Behavioral Barriers to Non-Migratory Movements of Birds

Home , Kinglet, Sylviorthorhynchus

ANN. ZOOL. FENNICI Vol. 39 • Barriers to non-migratory movements of birds 275 Ann. Zool. Fennici 39: 275–290 ISSN 0003-455X Helsinki ¶¶¶ 2002 © Finnish Zoological and Botanical Publishing Board 2002

Behavioral barriers to non-migratory movements of birds

Rebecca J. Harris & J. Michael Reed*

Department of Biology, Tufts University, Medford, MA 02155, USA (*e-mail: [email protected])

Received 4 January 2002, accepted 10 March 2002

Harris, R. J. & Reed, J. M. 2002: Behavioral barriers to non-migratory movements of birds. — Ann. Zool. Fennici 39: 275–290.

Although effects of physical barriers to animal movement are well established, the behavioral inhibition of individuals moving across habitat gaps, ecotones, and inter- patch (matrix) habitat has received little attention. Birds are often cited as a taxon in which movements should not be disrupted by gaps in landscape connectivity. Here we synthesize evidence from the literature for behavioral inhibition of movements by birds, and ﬁ nd that a wide variety of behavioral inhibitions to movements have been observed. We also present a model for describing edge or gap permeability that incorporates the propensity of an individual to cross an ecotone or enter a gap, and the effect of gap width. From published observations, we propose ﬁ ve ecologically based patterns of behavioral inhibition of bird movements as hypotheses: that habitat specialists, understory-dwellers, tropical species, solitary species, and non-migratory species are more inhibited than are species that are their ecological counterparts. Understanding what animals perceive as impediments to movement will contribute to efforts to maintain populations through landscape design, and will allow us to predict the types and degrees of habitat fragmentation that will cause persistence problems for various species.

Introduction cal constraints, such as the need to acquire fat reserves before crossing a water body or desert Physical barriers to movements such as dis- (Baker 1978, Akesson 1999). Our interest is in persal and migration are widely recognized for relatively subtle barriers to movements, where birds. For example, movements can be inhibited an animal is physically capable of crossing a by large geographic features, such as mountain particular distance or habitat but does not do so, ranges (Baker 1978, Leshem & Yomtov 1996), indicating a behavioral restriction of movement which can play a role in speciation (Nelson & (e.g., Mayr 1942, Ehrlich 1961). We also focus Rosen 1981, Gascon et al. 2000). Long-distance on movements at the landscape scale, typical of movement also can be restricted by physiologi- dispersal, rather than at the larger spatial scale of 276 Harris & Reed • ANN. ZOOL. FENNICI Vol. 39 migration. In this paper we refer to barriers that The impacts of habitat fragmentation and the restrict movement behaviorally as behavioral potential ameliorating effects of corridors would barriers to movement. be better predicted by understanding the behav- Some bird species migrate vast distances iors inherent in dispersal (Lima & Zollner 1996, over unfamiliar terrain, yet the same species Yahner & Mahan 1997, Sutherland 1998, Haddad can have relatively short dispersal distances 1999, Reed 1999, 2002). The importance of an that are affected by behavioral decisions (Rol- organismʼs dispersal capability, independent of stad 1991, Villard & Merriam 1995, Reed et distance, only recently has been incorporated al. 1999). Evidence of behavioral barriers to into models of subdivided populations (Taylor movement of birds comes from a wide variety et al. 1993, Fahrig & Merriam 1994, Donovan of sources, ranging from species distribution et al. 1995). patterns (Willis 1974, Terborgh 1985, Cap- Our goals here are to quantify gap-crossing parella 1988, Robinson 1999) to observations abilities and types of behavioral barriers to non- of habitat gap crossing (Desrochers & Hannon migratory movements by birds. These move- 1997, St. Clair et al. 1998, Harris & Reed ments are primarily for dispersal, seasonal shifts 2001). These behavioral inhibitions often coin- in home range, and within home range move- cide with landscape features such as ecotones, ments, so they encompass local and landscape- habitat gaps, and matrix habitat types (Stamps level movements. et al. 1987, Hansson 1991, Hansen & di Castri 1992, Rickets 2001, Vandermeer & Carvajal 2001), and can vary among individuals within Barrier permeability a species (Fraser et al. 2001). Few ecologically based patterns across species of barriers to Potential behavioral barriers to dispersal gener- movement have been proposed. It has been sug- ally take the form of a gap in habitat. The barrier gested that Neotropical migrant forest birds are can be crossing the ecotone itself, or it can more sensitive to habitat loss and fragmentation be some feature of the gap habitat, such as its than are temperate resident species, in that they width or the type of habitat that makes up the are less likely to occupy fragmented landscapes gap. It is useful to characterize potential barriers (e.g., Blake 1986, Robbins et al. 1989). It has by their permeability, or the propensity of an been suggested that this sensitivity might be individual of a particular species to cross the due to a greater inhibition of movement across barrier (Stamps et al. 1987). Permeability can habitat unsuitable for breeding (Whitcomb et be key to persistence of populations in frag- al. 1981, Lynch & Whigham 1984, Machtans mented landscapes. Using a simulation model, et al. 1996). However, some forest-breeding, Stamps et al. (1987) found that even slight Neotropical migrants readily cross wide gaps in changes in edge permeability could alter migra- forests (Norris & Stutchbury 2001), and there is tion (i.e., dispersal) rates dramatically, which evidence that Neotropical migrants are less sus- strongly inﬂ uenced metapopulation persistence. ceptible to other potential effects of fragmenta- Barrier permeability has been described quali- tion such as lack of nesting cavities (Imbeau et tatively as ranging from soft, those that are al. 2001). crossed readily, to hard, which dispersing indi- Behavioral barriers to movement become viduals virtually never cross (e.g., Stamps et al. more important to population persistence when 1987). Wiens (1992) offers a model based on habitats are fragmented (Hanski & Gilpin 1991, diffusion equations to describe permeability of Wu et al. 1993, Daily & Ehrlich 1996, Reed a landscape to a dispersing individual, express- 1999). The practical advantage of understanding ing permeability in terms of “thickness” of the the behavioral basis of movement decisions is boundary and the contrast between adjacent that landscapes might be designed to allow spe- patches. cies to move among otherwise isolated patches Perceptions of ecotone and gap permeability, of habitat (Saunders & Hobbs 1991, Hansson and therefore predicted impacts of habitat frag- et al. 1992, Beier & Noss 1998, Reed 2002). mentation, can vary by species (Wiens 1995). ANN. ZOOL. FENNICI Vol. 39 • Barriers to non-migratory movements of birds 277

