less forcefully than above). (4) Shove: A chick used its wings or upper body to push another chick off balance. (5) Threat display: A chick raised itself up with wings spread and neck extended and oriented towards a sibling.
The following observations were made using a 15x telescope and spoken into a tape recorder:
(a) all occurrences of aggressive behaviour and the identities of the chicks involved (b) whether aggressive acts took place during feeding or non-feeding bouts (c) timing of prey deliveries by the male or female parent (d) species and size of each prey captured (relative to the parent's tail or body length; Stinson 1978) (e) number of bites allocated to A, B and C-chicks and to the male and female parent
(f) presence or absence of the male and female from the nest or from the nest temtory.
For each observation session, a brood aggression rate was calculated as: (i) the aggression rate during feeding periods and (ii) the aggression rate during non-feeding periods. A feed was considered to have started as soon as a fish was delivered to the nest and ended when a fish was completely eaten or once a parent released the fish from its grip and stopped feeding itself or its nestlings for more than five minutes. The aggression rate during a feed was calculated as the total number of aggressive acts observed during the feed (acts) divided by the feed duration (hour). Aggression rates for all feeds during an observation session were then averaged to estimate aggression rate during feeding periods for that day. Therefore, all feeds were weighted equally, regardless of feed duration. Aggression rates were calculated in this way so that brood aggression rates in natural feeds could be directly compared to those in experimental feeds (see methods, chapter 111) however aggression rates calculated with or without equal weighting of feeds were very similar. Aggression rate during non-feeding periods was calculated as the total number of aggressive acts observed during all non-feeding periods divided by the duration of non-feeding observations. Following Mock & Parker (1986). aggression rates were standardized across brood sizes by dividing the brood aggression rate by the number of sibling dyads within a brood (3-chick broods had three possible dyads; Zchick broods had only one). Brood aggression rates are therefore reported as actsldyadhour.
EXTENT,TIMING AND VARIATION IN SIBLING AGGRESSION I determined the number and proportion of different aggressive acts observed over the entire nestling period. All aggression categories were equally weighted in the calculation of aggression rates. The timing of aggressive behaviour during the nestling period was examined by plotting brood age ranges during observation sessions against mean brood aggression rates (i) during feeding periods, and (ii) during non-feeding periods. A Wilcoxon signed ranks test for paired comparisons and a Friedman test were used to compare the number of aggressive acts performed by ranked chicks within two and three-chick broods, respectively.
AGGRESSION AND FOOD ALLOCATION Food allocation was investigated by comparing the mean proportion of bites received during feeds by A, B and C-chicks over the entire nestling period. Because the number and proportion of bites differed sigmtlcantly between ranked chicks from different brood sizes, two and three-chick broods were separated for subsequent analyses. A Mann-Whitney test was used to compare the mean proportion of bites received by A and B-chicks in two-chick broods, proportions received by A, B and C-chicks in three-chick broods were compared using a Kruskal-Wallis test. To examine the influence of brood age on food allocation, feeding observations on 12 broods were divided into three periods, based on brood age: period one (0 to 19 days), period two (20 to 35 days), and period three (36 to 51 days). These age categories were selected for convenience based on the timing of observations; each brood had been observed at least twice within each of the age categories. The mean proportion of bites received by nestlings during each period was compared with Mann Whitney and Kruskal-Wallis tests for two and three- chick broods, respectively. Two methods were used to assess the influence of sibling aggression on food allocation. First, the mean proportion of bites received by chicks in feeds with and without aggression were compared. Mann Whitney tests were used for all comparisons and two and threechick broods were considered separately for this analysis. Secondly, the proportion of bites received by A, B and C-chicks in feeds with aggression only was correlated to the aggression rate per feed. Only feeds during period one (when most of the observed aggression occurred) were used for both of the above analyses.
AGGRESSION AND NESTLING GROWTH If nestling growth influences sibling aggression, one might expect the rate of aggression within broods to vary inversely with the relative growth of broods. To test this prediction, nestling weight (under 10 days) versus nestling age was plotted separately for all A, B and C- chicks in the 12 broods under observation. Residuals of weight versus age were averaged for each brood and then correlated to mean brood aggression rate. Residuals of A-chick weight versus age were also correlated to mean brood aggression rate. This analysis was not carried out for older broods, because five of the 12 sample broods reduced and these chicks were no longer available for weighing. NESTLING GROWTH AND MORTALITY One-way ANOVAs were used to compare clutch size, hatching success and fledging success between years for all successful nests (i.e. nests that fledged at least one chick). G- tests with Yates correction were used to test for differences in egg and chick mortality between years. Nestling growth rates were calculated from the slopes of linear regressions of nestling mass versus age during the period of linear growth (5 to 35 days). The influence of year, nestling rank and brood size at hatching on nestling growth rate were investigated.
PROXIMATE INFLUENCES ON SIBLING AGGRESSION The proximate influence of food amount (presumably mediated by nestling hunger) on sibling aggression was investigated by correlating the aggression rate per feed to the following hunger indices: time since last feed, number of brood bites in last feed, number of A-chick bites in last feed, cumulative daily brood bites up to the feed, and cumulative daily A-chick bites up to the feed. Also, the aggression rate during feeding periods for each brood was correlated to a number of provisioning indices: daily bite rate per brood, daily fish delivery rate per brood, and daily feed rate per brood. a-values were adjusted for simultaneous comparisons using the Bonferroni correction (Miller 198 1). Only feeds and observation sessions in period one were included in these analyses. Brood age asynchrony and brood weight asymmetry were used to assess the potential influence of competitive asymmetries between nestmates on brood aggression levels. Brood age asynchrony was defined as the difference in age between the oldest and youngest sibling estimated at the first visit after hatching. Brood weight asymmetry was calculated as the weight difference between the oldest and youngest sibling, divided by the A-chick's weight. Weight asymmetry was standardized in this way because sibling weight differences change as a function of nestling weight and age (Poole 1982, Steidl & Griffin 1991) and not all nestlings were weighed at exactly the same age. The A-chick's weight was used because it is expected to deviate least for its age. Both age asynchrony and weight asymmetry were regressed against mean aggression rate during feeding periods for broods in period one only. The proximate influence of brood size on sibling aggression was examined by comparing the mean aggression rates during feeds in two and three-chick broods. Only feeds during period one were included in this analysis. Some of the variables under consideration had distributions that deviated from normality or had unequal variances between groups. Therefore, non-parametric statistics were used for most analyses. Dunn's multiple comparison tests were used to identify which groups differed significantly in Kruskal-Wallis or Friedman tests. Since the probabilities are shared between groups in this test, two groups are considered to be significantly different at values of a > 0.05, depending on the number of groups being tested (Neave & Worthington 1988). All statistical analyses were performed using the SYSTAT software package (Wilkinson 1986).
RESULTS EXTENT,TIMING AND VARIATION IN SIBLING AGGRESSION During 380 hours of observation, 1190 aggressive acts were observed in 12 osprey broods (Table 2.1). Direct aggression was more common than threats. Hits were the most common form of aggressive act and these appeared to cause occasional minor epidermal injuries and loss of plumage. There was tremendous variation in aggression among broods, ranging from an occasional threat display or a weak pecking bout to the almost continuous attacks inflicted by the senior sibling in one brood (N56) on its junior nestmate. Of the total number of aggressive acts observed, 697 (58%) were performed by this brood which later fledged both young. Hits made up seventy percent of the aggression observed in brood N56 as compared to a mean 43 percent in other broods. Eighty-seven percent of all aggressive acts occurred during period one (ie. when broods averaged 0 to 19 days old). Table 2. 1. Number and proportion of aggressive acts performed by Osprey nestlings during observations of unmanipulated broods. Results with brood N56 excluded are shown in parentheses.