Even for the same type of ecotone, reduced con- habitat > 400 m across (Norris & Stutchbury trast in vegetation structure and increased under- 2001). It should be noted, however, that the story density in habitat gaps consistently reduce stimulus to cross gaps is different between these forest birdsʼ perceptions of edges as barriers. For studies; birds in Brooker et al.ʼs (1999) study example, DeGraaf (1992) found that edge avoid- were dispersing. Also, several species of win- ance by forest birds was most pronounced where tering residents in temperate woodlots crossed forest stands creating the edges were farthest large gaps, with median maximum distances of apart in age (i.e., greatest for mature forest-clear- up to approximately 550 m (Grubb & Doherty cut edges) (see also Machtans et al. 1996, Siev- 1999). ing et al. 1996, Harris & Reed 2001). The early Ecotone permeability that includes discon- successional habitats used by juvenile wood tinuities or gaps is not readily described by thrushes (Hylocichla mustelina), a forest spe- Wiensʼ (1992) diffusion model of landscape cies, exhibit dense understory and ground cover permeability. As an alternative, we offer a model (Anders et al. 1998, Vega Rivera et al. 1998). for describing ecotone or gap permeability: Clumped regenerating vegetation also appears to facilitate movement of forest birds crossing clearcuts (Machtans et al. 1996). One implica- (1) tion of these observations is that the transient nature of regenerating clearcuts should reduce their long-term impact on population persistence Here, h is a measure of the tendency of an indi- of forest birds compared to other habitat types vidual to attempt to cross a gap of a particular that fragment forests, such as agricultural fi elds width (g). This value can be a characteristic of and roads (Askins 1994). Comparable informa- a species or an individual, and will probably tion is unavailable for non-forest birds. differ depending on stimulus. Consequently if One infl uence on gap permeability not h < 1, then an individual will not always enter included in Wiensʼ (1992) model is gap width, a gap when it is encountered. For example, and whether or not there is a species-specifi c we estimated h for black-throated blue warblers threshold distance beyond which individuals (Dendroica caerulescens) by using song play- will not cross (Table 1). A threshold distance is back (Harris & Reed 2001). We found 33% of one where a small change in distance produces the individuals evaluated would not enter the an abrupt reduction in the probability of move- clearcut from where the playback emanated, so ment (With & Crist 1995). Investigations of h = 0.67. As b increases, an animalʼs tendency gap permeability to birds using song playback to cross gaps, once the gap is entered, increases. often result in a graded response with distance, Specifi cally, b is the gap size for which an as with habitat cover, with some maximum dis- animal has a 50% probability of moving if g = b tance individuals will not travel (e.g., Rail et al. and h = 1. Finally, n determines the steepness 1997). Desrochers & Hannon (1997), Rail et al. of the transition from high to low probability of (1997), and St. Clair et al. (1998) provide spe- crossing as gap width increases. As n approaches cies-specifi c data on the probability of crossing infi nity, the probability of crossing becomes a gaps of different widths. All speciesʼ probabili- step function with a threshold at the value g = ties declined with distance; although a thresh- b. This equation can describe a variety of gap- old was not generally apparent, most species crossing probability functions (Fig. 1), and it can exhibited a maximum distance beyond which be combined with other rules. For example, the they would not cross. Brooker et al. (1999) also above equation might describe the probability provide data on maximum gap distances some of crossing a gap up to a certain width, beyond species were recorded crossing. A forest species which an animal will not cross (Fig. 1B). The that appeared to be particularly uninhibited in model also can incorporate the underlying proc- crossing open habitat was the hooded warbler esses that might affect both h and b, such as (Wilsonia citrina), where males searching for season and life history characteristics. extra-pair copulations can cross a gap of open To demonstrate the application of this 278 Harris & Reed • ANN. ZOOL. FENNICI Vol. 39 — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — (1998), (1998) (1999) (1999) (1996) (1996) — — — — — — — — — — et al. et al. et al. et al. et al. et al. — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — mobbing call playback — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — —

— — — — — — — — — — — — — — — — — — — — — — — — — — elds dispersal indirect: banding Brooker elds dispersal banding Brooker indirect: elds dispersal indirect:

— — ﬁ ﬁ eld breeding direct observation: Sieving observation: eld breeding direct eld breeding direct observation: Sieving observation: eld breeding direct elds elds late summer–fall direct observation: summer–fall Desrochers & Hannon (1997) direct observation: Desrochers & Hannon (1997) elds elds fall–winter fall–winter direct observation direct observation Grubb & Doherty (1999) Grubb & Doherty (1999) elds fall–winter direct observation Grubb & Doherty (1999) elds fall–winter direct observation Grubb & Doherty (1999) elds fall–winter direct observation Grubb & Doherty (1999) elds fall–winter direct observation Grubb & Doherty (1999) elds fall–winter direct observation Grubb & Doherty (1999) elds fall–winter direct observation Grubb & Doherty (1999) — —

fi fi fi fi fi fi fi fi fi fi fi fi — — — — — — — — eld/ eld/

— — ﬁ ﬁ — — — — — — — — — — — — clearcuts, agricultural clearcuts; forest winter direct observation: St. Clair clearcuts; forest winter clearcuts, direct observation: St. Clair detour not available mobbing call playback — — — — — — — — — — — — — — — — — — — —

a b — — 20 open a a — —

20 open ≥ a — — / ≥ — — c (m) movement (m) — —

120 < 50

— — distance reason for — — — — — — — — — — — — — — ) — — — — ) with cover song playback

— — — — — — — — — — — — ) ) — — ) — — ) ) — — ) with cover song playback ) detour available mobbing call playback Bélisle & Desrochers (in press) ) — — ) ) ) — — ) mobbing call playback — — — — — — — — — — — — — — icker 600 clearcuts, icker 600

— — ﬂ — — Possible threshold distances to movement across gaps, reported in or estimated from published papers. anked tapaculo 0/ — —

— — ﬂ — — — — — — — — — — Pomatostomus superciliosus Eugralla paradoxa Colaptes auratus Picoides villosus Poecile carolinensis Sitta carolinensis ufted titmouse 115 clearcuts, able 1. — — Sitta canadensis Malurus pulcherrimus Scytalopus m. magellanicus Poecile atricapillus Cyanocitta cristata Melanerpes carolinus Picoides pubescens Baeolophus bicolor North temperate residents Black-capped chickadee 50 Species — Threshold Gap type Season or Method Reference

( White-breasted nuthatch 150 Red-breasted nuthatch 50 clearcuts, White-browed babbler 270 agricultural ( ( Magellanic tapaculo 0 ( Ochre- ( T —

( South temperate residents Blue-breasted fairy wren < 60 ( Northern ( ( Blue jay 175 clearcuts, ( Hairy woodpecker 400 clearcuts, Red-bellied woodpecker 200 clearcuts, ( Downy woodpecker 160 clearcuts, ( T ( ( Caroline chickadee 90 clearcuts,

ANN. ZOOL. FENNICI Vol. 39 • Barriers to non-migratory movements of birds 279 — — — — — — — — — — — 20 m or 0. — ≥ — sed by males — — — (1996) (1996) (1996) — — — — (1997) (1997) (1997) (1997) (1997) — et al. et al. et al. — — — — et al. et al. et al. et al. et al. — — — — — — — — — — — — — — — — — — umed that the threshold distance is — — — — — — — — — — — — — mobbing call playback — — — — — — — — — — — — — — — elds to neighboring woodlots was not diminished at distances of up —