Type of Aggressive Act Bite Hit Peck Shove Threat Total
-- Number of acts observed 165 700 105 46 174 11 9oa (55) (211) (87) (46) (94) (493)
Proportion of acts observed 0.14 0.50 0.09 0.04 0.14 1.00 (0.11) (0.43) (0.18) (0.09) (0.19) (1.00) a 697 aggressive acts (58%) were performed by a single twoshick brood (N56). The mean rates of aggression observed (i) during feeding periods and (ii) during non- feeding periods are presented in Figure 2. la and Figure 2.lb, respectively. Mean aggression rate during feeding periods and during non-feeding periods both rose and then fell with brood age. An extremely high rate of aggression during feeds was observed in brood N56 during the 10 to 19 day age range (230 acts/dyad/hour, an order of magnitude greater than any other brood) and this may have obscured a declining trend with brood age (Figure 2.la). When the above observation session was removed from the analysis, a significant linear decline in aggression rate with brood age was detected (y = 6.1 - 1. lx; r2 = 0.93, n = 6, P = 0.002) (Figure 2. lc). Mean aggression rates during feeding and non-feeding periods were positively correlated within broods (r = 0.94, n = 12, P < 0.001). These two measures of aggression did not differ significantly (Wilcoxon signed ranks test for paired comparisons: T = 16, n = 12, P = 0.08; two-tailed). The proportions of aggressive acts performed by chicks in two and three-chick broods are presented in Figure 2.2a and Figure 2.2b, respectively. A-chicks in two-chick broods performed eight times as many aggressive acts as B-chicks (Wilcoxon signed ranks test for paired comparisons: T = 12.5, n = 14, P < 0.01). In three-chick broods, the number of aggressive acts performed by ranked chicks differed significantly (Friedman's test, M = 8.23, n = 15, P < 0.02). A and Bchicks were two or more times as aggressive as C-chicks (Dunn's multiple comparison test, TAc = 2.29, P < 0.10; TBC = 2.66, P < 0.05) whereas aggression in A and B-chicks did not differ significantly (TAB = 0.37, P > 0.30). The number of aggressive acts directed at ranked chicks in three-chick broods differed significantly (Friedman's test, M = 9.43, n = 15, P < 0.01). Three times as much aggression was directed at B and C-chicks as compared to A-chicks (TAB = 3.02, P < 0.05; TAC = 2.29, P < 0.10).
B and C-chicks were equally likely to be targeted as victims (TBc = 0.64, P > 0.30). Figure 2.1. Mean aggression rate per brood during (a) feeding periods, (b) non-feeding and c) feeding periods with observation session two on brood N56 omitted. The scales for feeding and non-feeding periods are different because of the high rate of aggression observed in brood N56 during the 10 to 19 day age range. Brood sample sizes for each brood age range are shown in the upper section of each graph. Error bars indicate means f SE. Mean Brood Age
Mean Brood Age
0-9 10-19 20-27 28-35 36-43 44-51
Mean Brood Age 1 Aggression directed at: Nestling A Nestling B
A B Nestling Performing Aggression
1 Aggression directed at: Nestling A Nestling B 0 Nestling C
Nestling Performing Aggression
Figure 2.2. The mean proportion of aggressive acts performed by and directed at ranked chicks in six two and six threechick broods. AGGRESSION AND FOOD ALLOCATION During feeds early in the nestling period, the larger first-hatched chicks appeared to monopolize a position close to the parent distributing bites of fish. Older nestlings were usually fed first and junior siblings, particularly in three-chick broods, did not receive substantial amounts of food until senior chicks were satiated and moved away from the parent. This effect became less apparent as nestlings grew older. The pattern of food allocation within Osprey broods is shown in Figure 2.3. A and B- chicks in two-chick broods received 25-30 percent greater proportions of food compared to those in three-chick broods (Mann Whitney test: n = 75 and 79 feeds for two and three-chick broods, respectively; U = 3853, P = 0.001 for A-chicks; U = 4069, P < 0.0001 for B-chicks), therefore two and three-chick broods were considered separately in the following analyses. In two-chick broods, no difference was detected in the proportion of bites received by A and B- chicks (U = 3280, n = 75, P = 0.079, P = 0.55), however the test is inconclusive due to low statistical power. The proportion of bites received by A, B and C-chicks in three-chick broods differed significantly (Kruskal-Wallis test: H = 18.07, n = 79, P < 0.0001). Both A and B- chicks received a greater proportion of bites compared to C-chicks (Dunn's Multiple Comparison test, TAC = 4.24, P < 0.05 and TBc = 2.27, P < 0.05, respectively); proportions received by A and B-chicks were not different (TAB = 1.62, P > 0.30).
The proportion of bites received by chicks in different age categories is presented in Figure 2.4a and 2.4b for two and three-chick broods, respectively. In two-chick broods, A-chicks received a third more bites compared to B-chicks during period one (Mann Whitney test: U = 1026, n = 38, P = 0.002). During period two, the proportions received by A and B-chicks did not differ significantly (U = 210, n = 19, P = 0.397), although the A-chick's proportion tended to be greater. In the last period, this trend was reversed, with B-chicks receiving a greater proportion of food than A-chicks (U = 88, n = 18, P = 0.018). In three-chick broods, the proportion of bites received by A, B and C-chicks differed only during the first period A B C Nestling Rank
b Figure 2.3. The mean proportion of bites received by chicks in six two and six three-chick k broods. Error bars indicate means f SE. H ChickA H Chick B
0- 19 20 - 35. 36- 51 Brood Age
ChickA ChickB 0 Chick C
0- 19 20 - 35 36 - 51 Brood Age
Figure 2.4. The mean proportion of bites received by chicks in different age categories for two and three-chick broods. Error bars indicate means f SE. (Kruskal-Wallis test: H = 32, n = 43, P < 0.0001), with A-chicks receiving 1.5 and 2 times as many bites as B and C-chicks, respectively (Dunn's Multiple Comparison test, TAB = 2.73, P < 0.05 and TAC = 5.65, P c 0.05). B-chicks took 1.5 times as many bites as C-chicks (TBC= 2.73, P < 0.05). A trend of food allocation parallelling nestling size hierarchies was
observed also during period three (H = 1.15, n = 15, P = 0.564). In period two, no differences were detected and the trend was opposite to that expected on the basis of initial nestling size (H = 0.48, n = 21, P = 0.786). During feeds with aggression, senior siblings kept their junior nestmates away from the ------w ------___ _ I _--- -- parent distributing bites. Once they secured a position beside the parent and had unrestricted _------access to food, aggression usually ceased. However, if junior siblings continued to approach the parent, begged loudly or contested access to food, then senior chicks would persist and occasionally forced their junior nestmates to the nest edge. Parents did not intervene in these disputes. Once senior chicks were satiated, they generally moved away from the parent and junior chicks had uncontested access to food The proportion of bites received by nestlings in feeds with and without aggression is shown in Figure 2.5a and Figure 2Sb, for two and three-chick broods, respectively. In two- chick broods, no difference was detected in the proportions received by A-chicks in feeds with and without aggression (Mann Whitney test: n = 12 and 24 feeds with and without aggression, respectively; U = 188, P = 0.144, p = 0.74). Similarly, no differences were detected for A, B and C-chicks in three-chick broods (n = 13 and 28 feeds with and without aggression, respectively; U = 146, P = 0.306, p = 0.79 for A-chicks; U = 198, P = 0.653, P = 0.76 for B- chicks; U = 204, P = 0.536, P = 0.88 for C-chicks). However, all of the tests have low statistical power, so null hypotheses cannot be accepted with confidence. Only in three-chick : broods was there a tendency for A-chicks to receive a greater share in feeds with aggression ! 1 than without. feeds without aggression feeds with aggression
A B Nestling Rank
feeds without aggression feeds with aggression
Nestling Rank
Figure 2.5. The mean proportion of bites received by chicks in feeds with and without aggression for two and three-chick broods. Em>r bars indicate means f SE. Correlations of the aggression rate per feed to the proportion of bites received by A, B and C-chicks are given in Figure 2.6a, Figure 2.6b and Figure 2.6~-respectively. There was a positive correlation between the aggression rate observed during feeds and the A-chick's share (y = -74.8 + 202.8~;r = 0.50, n = 38, P = 0.001) and a negative correlation with the B and C- chick's shares (y = 99.4 - 170.0~; r = 0.38, n = 38, P = 0.020 for B-chicks; y = 8.2 - 11.2~; r = 0.27, n = 20, P = 0.240 for C-chicks). Overall, the shares received by ranked chicks explained only a small proportion of the variation in aggression and the significant results for A and B-chicks can be attributed to a few extremely aggressive feeds. When the latter outliers were removed, no trends were apparent.
AGGRESSION AND NESTLING GROWTH Correlations of mean brood aggression rate against brood and Achick weight-versus-age residuals are presented in Figure 2.7a and Figure 2.7b, respectively. Both correlations were negative, as predicted (r = 0.51, n = 12; P = 0.09 for broods; r = 0.54, n = 12; P = 0.068 for A-chicks), however, one brood (N56) with the lowest brood and A-chick residual was entirely responsible for these trends. The weight deficit for the N56 A-chick was 29 percent below average weight, more than double that of any other A-chick in the sample. Generally, relative growth explained little of the variation in aggression and it may only be a factor in chronically underweight broods. A greater sample of underweight broods would be required to verify any effect.
NESTLING GROWTH AND MORTALITY A summary of breeding variables for all successful nests is presented in Table 2.2. There were no significant differences between years in any of the breeding variables, however the sources of mortality between years were quite different (Table 2.3). The rate of whole clutch Figure 2.6. Correlations of aggression rate per feed to the propomon of bites received by A, B and C-chicks. Two and three-chick broods are combined (n = 18 two-chick feeds; n = 20 the-chick feeds). 0.2 0.4 0.6 0.8 1.O Proportion of Bites Received by A-Chick
0.0 0.2 0.4 0.6 0.8 1.O Proportion of Bites Received by B-Chick
0.0 0.2 0.4 0.6 .0.8 1.O Proportion of Bites received by C-Chick Brood Weight vs Age Residual
-60 -40 -20 0 20 40 60 Chick A Weight vs Age Residual
Figure 2.7. Correlations of mean aggression rate per brood with brood and A-chick weight-versus-age residuals. Data from two and three-chick broods are combined. Table 2.2 Summary statistics by year for Osprey breeding variables. Statistics presented are mean f S.E per successful nest. Sample sizes are shown in parentheses.
Breeding Variable 1989 1990 F~ P
Clutch size 2.86 k .07 (221b 2.74 f .08 (31)' 1.14 0.291
Brood size at hatching 2.50 k .18 (16)~ 2.45 f .14 (291e 0.056 0.814 Brood size at fledging 2.14k .16 (22) 2.06f .13 (31) 0.119 0.732 a One-way ANOVA 30 active nests; 8 nests failed 34 active nests; 3 nests failed 22 successful nests; 6 nests excluded because mortality could not be conclusively attributed to late egg loss or early chick loss 3 1 successful nests; 2 nests excluded because mortality could not be conclusively attributed to late egg loss or early chick loss Table 2.3 Sources of Osprey egg and chick mortality by year. Statistics presented are number per active nest, unless otherwise indicated. Rates are shown in parentheses.
Type of Mortality 1989 1990 G~ P
Whole clutch losses 6 / 30 (.20) 2 / 34 (.06) 1.89 > 0.10 Whole brood losses 2 130 (.07) 1 / 34 (.03) 1 .05 > 0.25
Partial brood losses 2/16(.1~)~ 9/29(.31)C 1.17 > 0.25 (per successful nest) a G-test with Yates correction 22 successful nests; 6 nests excluded because mortality could not be conclusively attributed to late egg loss or early chick loss 31 successful nests; 2 nests excluded because mortality could not be conclusively attributed to late egg loss or early chick loss loss was three times higher in 1989 and the rate of brood reduction was more than three times higher in 1990. Associated with the tendency for greater brood reduction in 1990 was a significantly lower rate of nestling growth in that year (One-way ANOVA: F = 4.208, P = 0.044; Table 2.4). No effect of chick rank or brood size on growth rate was detected in
either year (chick rank: F = 0.192, P = 0.826 in 1989 and F = 0.832, P = 0.445 in 1990;
brood size: F = 0.836, P = 0.445 in 1989 and F = 0.451, P = 0.507 in 1990). Only two of the 11 victims of brood reduction were recovered. One of these two chicks had a slight head wound. In 10 of the 11 cases of partial brood loss, the victim was the last- hatched chick. Pooling cases across years, there was no significant difference in brood reduction between two and three-chick broods (30 percent of three-chick broods versus 20
percent of two-chick broods; G-test: G = 0.322; P > 0.05). In 10 of the 11 nests where brood reduction occurred, an approximate nestling age at death could be determined (chicks were estimated to have died half way between weekly nest visits in cases where a better estimate was not available). Based on these estimates, the oldest chick died at 19 days of age and seven of the 10 chicks died under 12 days of age. Most brood reduction therefore occurred during the 0 to 19 day age range, before nestling food intake reached a peak (Figure 2.8). The increase in food intake rate shown in Figure 2.8 is conservative in that it is based on the assumption that bite size remains constant throughout the nestling period. In fact, females appeared to increase the size of bites they fed to their nestlings as nestlings grew larger.
PROXIMATE INFLUENCES ON SIBLING AGGRESSION The correlations of brood aggression rates with hunger and provisioning indices are presented in Table 2.5 and Table 2.6, respectively. Three of the eight correlations were in the direction predicted if hunger is proximally linked to aggression. Very little of the variation in aggression was explained by any of the indices and none of the correlations were significant. Table 2.4. Summary of weighted mean growth rates (grams/day) of ranked Osprey chicks during 1989 and 1990. Nestling sample sizes are shown in parentheses. Data from broods of one, two and three chicks are combined (1989: n = 2,5 and 6 one, two and threechick broods, respectively ; 1990: n = 3, 10 and 4 one, two and threechick broods, respectively).
Year A-chicks B-chicks C-chicks Overall Mean Brood Age Range
Figure 2.8. Mean bite rate per brood versus brood age range. Error bars indicate means f SE. Data from two and three-chick broods are combined Table 2.5. Results of correlations of brood aggression rate per feed with hunger indices for n feeds. Data from two and three-chick broods are combined
Hunger indices r P n
Time since last feed -0.125~ 0.352 58 Number of brood bites in last feed -0.040 0.766 58
Number of A-chick bites in last feed 0.058~ 0.665 58
Cumulative number of brood bites 0.01 la 0.926 8 1 before feed
Cumulative number of A-chick bites 0.083~ 0.464 8 1 before feed
a Sign of the correlation coefficient is opposite the direction predicted. Table 2.6. Results of comlations of mean daily aggression rate per brood during feeding periods with provisioning indices for n observation sessions. Data from two and three-chick broods are combined
Provisioning indices r P n
- - Daily bite rate -0.033 0.885 22 Daily fish delivery rate -0.136 0.545 22
Daily feed rate 0.086~ 0.704 22
a Sign of correlation coefficientis opposite the direction predicted. These results suggest that hunger has at most a weak effect on aggression under the observed feeding conditions. Regressions of brood age asynchrony and brood weight asymmetry against aggression rate are shown in Figure 2.9a and Figure 2.9b, respectively. Weak inverse relationships were found between aggression rate and age asynchmny (y = 32.6 - 7.15~; r2 = 0.14, n = 12,
P = 0.232) and between aggression rate and weight asymmetry (y = 37.3 - 69.1~;r2 = 0.13, n = 12, P = 0.253). In both cases, trends were sensitive to one brood (N56) and no trends were apparent when this brood was removed from the analyses. Mean agggression rates in broods of two and three-chicks are presented in Figure 2.10. Aggression rates during feeds were not different in two and three-chick broods (Mann Whitney test: U = 953, P = 0.239, n = 38 and 44 feeds for two and threechick broods, respectively). When brood N56 is omitted from the analysis, aggression rates per dyad were only slightly higher in two-chick broods.