ﬁ — — — — ection point or median gap distance crossed. —

— ﬂ alone or with offspring alone copulations — — — — — — — — — — — — — elds late summer: males direct: transmitters Bayne & Hobson (2001) elds summer: males direct: transmitters Norris & Stutchbury (2001) —

eld breeding direct observation: Sieving observation: eld breeding direct eld breeding direct observation: Sieving observation: eld breeding direct eld breeding direct observation: Sieving observation: eld breeding direct elds summer–fall direct observation: Desrochers & Hannon (1997) ﬁ ﬁ —

fi fi fi fi — — — — eld/ eld/ eld/

— fi fi fi — — — — elds, clearcuts summer–fall elds, clearcuts direct observation: summer–fall Desrochers & Hannon (1997) direct observation: Desrochers & Hannon (1997) — dispersal movements were inhibited.

ʼ — trails, dirt roads, ﬁ summer ﬁ trails, dirt roads, direct observation: Rail summer clearcuts, direct observation: Rail roads, powerline gaps late summer: males direct: transmitters Bayne & Hobson (2001) trails, dirt roads, summer direct observation: Rail

agriculture — — — — — — —

—

— a —

d — e — 20 open 20 open 20 open

— ≥ ≥ ≥ — — 30 < 25

100–300 agriculture

— — — — — — — — ) with cover song playback — — — — — — — — ) with cover song playback — ) mobbing call playback ) clearcuts song playback clearcuts ) song — ) clearcuts mobbing call playback — ) clearcuts mobbing call playback ) with cover song playback s resident patch. — ʼ — ) clearcuts mobbing call playback ) clearcuts song playback song clearcuts ) — ) mobbing call playback ) seeking extra-pair seeking ) — — — — — — s thrush 50 wiretail 0/ — ʼ ʼ — — — — — — — alues were reported as threshold distances or extrapolated from data using in alues were reported as distances above which birds — Vireo olivaceus Wilsonia citrina ellow-rumped warbler 35–40 trails, dirt roads, summer direct observation: Rail — Seiurus aurocapillus Regulus satrapa Catharus ustulatus Dendroica virens Dendroica coronata Sylviorthorhynchus desmursii Pteroptochos tarnii Scelorchilius rubecula Zonotrichia albicollis V V Threshold values were not measured; about 30% of birds crossed small roads or powerline gaps, and the minimum gap distance cros Threshold values were not measured; the frequency of movements across open Threshold values were not measured; birds either approached speakers at a maximum distance of 20 m or not, therefore, it is ass Swainson ( Ovenbird ( 40–50 Y ( Long-distance migrants (Neotropical) Red-eyed vireo ( Black-throated green warbler ( ( 25 30 trails, dirt roads, summer direct observation: Rail

Hooded warbler 465 Short-distance migrants (north temperate) ( White-throated sparrow 65–70 ( Des Murs ( Black-throated huet-heut 0/ Golden-crowned kinglet 40 Chucao tapaculo 0/ ( a b c d e 465 m from a male without young was 100–300 m.

( — 280 Harris & Reed • ANN. ZOOL. FENNICI Vol. 39

) h =1,b = 50, n =2 1 1 Inhibited habitat generalist: h = 0.8, b =0 0.9 0.8 0.8 h = 0.7, b = 50, n =2 0.6 0.7 )

g 0.6

( h =1,b =3,n =2 f 0.4 ) C 0.5 ( B 0.4 0.2 0.3 0 0.2 0102030405060708090100 0.1 Gap width (g) * 0 1 0102030405060708090100 Gap width (C) 0.8 h =1,b = 50, n =10

0.6 Fig. 2. Data from Rail et al. (1997) on gap crossing )

g if g < 50, h = 0.7, b = 50, n =2 probabilities, relative to gap width, by golden-crowned ( f 0.4 if g > 50, f(g) = 0 kinglets (Regulus satrapa) (closed diamonds), and our fi t to these data using Eq. 1, and parameter 0.2 values of h = 0.9, b = 34.7, and n = 2 (open diamonds). 0 0102030405060708090100 Gap width (g) well (r2 = 0.94; Fig. 2). Because we only guessed Fig. 1. — A: Gap-crossing probabilities for four differ- at an appropriate value for n, we reran the model ent combinations of values for parameters in Eq. 1. — B: gap-crossing probability as step functions using with n = 1.5, 3, and 4, and found model fi ts of 2 a single set of values in Eq. 1 (upper curve) and as r = –0.7, 0.94, and 0.93, respectively. Conse- multiple rules for gap crossing based on distance quently, n = 2–3 is the best fi t. (lower curve).