DISCUSSION Previous studies have yielded conflicting results with respect to the prevalence of sibling aggression among nestling Ospreys and its role in brood reduction. I investigated the extent, variation and timing of sibling aggression in the Kootenay Osprey population. I attempted to relate the pattern of sibling aggression to trends in nestling feeding behaviour, nestling growth and the pattern and timing of mortality within broods. I also examined whether sibling aggression is influenced by one or more potential proximate cues. Most of the aggression observed during this study occurred early in the nestling period when broods were less than 20 days old. Similarly, feeding disadvantages to last-hatched chicks were only evident when broods were less than 20 days old. As expected, senior chicks chick broods), however not all aggression was directed towards the youngest member of a brood, B and C-chicks in three-chick broods were both likely to be targeted as victims. Aggression Rate During Feeding Periods Aggression Rate During Feeding Periods (acts/dyad/hr) (actsldyadlhr)
CI 0 8 8 Mean Aggression Rate During Feeds (acts/dyad/hr)
0 E; s In broods under three weeks of age, feeding was hierarchical with senior chicks having greater access to incoming food. These findings are similar to those of Poole (1982) and Forbes (1991). When feeds with and without aggression were compared, no differences in food allocation were detected. Only in three-chick broods was there a tendency for the senior chick's relative share to increase with aggression. The mean daily bite rate of two and three-
chick broods did not differ significantly in this study (Mann Whitney: U = 229, n = 22 and 23 observation sessions for two and three-chick broods, respectively; P = 0.586). Similarly, several other studies have found no difference in delivery rates to two and three-chick broods, suggesting that nestlings from two-chick broods receive more food, on average (Stinson 1978, Poole 1982, Jamieson et al. 1983, Eriksson 1986). It is possible that B-chicks in two-chick broods still received a substantial share of food in feeds with aggression, once A-chicks were satiated. Such an effect would offset the influence of aggression on food allocation in two- chick broods. Poole (1982) and Forbes (1989) found that senior siblings received a significantly greater proportion of bites at feeds with aggression than at feeds without it. However, Forbes used provisioning data over the entire nestling season to make this comparison. Poole's (1982)
comparisons were based on nestlings ranging from 1-4 weeks of age. If I similarly include all brood age ranges, then senior chicks in two and three-chick broods obtain a greater proportion in feeds with aggression than in feeds without (Mann Whitney test: U = 560, P = 0.55 for two- chick; U = 506, P = < 0.05 for three-chick). However, the influence of aggression on food allocation is confounded with the effect of nestling age on food allocation in these analyses. By limiting my evaluation to broods in the range of 0 to 19 days, I have avoided this problem, however, the statistical power of the tests is low. When the quantitative influence of aggression was considered, aggression rate was positively correlated to the A-chick's share and negatively correlated to the B and C-chick's share. The results are consistent with those obtained by Poole (1984) during brood enlargements. Nestlings in his experimentally enlarged broods were more aggressive, senior siblings received larger shares and junior siblings smaller shares, as compared to natural broods. The trends observed in this study were the result of a few extremely aggressive feeds and the fact that the association was detected only using the latter method suggests that it is biologically weak. However, I was only able to observe broods once a week and therefore cannot rule out the possibility that aggressive interactions which took place prior to observation sessions established dominance. Indeed, even during early sessions, threat displays by senior siblings were very effective at eliciting submissive responses (e.g. crouching, retreating to the nest edge) from junior nestmates. It is possible that early aggressive exchanges may have led to these responses. This would have diminished the need for senior siblings to use overt aggression (unless dominance was contested) and would have made the proximate influence of aggression on food allocation diffkult to discern using my sampling technique. A large sample of feeds on very young broods may be required to provide an unequivocal demonstration of an effect of aggression on food allocation. The extremely high rate of aggression observed in brood N56 was associated with a 29 percent weight deficit in the A-chick (based on a regression of weight versus age for all A- chicks in the sample), more than double the weight deficit of any other A-chick. A similar influence of weight deficit on aggression levels has been reported in facultatively siblicidal Blue-footed Boobies (Drummond et al. 1986, Drurnmond & Garcia Chavelas 1989). Drummond and Garcia Chavelas showed that senior chick aggression increased with food deprivation and intensXied when the senior chick's weight dropped 20-25% below potential (i.e. below the mean weight of same age senior chicks in the year with fastest recorded growth), suggesting a threshold effect. A similar threshold effect may operate in the case of Ospreys. Broods with A-chick weight deficits of under 10 percent showed no tendency for elevated aggression. Another interpretation of the large weight deficit of the N56 A-chick is that it paid a cost, in terms of reduced weight gain, for its substantial investment in aggression. My data do not allow me to address quantitatively, whether senior chicks pay a fitness cost for their aggression. In the case of brood N56, the senior chick invested considerable time attacking its sibling. It is notable that neither of two chicks from this nest had fledged by 59 and 60 days of age respectively, whereas the mean fledging age for 18 fledglings I monitored in 1989 was 55.5 f 2.1 days. A similar delay in fledging associated with aggressive broods was found by McLean & Byrd (1991). Poole (1989) showed that young Ospreys fledging late in the season have a significantly lower probability of surviving to breeding age. One cost of aggression (or failing to kill a sibling when food is insufficent for the entire brood) may be a reduced probability of surviving to breed. One other factor distinguishing brood N56 was that it occupied the only newly established nest in the sample. The female parent at this nest was a banded four year-old, and likely breeding for the first time. According to Poole (1989). inexperienced parents fledge fewer young as compared to experienced breeders, presumably as a result of lower provisioning rates. I was unable to detect any significant difference in provisioning rates between this brood and other broods in my sample based on 8 to 10 hour watches at weekly intervals. However, if the N56 male was similarly inexperienced, this would be consistent with the relatively poor nutritional condition of the senior chick and the brood as a whole. Brood reduction in this study was consistent with the proposed advantages of Lack's hypothesis. Junior chicks were at an initial size disadvantage because of asynchronous hatching and more aggression was directed at junior chicks than at their senior nestrnates. The combined effect of smaller size and greater harassment led to significant feeding disadvantages for junior chicks. All of these disadvantages led to over 90 percent junior chick biased mortality. All brood reduction occ- when victims were less than 20 days old. Similarly, in three other studies of brood reduction in Ospreys, mortality occurred during the first 20 days (Hagan 1986, McLean & Byrd 1991, Steidl & Griffin 1991). This was the same age range in which food was skewed to senior chicks and in which almost all of the aggression occurred. Therefore, mortality appeared to be an outcome of aggression-mediated starvation. A trend of greater brood reduction in 1990 was associated with a lower mean rate of nestling growth in that year, suggesting that brood reduction occurred in response to food limitation. Because chicks died relatively early, parents expended minimal effort on the victims and junior chicks that did survive grew as fast as their senior nestrnates, suggesting that they were of similar quality. To determine why the rate of brood reduction tended to be higher in 1990, I compared the number of rain days and the mean daily rate of precipitation in the months of April to July. Forbes (1989) showed that the number of rain days from April to July is negatively associated with mean number of young Ospreys raised per successful nest. Although the number of rain days was identical in the two years (ie. 60 days), the mean daily rate of precipitation in 1990 was more than twice the rate reported in 1989 (ie. 4.75 versus 2.05 Wday, respectively). No windspeed measurements were available for my study area but Forbes (1989) found that precipitation was negatively correlated with temperature and positively correlated with windspeed and water surface conditions in Creston. The latter conditions impair Osprey foraging success (Grubb 1977, Machmer & Ydenberg 1990). The higher rate of brood reduction in 1990 may be partly attributable to the effects of wetter weather on the foraging success of parents and the energetic demands of nestlings themselves. During the age range when all mortality occurred, the mean bite rate to broods was slightly more than half of what it was when food intake peaked, at 20 to 35 days of age. This leads to an apparent paradox: junior siblings appeared to suffer to a variable extent from aggressively mediated food shortage which occasionally led to brood reduction. Yet, assuming food availability did not increase with the season, male Ospreys were able to work harder later on, in order to meet the greater demands of older young. This suggests that Osprey parents encourage brood reduction early on, both by withholding food from their cmntbrood and by not intervening in aggressive nestling interactions. Such parental restraint has been reported in other species (Drent & Dam 1980, Nur 1984, Gustafsson & Sutherland 1988, Mock & Lamey 1991) and may be part of an overall reproductive strategy in long-lived birds, which maximizes lifetime rather than seasonal reproductive success. The lack of significant correlations between hunger indices and brood aggression rates is not surprising. Even under experimental conditions where food intake and inter-brood variation in aggression were controlled, aggression rates were only slightly higher in broods subjected to four hours of food deprivation than in satiated broods (see chapter 111). During watches of natural broods, inter-brood variation in aggression was large and differences in the recent feeding history of broods could not be controlled. I attempted to minimize the latter by initiating observations at dawn, before males began hunting, but I occasionally observed i broods eating in semi-darkness (possibly on fish leftover from the previous evening). The modal time between feeds during natural observations was 56 minutes, and broods were rarely deprived of food for more than four hours. All of the above factors could have obscured trends with hunger. More convincing support for the influence of hunger on aggression is presented in chaper III. The lack of association between provisioning indices and brood aggression rate may be a result of the sampling regime, rather than an actual absence of any trends. Considering that male Ospreys in Nelson deliver relatively few fish per day, 8 to 10 hour observation sessions may have been too short to detect real differences in provisioning between broods. Also, some day to day variation in delivery rates of a few closely observed males has been noted (Steeger, Green, pers. comm.) and my sessions may have been too infrequent to obtain representative provisioning rates. I detected no effect of within-brood competitive asymmetry on aggression rates, as I did in experimental broods (see chapter 111). It should be noted however, that the two broods which reduced from three to two chicks before I could begin observations are the two most synchronous broods in Figure 2.9a. This early reduction may have influenced the aggressive behaviour of the surviving chicks and the results are therefore difficult to interpret. I found no effect of brood size on aggression rates. The influence of potential proximate cues for aggression would be more appropriately evaluated in an experiment, where food amount, asynchrony or brood size could be manipulated and inter-brood variation in aggression could be controlled. In the next chapter, I experimentally evaluate the influence of food amount on sibling aggression.
CONCLUSIONS Brood reduction occurred at rates of 12 and 31 percent per successful nest in the two years of the study. Junior chick biased mortality was found in over 90 percent of the cases of partial brood loss. This pattern was an outcome of the initial size disadvantage and greater aggressive harassment suffered by junior chicks in the first 19 days of the nestling period. During this same period, food was skewed in favor of senior siblings and almost all aggression occurred. Therefore, mortality appeared to be an outcome of aggression-mediated starvation. A trend of greater brood reduction in 1990 was associated with slower nestling growth in that year, suggesting that mortality occurred in response to food limitation. I found little evidence that
I food amount, within-brood competitive asymmetry or brood size affected sibling aggression. g An anecdotal observation suggested that aggression in Ospreys is related to the growth and nutritional condition of the senior chick as has been reported in other species. cliumux A TEST OF THE FOOD AMOUNT HYPOTHESIS FOR SIBLING AGGRESSION IN NESTLING OSPREYS
Sibling aggression has been reported in a number of avian species (reviews in O'Connor 1978, Stinson 1979, Mock 1984a). The brood reduction hypothesis (Lack 1947, 1954,1968) is widely accepted as the adaptive explanation for the occurrence of such aggression in facultatively siblicidal birds. It proposes that females lay the clutch size that maximizes their reproductive success in years of high food availability. When food is insufficient, competitive asymmetries established through hatching asynchrony and exaggerated through sibling aggression are thought to facilitate rapid reduction of brood size to match prevailing food levels. The ultimate cause of sibling aggression is therefore widely considered to be an insufficient amount of food (Lack 1968, Proctor 1975, Braun and Hunt 1983, Mock et al. 1987a). The role of food shortage in regulating sibling aggression has received considerable attention (Proctor 1975, Stinson 1979, Braun & Hunt 1983, Mock et al. 1987a, Drurnrnond & Garcia Chavelas 1989) and a proximate link between food intake and aggression is often assumed (Skutch 1967, Proctor 1975, Brown et al. 1977). This view is expressed in the food amount hypothesis (FAH) (Mock et. al. 1987a), which proposes that sibling aggression varies inversely with the quantity of food delivered by parents, presumably mediated by nestling hunger. Most evidence in support of the FAH is circumstantial and can be summarized by the following observations: (i) aggression is temporally associated with feeding bouts (Braun & Hunt 1983, Jamieson et al. 1983, Groves 1984), (ii) satiated chicks exhibit a temporary increases during periods of inclement weather, when parental provisioning rates are reduced (Williams & Burger 1979, Poole 1982, Braun & Hunt 1983). Three studies have tested the FAH experimentally. Proctor (1975) reported increased nestling aggression rates when two-chick broods of South Polar Skua (Catharacta maccormicki) were artificially deprived of food, however there were problems with his experimental design (see Mock et al. 1987a, Drummond & Garcia Chavelas 1989). Drumrnond & Garcia Chavelas (1989) demonstrated proximate control of sibling aggression by food amount in two-chick broods of Blue-footed Booby (Sula nebomii). Aggression rates increased and senior chicks were offered a greater proportion of bites in broods where the necks of nestlings were taped to prevent ingestion, as compared to senior chicks in control broods. This effect intensified with prolonged food deprivation, when the mass of the senior chick dropped 20-25 percent below potential. This study concluded that aggression in senior chicks was proximately linked to food amount and mediated by chick hunger or nutritional condition. Sibling aggression rates in Great Egret (Casmerodius albus) broods were not influenced by experimental increases or decreases in food amount (Mock et al. 1987a). Even nestlings consuming ten times their normal daily food intake continued to be aggressive; hence the FAH was not supported in this species. Extensive observations on three Ardeid species have failed to demonstrate the inverse correlation between current food amount and aggression predicted by the FAH (Mock et al. 1987a,b). Nonsignificant positive correlations are consistently observed and increased vulnerabilty of junior egret siblings to starvation seems to be the important factor associated with food amount, rather than elevated aggression. Two patterns of siblicide have emerged from these investigations to date. Mock (1984a) distinguished these as conditional and probabilistic siblicide, based on the presence or absence of a proximate link betweeen food shortage and aggression, respectively. In the conditional system, sibling aggression is triggered by hunger and nestlings behave according to the simple rule: fight if hungry. Fighting is thought to affect food allocation through intimidation of junior siblings and results in senior siblings receiving a disproportionate share of food (Poole 1982, Fujioka 1985b, Ploger & Mock 1986, Parker et al. 1989, Drummond et al. 1986, Forbes 1989). Once senior siblings are satiated, fighting ceases. The conditional system allows senior siblings to modify their behavior based on current food availability, and minimizes the costs associated with aggression (e.g. time, energy, risk of injury). In the probabilistic case, aggression occurs irrespective of food intake level. However, the ability of the junior sibling(s) to withstand the aggression and survive is dependent on incoming food level. The probabilistic pattern appears to operate in Ardeids (Mock et al. 1987a). Mock et al. have proposed that the degree of within-year food stability determines which type of siblicidal pattern (conditional or probabilistic) is likely to evolve. If food availability is highly predictable from day to day, then a proximate link between food amount and aggression ensures that the latter is triggered only when food shortage is imminent. However, if short tern food availability varies unpredictably, as in Great Egrets (Mock et al. 1987a), then the
best strategy for a senior sibling may be to fight irrespective of current food amount, or to fight in response to cues other than food amount. Such cues might include: (i) competitive asymmetry between nestmates, associated with within-brood hatching asynchrony (Fujioka 1985b Mock & Ploger 1987, Forbes 1991), (ii) the ability to monopolize prey through aggression (Mock 1984b, 1985; Mock et al. 1987b), and (iii) brood size (Mock & Lamey 199 1). These cues are not mutually exclusive. In this study, I attempted an experiment to test the FAH in nestling Ospreys (Pandion haliaetus). Reliable information on short term food stability in Ospreys is not available at the present time. However, several observations suggest that aggression is proximately regulated by food amount and mediates food allocation in this species: (i) aggressive interactions are usually associated with feeding (Poole 1982, Forbes 1991, McLean & Byrd 1991), (ii) recently-fed chicks are less likely to be aggressive (Poole 1982, Forbes 1991), (iii) the rank order of aggression varies inversely with feeding rate in three colonies of Ospreys (Poole 1979, 1982) and (iv) senior siblings obtain a greater proportion of food at feeds with aggression than at feeds without (Poole 1982, Forbes 1989). The predictions tested are: (1) broods should be more aggressive when they are hungry than when they are satiated and (2) senior (A) siblings should take a greater share of the total available food offered when they are hungry than when they are satiated. I also examine the roles of other proximate factors that may influence aggression: relative nutritional condition of brood members, competitive asymmetries between nestmates and brood size.