Other behavioral barriers to model, we fi t Eq. 1 to gap crossing probabilities movement by golden-crowned kinglets (Regulus satrapa) published by Rail et al. (1997), using data we Until now, we have focused on habitat gaps extracted from their Fig. 2b. From their graph, a and ecotones as potential behavioral barriers 50% probability of crossing a gap corresponds to to movement. Most of the available evidence a gap of ~31.25 m; this value would be b if h =1. comes from forest species whose movement is However, kinglets in this study had a response restricted by forest/non-forest transition zones. rate of 90% to tape playback from 100 m (Rail There are also examples of habitat gaps or et al. 1997), so h = 0.9. Consequently we stand- changes in habitat structure within forest that ardize b using h, so b = 31.25/0.9 = 34.7. From result in a behavioral barrier to movement the plot, the relationship between probability of (Enoksson et al. 1995). The Bachmanʼs sparrow crossing and gap width is shallow, so we let n = (Aimophila aestivalis) is a songbird that special- 2 as a fi rst estimate (see Fig. 1A). From this, Eq. izes in open longleaf pine forest maintained by 1 becomes fi re or other disturbance. For this species, closed canopy forest is a barrier that can inhibit disper- . sal between patches of suitable breeding habitat (Dunning et al. 1995). Some open-habitat specialists such as yellowhammers (Emberiza cit- When we calculate predicted gap crossing prob- rinella) avoid moving near forest edges, just as abilities using the distance values corresponding some forest-interior species do (Hansson 1983). to the data points in Rail et al.ʼs (1997) fi gure, Rivers are barriers to dispersal for some spe- we fi nd that this model fi ts the presented data cies, often forming borders of avian speciesʼ and ANN. ZOOL. FENNICI Vol. 39 • Barriers to non-migratory movements of birds 281 subspeciesʼ distributions, particularly in the trop- Dispersal, home range, and post- ics (Mayr 1942, Ford 1978, Capparella 1988, fl edging movements Gascon et al. 2000). The impeding effects of water boundaries on avian movement may be Details of avian movements during dispersal are more prevalent than is apparent from speciation limited, but we fi nd evidence in the literature events, however, since genotypic changes have for the existence of behavioral barriers to both been identifi ed across water boundaries even natal and breeding dispersal for forest species. where phenotypic differences are not obvious Much of this comes from movements detected (Capparella 1988). Colonization and extinctions in connected (intact) landscapes, with cor- of species on islands also provide evidence that responding lack of movement in unconnected water inhibits movements of temperate (Grubb habitat interpreted as evidence of a barrier. For & Pravosudov 1994) and tropical (Robinson example, riparian buffer strips can enhance 1999) bird species. Avian extinctions on Barro movements of juveniles and adults dispersing Colorado Island, Panama, indicate that water through fragmented boreal forests (Machtans can be a barrier to immigration (Willis 1974, et al. 1996, Desrochers & Hannon 1997). In Karr 1982, Robinson 1999). Another example some studies, forest birds often take woodland of extinctions on islands comes from a human- routes rather than crossing clearcuts, even when made lake in South Africa, where islands lost the path through the forest is two to three times bird species while land-bridge islands main- longer than the direct route (Desrochers & tained all of their original species, even where Hannon 1997, Bélisle & Desrochers in press). bridges consisted of a thin strip of muddy lake The authors hypothesized that predation risk bottom (Dean & Bond 1994). For some species, may be a driving factor in a birdʼs decision to water appears to be a more impermeable bar- take the longer, less exposed route. The preda- rier to movement than is land of similar width. tion risk hypothesis may be particularly relevant Machtans et al. (1996) observed birds fl ying when evaluating birds dispersing with young; across a 100–300 m-wide lake less frequently male ovenbirds (Seiurus aurocapillus) with fl edg- than across a 200 m-wide clearcut, and forest lings were much more reluctant to cross large specialists were never observed fl ying over the (100–300 m) gaps than males dispersing alone lake (see also Hodges & Krementz 1996). (Bayne & Hobson 2001). Two Australian habitat Finally, social interactions can be behavioral specialists, the blue-breasted fairy wren (Malu- barriers to movement, particularly to dispersal rus pulcherrimus) and the white-browed bab- (Brandt 1985). Intraspecifi c competition can bler (Pomatostomus superciliosus), readily used reduce movements of songbirds via territoriality, corridors of native vegetation during dispersal aggression, and social dominance (e.g., Sherry & (Brooker et al. 1999). In studies of fragmented Holmes 1996), and interspecifi c competition can and intact forests at a larger spatial scale, disper- limit movements through corridors and into edge sal patterns are altered by fragmentation. Juve- habitat (Catterall et al. 1991, Bentley & Catterall nile crested tits (Parus cristatus) in fragmented 1997). Even local song dialects have the potential habitat delay dispersal compared to those in to inhibit immigration (Baker & Mewaldt 1978, continuous forest (Lens & Dhondt 1994). In a Baker 1981, Chilton & Ross 1996, Miyasato & study of wood nuthatch (Sitta europaea) disper- Baker 1999). Particularly for