METHODS I performed a controlled feeding experiment on ten broods of ospreys (six in 1989; four in 1990) in June and early July of each year. Broods were selected based on the accessibility of their nests and nestling age. Nestling ages ranged from 5-13 days at the beginning of the experiment. Nestlings beyond 16 days of age became less willing to accept food during treatments and therefore were not used. In each year, two nests were chosen as viewing nests because of their superior visibility from a vantage point and their accessibility by boat. Each of the ten experimental broods was exchanged with the brood from one of these viewing nests for the duration of the two-day experiment. As a result of these brood transfers, Osprey pairs were never without at least one nestling during the course of the experiment. Parents returned to their nests without delay following brood transfers and they readily accepted foster chicks. At the beginning of each experimental treatment, an entire brood was removed from the viewing nest and kept nearby in captivity for four hours. This duration corresponded approximately to the time it took for a 1-2 week old nestling to empty a 314-full crop (pers. obs.). During this time, viewing nest parents were left with a chick from the original viewing nest brood. Immediately following removal, each member of an experimental brood was weighed to the nearest 1 or 5 g, using a 100 or 1000 g pesola, respectively. Culmen lengths were measured to the nearest 0.1 mm with Vernier calipers and each nestling's crop content was estimated to the nearest 118 by palpitation. Nestlings were then marked with water-based paint on their heads and necks. Colors within a brood were assigned according to nestling mass and proceding from heaviest to lightest chicks, nestlings received red, green and yellow markings, respectively. Culmen lengths were used to rank nestlings when masses were indistinguishable. The experimental brood was then placed in a ground level stick nest lined with moss and shielded from wind and rain. During cool periods, chicks were covered with a blanket to prevent chilling. In the last half hour before a brood was transferred to the viewing nest, each brood member was again weighed and its crop content estimated. The brood was then subjected to one of two treatments: && - Each brood member was hand-fed small bites of fish to satiation. The number of bites taken and the feed duration for each nestling were recorded. Mock feed - A single piece of fish was held out to chicks to get their attention and then each nestling was mock-fed (ie. feeding was simulated by repeatedly placing the experimenter's fingers to the chicks mouth) the same number of times that it consumed bites during feed treatments. Care was taken to equalize the duration of handling in feed and mock feed treatments. Immediately following feeds and mock feeds, nestlings were weighed and their crop contents were again estimated. After four hours in captivity, the experimental brood was transferred to the viewing nest r and the viewing nest chick was removed. A pre-weighed whole fish was also supplied to the I viewing nest at this time. Usually females began feeding the supplemented fish to their brood almost immediately. Only trials in which feeds were initiated within 60 minutes of the brood transfer (i.e. 95%) were included in the results; two trials were excluded. As soon as the experimental brood was placed on the viewing nest, the following observations were made from a vantage point 50-150m from the viewing nest using a spotting scope and a commentary spoken into a tape recorder: (a) all occurrences of aggressive behavior (categorized as bites, hits, pecks, shoves, threat displays; see chapter I1 for a description of aggression categories) and the identities of the chicks involved (b) number of bites allocated t A, B and C-chicks and to the female and male parent
(c) presence or absence of the female and male from the nest or nesting temtory
Observations ended once a fish was completely eaten or when the parent released the fish from its grip and had stopped feeding itself and its nestlings for more than five minutes. In 1989, each experimental brood received three treatments per day, for a total of six treatments over a two-day period. Treatment orders were reversed on the second day of the experiment. Unfavorable weather, dwindling daylight and my restriction on nestling ages made it impossible to complete all treatments for each brood. In 1990, each brood received two treatments per day, for a total of four treatments over two days and all treatments were completed. In all, 40 treatments (21 real; 19 mock) were completed on ten broods over the two-year period. A shortage of accessible 3-chick broods made it necessary to use both 2- and 3-chick broods (n = 4 for 2-chick broods; n = 6 for 3-chick broods). Brood aggression rates were calculated as total number of aggressive acts (acts) divided by the observation duration (hr). Following Mock & Parker (1986). aggression rates were then standardized across brood sizes by dividing the brood aggression rate by the number of sibling dyads within a brood (3-chick broods had three possible dyads; 2-chick broods had only one). Brood aggression rates are therefore reported as acts/dyad/hour. To test if some aspect of the experiment itself (e.g. handling) influenced aggression levels in experimental broods, 8-10 hours of natural control observations were completed on the four experimental broods in 1990. These sessions were conducted one day before each brood was tested in the experiment, and the following observations were recorded: a) all occurrences of aggressive behavior (categorized as bites, hits, pecks, shoves, threat displays and other) and the identities of the chicks involved (b) the context in which aggressive acts took place (ie. feeding versus non-feeding bouts, duration since last feeding) (c) timing of prey deliveries by the male or female parent (d) species and size (estimated relative to the parent's tail length; Stinson 1978) of each prey captured (e) number of bites allocated to A, B and C-chicks and to the male and female parent
HUNGER,FOOD ALLOCATION AND AGGRESSION A Wilcoxon signed ranks test for paired observations was used to test whether nestlings were more aggressive, on average, following mock feed treatments, when they were presumably hungrier. A Mann Whimey test was performed to assess whether brood aggression rates during experimental treatments and natural control observations were significantly different. The proportions of bites received by A, B and C-chicks and females were calculated for all feeds. Since the size of supplemented fish supplied to females during the experiment was not standardized, proportions were averaged, resulting in a mean feed and mean mock feed proportion for each brood member and female. This ensured that feeds of all meal sizes were equally weighted in the results. Wilcoxon signed ranks tests for paired comparisons were used to compare the proportions taken by nestlings and females in feed and mock feed treatments.
AGGRESSION AND RELATIVE NUTRITIONAL CONDITION If hunger is proximately linked to aggression, the relative nutritional condition of nestlings (which presumably reflects past hunger) should be inversely related to the intensity of aggression. To test this prediction, nestling weight versus age (measured on the first morning
49 of each nestling's participation in the experiment) was plotted separately for all experimental A, B and C-chicks. Residuals of weight versus age were averaged for each brood and then correlated to mean brood aggression rate in feed and mock feed treatments. Residuals of A chick weight versus age were also correlated to mean brood aggression rate.
AGGRESSION AND COMPETITIVE ASYMMETRIES BETWEEN NESTMATES Brood age asynchrony and brood weight asymmetry were used to estimate the influence of competitive asymmetries between nestmates on brood aggression levels. Brood age asynchrony was calculated as the difference in estimated age between the oldest and youngest member of a brood. Nestling ages were estimated from a regression of culmen length (x) on age (y) derived fiom known-age chicks (y = -13.6 + 1.5~.r2 = 0.96, n = 40, Steeger 1989; cf. Poole 1984). Brood weight asymmetry was calculated as the weight difference (measured on the first morning of the experiment) between the oldest and youngest member of a brood, divided by the A-chick's weight. Weight asymmetry was standardized in this way because sibling weight differences vary according to nestling weight and age (Poole 1982, Steidl & Griffin 1991), and brood ages differed slightly when broods were tested. The A-chick's weight was used because it is expected to deviate least for its age. Regressions of brood aggression rate against brood age asynchrony and brood weight asymmetry were computed for feed and mock feed treatments.
AGGRESSION AND BROOD SIZE A two-way ANOVA with repeated measures was used to investigate the potential effect of brood size on aggression levels in feed and mock feed treatments.
Some of the variables under consideration had distributions that deviated from normality or had unequal variances between groups. In these cases, non-parametric statistics were employed. Dunn's multiple comparisons were used to identify which groups differed significantly in Kruskal-Wallis tests. Since the probabilities are shared between groups in this test, two groups are considered to be significantly different at values of a > 0.05, depending on the number of groups being tested (Neave & Worthington 1988). All statistical analyses were performed using the SYSTAT software package (Wilkinson 1986).
RESULTS HUNGER,FOOD ALLOCATION AND AGGRESSION A summary of aggression rates in feeds following feed and mock feed treatments is presented in Table 3.1. Broods were slightly but significantly more aggressive, on average, following mock feed treatments, when they were presumably hungrier (Wilcoxon signed ranks test for paired comparisons, one-tailed, T = 1, n = 9, P < 0.01). Three lines of evidence suggest that there was nothing about the experiment itself which systematically influenced brood aggression levels: (i) aggression rates in feeds following experimental treatments (pooled across broods) were not significantly different from aggression rates in natural control feeds (Mann Whitney test: n = 16 and 17 experimental and control feeds respectively, U = 152, P = 0.543). (ii) no systematic difference in mean within- brood aggression rates was detected between natural and experimental feeds (Figure 3.1) and (iii) mean feed and mock feed aggression rates in 10 experimental broods were comparable to mean aggression rates in 12 unmanipulated broods under 20 days of age (feed treatments versus feeds in unmanipulated broods: U = 63, P = 0.843; mock feed treatments versus feeds in unrnanipulated broods: U = 28, P = 0.065; see Chapter 11). Therefore the results of this experiment should apply to broods under natural conditions. The proportions of bites taken by A, B & Cchicks and females in feeds following feed and mock feed treatments are shown in Appendix 1 and are presented schematically as frequency distributions in Figure 3.2. A dominance hierarchy was evident in feeds following feed ICY IIOOCY I I I I 0 d natural feeds experimental feeds
N37 N29 N38 N30 Nest #
Figure 3.1. Comparison of mean aggression rate per brood in f& following experimental treatments versus natural control feeds. Actual values are shown above each bar. PE
Frequency of Feeds Frequency of Feeds
OWPOr00Z OWPOlocS
Frequency of Feeds Frequency of Feeds treatments with ranked chicks receiving different proportions of the total available bites (Kruskal-Wallis test, n = 21.21, and 13 feeds for A,B and Cchicks respectively, H = 9.99, P < 0.01). The shares obtained by A-chicks were 1.5 and 2 times greater than the shares received by B and C-chicks, respectively (Dunn's Multiple Comparisons test: TAB= 2.70,
b P < 0.05; TAC = 2.66, P < 0.05). The shares received by B and C-chicks were not different t 1 (TBC = 0.30, P > 0.30). Proportions received by ranked chicks also differed significantly t following mock feed treatments (Kruskal-Wallis test, n = 19.19 and 12 feeds for A,B and C-chicks respectively, H = 7.66, P = 0.022). The shares taken by A-chicks were 1.5 times greater than C-chick shares (TAc = 2.73, P < 0.05) however the shares taken by A and
B-chicks and by B and C-chicks did not differ (TAB= 1.57, P > 0.30 and TBC = 1.35,
P > 0.30, respectively). Senior siblings took a slightly greater average proportion of the total available bites in feeds following mock feed treatments, as compared to following feed treatments (Wilcoxon signed ranks test for paired comparisons, one-tailed, T = 8, n = 9, P < 0.05). Furthermore, the share
taken by B-chicks was 1.5 times greater following mock feed treatments (T = 0, n = 9, P < 0.005). C-chicks did not receive a smaller average share of bites following mock feed treatments (Figure 3.3). The proportions received by C-chicks were either not different or greater than the proportion taken after feed treamnts when broods were considered separately. Pooling all threechick broods, no difference was detected in the proportions taken by C-chicks
(Mann-Whitney test: U = 55, n = 13 and 12 feed and mock feed treatments respectively, P = 0.200, P = 0.80), however the power of the test is low. Females compensated for the hunger of their broods following mock feed treatments (Figure 3.2) by taking a 45 percent smaller average share of the available food as compared to
following feed treatments (Wilcoxon signed ranks test for paired comparisons, T = 0, n = 9, P c 0.005). This trend was present in all broods tested and in three of the broods, the decrease 1 feed treatments 0 mock-feed treatments
2 5 7 8 9
Experimental Brood #
Figure 3.3. Mean proportion of bites received by C-chicks in feeds following feed and mock feed treatments. Error bars indicate means f SE. was greater than 50 percent. This result was produced by the four females from the viewing nests. The average proportion taken by each female following mock treatments was much smaller than the share taken following feed treatments, when all broods tested with the same female were pooled (Figure 3.4). Females tended to take fewer bites in feeds following mock feed treatments, however the differences between feed and mock feed treatments were not significant (T = 16, n = 9, P = 0.173). Females also compensated by distributing a greater number of bites in feeds following mock feed treatments (Figure 3.5). Meal sizes (ie. total number of bites) were 45 percent larger, on average, following mock feed treatments, compared to feed treatments (T = 0, n = 9, P < 0.005).