colonial-nesters, the sal, adults were less likely to disperse from their absence of conspecifi cs at an otherwise suitable territories in fragmented habitat than were adults breeding site also can be enough to discourage in intact habitat (Matthysen & Currie 1996). The movement to that site, thus acting as a behavioral authors proposed higher search costs for suitable barrier to dispersal (Smith & Peacock 1989, habitat and perceived increased mortality risk Reed & Dobson 1993, Reed et al. 1999). Conspe- among birds that ventured into open areas to cifi c attraction, and even heterospecifi c attraction move between fragments; the latter hypothesis (Mönkkönen et al. 1996, 1999), also may attract is a behavioral barrier to dispersal. Behavioral birds across an otherwise unsuitable habitat gap. constraints on adults crossing open habitat has 282 Harris & Reed • ANN. ZOOL. FENNICI Vol. 39 been reported for other species as well. For tat between patches. Again, this provides infor- example, breeding dispersal of American robins mation on what gap size and matrix type did not (Turdus migratorius) is greater between con- constitute a barrier to movement. nected sites than between unconnected sites, Similar examples of habitat types unsuit- suggesting woody draws act as corridors or step- able for breeding being included in home ranges ping stones (Haas 1995). come from forest-interior birds that sometimes Dispersal can be a product of expanding use snags and remnant trees in open areas as home range movements followed by a shift in perches (e.g., McClanahan & Wolfe 1993, King activity and subsequent home range retraction et al. 1997). Grubb & Doherty (1999) studied (Rolstad 1991). Consequently, we investigated inter-patch movements within the home ranges home range behaviors to determine what evi- of eight temperate-forest species during late dence was available for behavioral barriers. A fall and winter. They found variability among home range is the area within which an animal species in gap-crossing tendencies, with large forages, breeds, and otherwise spends time birds such as blue jays (Cyanocitta cristata) during a given time interval (Baker 1978). Stud- and red-bellied woodpeckers (Melanerpes caro- ies of home range use are not normally capable linus) more likely to cross, and song sparrows of providing evidence of behavioral barriers to (Melospiza melodia) the least likely. They also movement, per se; rather, they provide indirect found that larger gaps were crossed in the fall evidence. Specifi cally, the inclusion of a habitat than in early or late winter, and suggest that fall gap in a home range demonstrates that the gap movements might have been dispersal to winter- size and its matrix habitat do not constitute a ing areas while later movements were not. It is behavioral barrier to movement. For example, important to distinguish between dispersal and individuals sometimes include unusable habitat home range movements because if a species is within their home range, regularly crossing gaps more prone to behavioral inhibition of move- in suitable habitat (Ims 1995, Grubb & Doherty ment during the breeding season than at any 1999). Birds might also move across ecotones other time, landscape structure of breeding habi- and habitat gaps to seek extra-pair copulations tat might limit local population size. (Norris & Stutchbury 2001). These observa- Field studies show that behavioral barri- tions constitute evidence that the ecotones or ers to movement apparently change by season, gaps being crossed do not constitute barriers being more restricted during the breeding season to movement. Conversely, the observation of and less so post-fl edging. Late in the breeding habitat patches in fragmented landscapes not season, non-riparian songbirds use shrub cor- being included in home ranges (e.g., Matthysen ridors to travel to riparian areas, using uncon- & Currie 1996, Schmiegelow et al. 1997) is nected riparian habitat minimally (Dmowski not suffi cient evidence of a barrier to dispersal & Kozakiewcz 1990). During the post-breed- because exclusion could be due to other factors, ing season, forest-interior species, such as such as reduced habitat quality (Recher et al. wood thrushes, readily use and move through 1987, Bierregaard et al. 1992), predation risk early- and mid-successional habitats, which are (Lima & Dill 1990), or energetic ineffi ciency avoided in other seasons (Anders et al. 1998, (Redpath 1995). Habitat borders also can serve Vega Rivera et al. 1998). However, the most as conduits for movement, with increased activ- open habitats, such as fi elds and clearcuts, still ity near edges potentially indicating a boundary act as barriers to movements of these thrushes. effect of those edges (Desrochers and Fortin Several species that breed exclusively in inte- 2000). Possibly the best evidence from home rior forest expand their habitat use during fall range studies of what does and does not con- to include a wide variety of habitats, including stitute a behavioral barrier to movement comes recent clearcuts, presumably due to a concomi- from comparative and experimental studies. Ims tant shift to frugivory. In fact, a fall survey in the (1995) experimentally fragmented a landscape, southeastern U.S. showed the greatest abundance and found home range sizes of capercaillie of Neotropical migrant interior-forest specialists (Tetrao urogallus) expanded to encompass habi- in large (80 m-wide) clearcut gaps (Kilgo et al. ANN. ZOOL. FENNICI Vol. 39 • Barriers to non-migratory movements of birds 283