AGGRESSION AND RELATIVE NUTRITIONAL CONDITION Correlations of mean brood aggression rate against mean residual of A, B and C-chick weight versus age were not significant (r = 0.49, n = 9; P = 0.153 for feed treatments;
r = 0.49, n = 9; P = 0.185 for mock feed treatments) (Figure 3.6a). Mean brood aggression rate correlated with the residual of A-chick weight versus age was also not significant
(r = 0.31, n = 9; P = 0.378 for feed treatments; r = 0.28, n = 9, P = 0.463 for mock feed treatments) (Figure 3.6b). In both cases, the trends were opposite to those expected based on the significant positive elations ship between hunger and aggression.
AGGRESSION AND COMPETITIVE ASYMMETRIES BETWEEN NESTMATES The influence of brood age asynchrony and brood weight asymmetry on brood aggression k i levels is presented in Figure 3.7a,b and Figure 3.8a,b, respectively. Brood aggression rate i and brood age asynchrony were inversely related in feeds following feed and mock feed treatment. Neither regression was significant (y = -10.7 x + 54.2 ; r2 = 0.30, n = 10, F = 3.38, P = 0.103, P = 0.64 for feed treatments; y = -12.7 x + 73.1 ; r2=0.38, n = 9, F = 4.37, P = 0.075, $ = 0.53 for mock feed treatments), however the power of the tests is low. feed treatments H mock feed treatments
Viewing Nest #
Figure 3.4. Mean proportion of bites taken by four viewing nest females in feeds following feed and mock feed treatments. Error bars indicate means f SE. feed treatments mock feed treatments
Meal Size Range (# of bites)
Figure 3.5. Frequency distribution of meal sizes in feeds following feed and mock feed treatments. 2009 feed heanent 0 mock feed treatment i 150 - 0 a .rl V1 8 e g 100- M 2 3 0 50 - 0 E a@ 0 0 3o I I I -40 -20 0 20 40 60 Mean Brood Weight vs Age Residual
feed treatment mock feed treatment
Nestling A Weight vs Age Residual
Figure 3.6. Correlations of mean aggression rate per brood with brood and A-chick weight- versus-age residuals. 150 - . loo - 50 - . . e 0 0-4 I +, . I 0 2 4 6 8 Brood Age Asynchrony (days)
0 2 4 6 8 Brood Age Asynchrony (days)
Figure 3.7. Regressions of brood age asynchrony with mean aggression rate per brood in feeds following feed and mock feed treatments. Mock Feed Treatment Aggression Rate Feed Treatment Aggression Rate (acts/dyad/hr) (acts/dyad/hr) A significant inverse relationship between brood aggression rate and brood weight asymmetry was found in feed and mock feed treatments (y = -137.4 x + 71.8 ; r2 = 0.40, F = 5.42, n = 10, P < 0.05 for feed treatments; y = -171.95 x + 93.6 ; r2 = 0.51, F = 7.38 , n = 9, P < 0.05 for mock feed treatments).
AGGRESSION AND BROOD SIZE Mean aggression rates in feeds following feed and mock feed treatments as a function of brood size are shown in Figure 3.9. A two-way ANOVA with repeated measures indicated a significant effect of the treatment on aggression rate (F =13.21, n = 9, P < 0.01), however no significant difference was detected as a function of brood size (F = 1.54, n = 9, P = 0.255).
The interaction between brood size and treatment was also not significant ( F = 0.59, n = 9, P
= 0.469).
DISCUSSION The FAH proposes that sibling aggression varies inversely with the quantity of food delivered by parents, possibly mediated by nestling hunger. Assuming that senior siblings are more likely to use aggression (as was found in this study and studies of other facultatively siblicidal species), the FAH generates two predictions. Broods should be more aggressive when they are hungry than when they are satiated and senior siblings should take a greater share of the total available food offered when they are hungry than when they are satiated. I tested these predictions in an experiment using nestling Ospreys. I also investigated the influence of other proximate factors on sibling aggression in this species.
HUNGER, FOOD ALLOCATION AND AGGRESSION Both of the predictions of the FAH were supported in the experiment. Broods were slightly but significantly more aggressive at feeds when they were hungry than when they were B H feed treatment 2 50 H mock feed treatment
.w8
Meam 4z 30 1 20 m 8 El 10 3 0 2 3 Brood Size
Figure 3.9. Mean aggression rate per hodof two and three-chick broods in fccds following feed and mock feed treatments. Error bars indicate means + SE. satiated, and senior siblings took a significantly greater share of available food at feeds when they were hungry than when they were satiated. A surprising result was that the proportions of food received by junior chicks (B and C-chicks) did not decrease as a result of the increased proportion taken by A-chicks, when broods were hungry. Instead, females compensated for the hunger of their broods following mock feed treatments, by taking a smaller share of available food for themselves. Female food intake was not controlled during this experiment so females could potentially make up for their own food deficit between treatments. However, the willingness of Osprey females to sacrifice their food intake for their brood has been demonstrated during experimental brood enlargements (Poole 1984). In Poole's study, females with enlarged broods took significantly smaller proportions of food and suffered weight loss, compared to females with normal broods. Females also compensated for the hunger of their broods following mock feed treatments by increasing the total number of bites distributed to their brood. Supplemented fish were assigned randomly to treatments, so the larger meal sizes following mock feed treatments were not due to differences in fish size. Females were simply more likely to feed an entire fish to their broad in one bout following mock feed treatments. Most likely, this was because their nestlings solicited food more persistently when hungry. The option of withholding a portion of fish following feed treatments, would not be available to females under more food-limited conditions. In this experiment, females effectively buffered their broods against short term (four-hour) food deprivation and junior chicks suffered no disadvantage in terms of the proportion of food they received. However, more prolonged or frequent food shortages would eventually result in a decline in nestling food intake. Such shortages are expected to affect junior siblings disproportionately because of the size-based feeding hierarchy found in Ospreys (see chapter 11) and because of the increased aggression associated with food shortage. In other studies, aggression has been observed to exaggerate the feeding advantage of senior siblings at the expense of junior nestmates (Poole 1982, Fujioka 1985b, Ploger & Mock 1986, Drummond et al. 1986, Forbes 1989). An anecdotal account of an Osprey female feeding her two chicks after a prolonged storm (Forbes 1990a) supports these expectations. The female and senior sibling devoured a fish while the emaciated junior sibling received no food and died shortly afterwards. Similarly, Poole (1982) reported increased sibling aggression, increased nestling loss, and severely curtailed growth rates of junior siblings following a twoday rainstom.
AGGRESSION AND RELATIVE NUTRITIONAL CONDITION The relative nutritional condition of entire broods and of A-chicks alone showed a weak positive correlation with brood aggression rate in feeds following feed and mock feed treatments. These trends are opposite to those expected based on the positive relationship between hunger and aggression. A single brood was responsible for the direction of both correlations. This two-chick brood was by far the most aggressive of the ten experimental broods and was also the most synchronous, in terms of age. The potential influence of brood asynchrony on sibling aggression is discussed below. Excluding this brood from the analysis, all correlations show a nonsignificant negative trend. A similar trend was obtained between brood aggression rate and weight-versus-age residuals in chapter 11. A larger sample of broods is required to adequately address the influence of nutritional condition.
AGGRESSION AND COMPETITVE ASYMMETRIES BETWEEN NESTMATES The effect of hunger in this system was manifested by small but consistent increases in aggression within broods following mock feed treatments. However, the natural variation in aggression rate observed between broods was very large. Other factors which might explain some of the variation between broods are the degree of competitive asymmetry between nestmates (ie. within-brood age asynchrony or weight asymmetry) or the number of within- brood competitors. A trend of decreasing aggression with an increase in brood age asynchrony and brood weight asymmetry was apparent following both feed and mock feed treatments, however only the decline with weight asymmetry was significant. Weight asymmetry seems more appropriate as an indicator of within-brood competition, since nestlings of the same age can vary tremendously in size @ers. obs.) and early dominance appears to be determined on the basis of size and associated motor skill advantages. The sigmficant effect of weight asymmetry on aggression is tenuous because of its sensitivity to a few synchronous broods, which were highly aggressive. However, removing the most aggressive brood from the analysis does not affect the interpretation of the results. (In fact, more of the variation in aggression is explained by weight asymmetry when this brood is excluded). The inverse relationship between aggression rate and brood asymmetry is consistent with studies that have experimentally manipulated asynchrony and subsequently measured aggression levels. Artificially synchronous Cattle Egret broods were significantly more aggressive than asynchronous controls (Fujioka 1985b, Mock and Ploger 1987). A similar increase in sibling aggression was noted in artificially synchronous Jackass Penguin (Spheniscus demersus) broods, although only qualitative aggression observations are available for the latter study (Seddon and Van Heezik 1991). In Ospreys, artificially asynchronous broods were significantly less aggressive than unrnanipulated broods (Forbes 1991). From the perspective of a senior sibling, the degree of within-brood competitive disparity may be an effective cue on which to base its aggression policy. If the competitive ability differs substantially among siblings and senior siblings can maintain unrestricted access to food through dominance alone, then aggression is superfluous. Through clemency, senior siblings can minimize the costs of aggression and can potentially increase their inclusive fitness through the survival of junior nestmates, when food is abundant (Forbes and Ydenberg 1992). If, on the other hand, siblings are competitvely similar, senior siblings should use aggression, both to ensure unrestricted access to food and to eliminate junior siblings, when resources appear insufficient for the entire brood. This study and the studies discussed above support the "sibling rivalry reduction hypothesis" (Hamilton 1964, Hahn 1981). It proposes that asymmetry in competitve ability among siblings reduces aggressive interactions among them, thereby minimizing the energy expended on sibling competition.
AGGRESSION AND BROOD SIZE Several studies investigating sibling aggression in nestling Ospreys have noted that aggression is more common in three-chick broods (Poole 1982, Jamieson et al. 1983, Steidl& 1991). This same trend has also been observed in other facultatively siblicidal species (Mock & Parker 1986, Sullivan 1988). Mock & Lamey (1991) have recently demonstrated that nestling Cattle Egrets use brood size as a proximate cue for aggression (independent of current food amount), with larger broods exhibiting greater aggression. In this study, the effect of hunger on aggression rate tended to be greater in three-chick broods, however the interaction between the treatment and brood size was not significant. There was no significant difference in aggression rates between two and three-chick broods. In fact, aggression in two- chick broods tended to be higher, even when rates were not standardized according to the number of dyads. These results suggest that brood size per se is not an important determinant of sibling aggression in Ospreys.
CONCLUSIONS As in other facultatively siblicidal species, Osprey nestlings occasionally use aggression to establish and maintain access to food. Broods in this study were slightly but significantly more aggressive when hungry, indicating that sibling aggression in nestling Ospreys is influenced at the proximate level by food shortage. Senior siblings secured a greater proportion of food at feeds when they were hungry, however this was not achieved at the expense of their junior nestmates. Instead, females compensated for the hunger of their broods in the short term, by sacrificing a significant share of their own food intake. Ospreys exhibit the pattern of conditional siblicide found in Blue-footed Boobies, showing flexibility in aggression with respect to current food amount. The advantages of such a system are that it is reversible (ie. junior nestlings can regain condition if food availability increases) (Proctor 1975) and that the costs of unnecessary fighting are minimized. A food-mediated siblicidal strategy is reasonable when food varies in a predictable manner from day to day (Mock et al. 1987a). It remains to be established whether within-season variation in food availability is small in this population of Ospreys relative to other facultatively siblicidal systems. The level of variation in aggression found between broods in this study was large. Results presented here suggest that Ospreys are sensitive to the level of within-brood competitive asymmetry and that this explains a considerable part of the variation in aggression between broods. These findings are consistent with studies that have manipulated nestling asynchrony and subsequently monitored sibling aggression rates. l2uwEuY GENERAL CONCLUSIONS
In chapter 11, I examined the natural pattern and timing of sibling aggression and brood reduction within the Kootenay Osprey population. The natural variation in aggression between broods was large and virtually all of the aggression observed during this study occurred when broods were less than 20 days old. When I examined the pattern of aggression and food allocation within bas,I found that senior siblings were more likely to use aggression and they concomitantly received a much greater share of food than their junior nestmates. After about 20 days of age, senior siblings rarely used aggression. From then on, they received equal or smaller (in two-chick broods) shares relative to their junior nestmates. This suggests that through aggression, senior siblings commanded a disproportionate share of the available food to the detriment of other brood members. I attempted to verify this directly, however my data only provide weak support for the link between aggression and food allocation. My observations suggest that aggression may have a protracted influence on the behaviour of junior siblings and this effect may diminish the need for senior siblings to use overt aggression as nestlings age. I suspect that only intensive observations of very young broods will provide unequivocal support for an effect of aggression on food allocation. In over 90 percent of all cases of partial brood loss, mortality involved junior chicks and all known brood reduction occurred when nestlings were under 20 days old. Based on the pattern of food allocation and aggression with nestling age, I conclude that mortality was an outcome of aggression-mediated starvation. Brood reduction occurred at rates of 12 and 31 percent in 1989 and 1990, respectively. Nestlings grew more slowly in 1990, suggesting that reduction occurred in response to food limitation. A generally wetter breeding season in 1990 may have contributed to this result. In chapter 111, I used the results of a controlled feeding experiment to test the food amount hypothesis (Mock et al. 1987a) in nestling Ospreys. The results of my experiment supported both predictions of the FAH. Broods were slightly but significantly more aggressive at feeds when they were hungry than when they were satiated. Also, senior siblings received a greater share of the total available food at feeds when they were hungry than when they were satiated. The additional food taken by hungry senior siblings did not come from the shares of younger nestmates. Instead, females compensated for the hunger of their broods, both by taking smaller shares of food for themselves and by increasing the meal sizes they fed to their broods. As a result of these adjustments, nestlings suffered no feeding disadvantages and only marginal increases in aggression, under four-hour food deprivation. However, if food shortages are more prolonged or frequent, I anticipate that females will be less willing to compromise their own food intake and that sibling aggression will escalate. Subject to greater physical harassment and smaller shares of a diminishing food supply, last-hatched chicks are expected to succumb disproportionately, as found in this study. Results presented here are consistent with the interpretation that sibling aggression in Ospreys is proximately related to incoming food ,amount and mediated by hunger and possibly past nutritional condition. Such a supply-based mechanism for regulation of aggression has several advantages. Senior siblings can track changes in provisioning levels and update their decision of whether or not to commit siblicide accordingly. If feeding conditions improve dramatically, a junior sibling suffering from sublethal abuse may still recover and enhance the indirect component of the senior chick's inclusive fitness (Proctor 1975). I used brood age asynchrony and brood weight asymmetry to examine the effect of competitive asymmetries between nestmates on sibling aggression levels. In experimental broods, aggression tended to decrease with an increase in age asynchrony and weight asymmetry, however only the latter decline was significant. This effect is consistent with studies that have manipulated asynchrony and suggests that senior siblings modify their aggression policies based upon the perceived level of competition within their brood. I did not detect these trends during observations of natural broods. I also found no effect of brood size on brood aggression levels (per sibling dyad) in natural or manipulated broods. Unlike other facultatively siblicidal species (Mock & Lamey 1991), Ospreys do.not appear to predicate their aggressive tendencies on the number of consumers. LITERATURE ClTED
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Aspects of the breeding biology of the Imperial cormorant, Phalocrocorax atriceps, at Marion Island. Le Gerfaut 69: 407-423. Appendix 1. Proportion of bites taken by A$ & C nestlings and females in fcods following feed and mock feed treatments. Proportions are shown in the fom : A I B I C. The proportions taken by females appcar in the second line of each row. Brood Fcedla Mock1 Fced2 Mock2 Feed3 Mock3 Mean Feed Mean Mock
a fvst trial of feed treatment