1999). Species that treat open areas as barriers less, one consequence of behavioral barriers to to movement during the breeding season, par- movement is that fragmenting a landscape can ticularly Neotropical migrants, readily use open have a greater impact on species persistence areas during winter months as well (Hutto 1985, than previously thought. Models of landscape Böhning-Gaese et al. 1993). Bentley & Catter- connectivity (e.g., Metzger & Decamps 1997, all (1997) hypothesized that increased use of With et al. 1997, With & King 1999, reviewed remnant habitat by wintering migrant bushland by Tischendorf & Fahrig 2000) would be birds in Australia indicates a greater fl exibility improved by incorporating behavioral inhibi- in moving across open space during the winter tions to movement to accurately assess potential than during the breeding season. Gap-crossing dispersal. For example, models of habitat loss experiments on resident birds in logged boreal and fragmentation predict thresholds in habitat forests reveal a similar pattern (Desrochers & loss below which persistence declines dramati- Hannon 1997, St. Clair et al. 1998), although cally (e.g., Gardner et al. 1987, Keitt et al. 1997, seasonal differences are not always evident Hill & Caswell 1999; see Fahrig 1998 for model (Bélisle & Desrochers in press). predicting thresholds to be uncommon). Empiri- cal support of critical threshold relationships between habitat cover and patch occupancy Discussion or population density, however, is debated (Andrén 1994, Bender et al. 1998, Mönkkönen Birds are often cited as a taxon in which move- & Reunanen 1999). Incorporating behaviorally ments should not be disrupted by gaps in the based gap permeability might decrease disparity structural connectivity of the landscape (e.g., between theoretical and empirical results. Bennett 1990, With et al. 1997). Based on our Corridors might ameliorate the effects of literature review, however, a wide variety of habitat fragmentation and behavioral barriers behavioral inhibitions to movements by birds to movement (e.g., Noss 1987, Beier & Noss have been observed. Ecotones, gaps in habitat, 1998), but this solution must be tested. In addi- and water commonly act as behavioral barriers tion, corridors are not the only potential way to to movement by birds. We were surprised, how- reduce the impacts of habitat fragmentation. Our ever, to fi nd no evidence of paved roads acting review indicates that for forest species, organ- as barriers to avian movement, beyond the obvi- izing a landscape to reduce contrasts between ous physical risks to crossing (e.g., Mader 1984, adjacent patches and maintain gap distances Massemin et al. 1998). Roads are known to be below species-specifi c crossing thresholds barriers to movement for other taxa (e.g., Oxley would encourage more movement across the et al. 1974, Baur & Baur 1990, Trombulak landscape. In a recent experimental study, & Frissell 2000), but road avoidance by birds Bélisle et al. (2001) displaced black-throated appears to be a function of low quality habitat blue warblers, ovenbirds, and black-capped for breeding rather than inhibition of movement chickadees (Poecile atricapillus), all forest spe- (Reijnen et al. 1997). cies, from territories in landscapes that varied The mechanisms driving observed behavio- in their forest cover and fragmentation. They ral inhibitions to cross landscape features that do found greater return rates correlated with forest not constitute physical barriers are unknown. It cover, but not with mean inter-patch distances, is possible that observed behavioral inhibitions ostensibly demonstrating behavioral constraints to crossing gaps of open habitat are geneti- to movement at the landscape level caused by cally based (cf. Fraser et al. 2001) ancillary to habitat fragmentation. In another study that con- adaptations for other purposes, such as habitat trolled for forest cover, a more complex pattern and foraging specialization (Huhta et al. 1999), was observed. Bélisle and St. Clair (2001) found lack of behavioral fl exibility (cf. Sol & Lefebvre whether or not landscape features such as rivers 2000), and neophobia (Greenberg 1983, 1989, acted as a movement barrier depended on migra- Schaden 1993), or from direct pressures like tory strategy and perhaps on navigational ability. predator avoidance (Lima & Dill 1990). Regard- The importance of habitat cover is supported by 284 Harris & Reed • ANN. ZOOL. FENNICI Vol. 39 a study by Ricketts (2001), where matrix habi- patches. Generally, movement patterns relative tat affected inter-patch movement in four of six to landscape features assessed by indirect meth- taxa of butterfl y, showing another physiologi- ods must be inferred, and these assumptions cally mobile group to exhibit behavioral inhibi- play critical roles in determining connected- tion to movements. ness of a landscape (Metzger & Décamps 1997, It is important to realize that reported gap- With & King 1999). However, given adequate crossing tendencies and patterns (Table 1 and replication, and perhaps experimental landscape above) can be affected by landscape features, manipulation, indirect methods can provide evaluation methods, and the motivational state strong evidence for or against behavioral inhibi- of the individual, as well as by the reason for tions to movement. moving (e.g., foraging, dispersal, seeking extra- Direct methods can be as simple as observ- pair copulations). This has the potential to hide ing unmarked individuals fl ying across ecotones ecologically signifi cant patterns among species. or habitat gaps (e.g., Wegner & Merriam 1979) Movements can be inferred from indirect evi- or as sophisticated as tracking individuals using dence or observed directly (Turchin 1998). The satellite radio transmitters (Brodeur et al. 1996, different methods used for assessing movement Britten et al. 1999). Possibly the most common vary in their capacity to provide details of approach is to sequentially survey populations for what might constitute a barrier to dispersal, individually marked individuals to quantify dis- and may work only seasonally. For example, persal events across different landscape features. song playback during the breeding season would These longitudinal studies provide excellent data target only movements associated with territo- on short-distance movements, but become more rial defense. Because this literature has been diffi cult as distance increases due to the area reviewed recently (Desrochers et al. 1999), we researchers are required to search (Barrowclough will only highlight key issues. 1980, Faaborg et al. 1998). A less intensive Assumptions are often made about move- method to determine if behavioral barriers to ment paths and about responses to experimental movement exist is to use recorded playback of stimuli, and limitations exist for all methods of conspecifi c songs or alarm calls to assess pro- evaluating movement. However, these should pensity to cross ecotones or enter different types not deter research on behavioral inhibitions to of habitat. However, responses are dependent on movement because suitable experimental design the motivation provided by the stimulus, such can allow strong arguments to be made for the as a bird showing a territorial reaction (e.g., presence or absence of inhibitions. Indirect Titus & Haas 1990). Consequently, if a suitable methods for assessing movement across vari- playback is determined, a great deal of relevant ous habitats include censuses, capture-recapture information can be gathered quickly (Desroch- studies, and genetic comparisons. Indirect meth- ers & Hannon 1997, St. Clair et al. 1998). ods often provide a large amount of data per unit Recent research using ground-based radio effort, and can be done at a large spatial scale, telemetry (Bélisle & St. Clair 2001, Norris & but normally cannot reveal the movement path Stutchbury 2001) and return patterns of displaced taken. For example, census data provided the birds (Bélisle et al. 2001) provide excellent basis for early investigations of corridors facili- information on landscape use, although behav- tating movement, where movement through a ioral decisions during movement are unknown. corridor was inferred from species composi- Based on our literature review, we fi nd some tions in connected patches, but there was no evidence of ecologically based patterns in behav- evidence that birds would not have moved in the ioral barriers to movement. We present these pat- absence of corridors (MacClintock et al. 1977). terns as hypotheses because studies are sparse, Schmiegelow et al. (1997) improved on this so the evidence for each hypothesis is only sug- experimental design by comparing species rich- gestive. Consequently, these hypotheses should ness in connected and unconnected areas, and be viewed as heuristic tools to stimulate interest they supported the idea that corridors facilitate in research on behavioral barriers to movement. movement of birds between otherwise isolated Hypothesis 1: Habitat specialists are more ANN. ZOOL. FENNICI Vol. 39 • Barriers to non-migratory movements of birds 285 likely than are habitat generalists to be inhib- ers to movement in the tropics in the form of ited in crossing ecotones or habitat gaps. Stud- competitive interactions, particularly where the ies of forest birds show that habitat specialists centers of multiple speciesʼ distributions overlap are unlikely to cross large gaps between forest (Terborgh 1985). If this hypothesis were sup- patches (Desrochers & Hannon 1997, Rail et ported, it would imply that habitat fragmenta- al. 1997, St. Clair et al. 1998), and when habi- tion has a greater impact on tropical species than tat is fragmented, corridors increase movement on temperate species. Again, specializations are and colonization rates (Machtans et al. 1996, likely play an additional role in this sensitivity Schmiegelow et al. 1997, Brooker et al. 1999). to fragmentation in tropical species; Neotropi- In one study, forest specialists were more reluc- cal forest birds are generally more specialized tant to cross large gaps than were generalist in their foraging techniques and use narrower species such as white-throated sparrows (Zonot- habitats and microhabitats than temperate forest richia albicollis) (Rail et al. 1997). Habitat spe- birds (Willis 1974, Stouffer & Bierregaard 1995, cialists are defi ned as such because they restrict Robinson 1999). their habitat use. Consequently, these species Hypothesis 4: Species that tend to be socially might be relatively sensitive to habitat fragmen- solitary are more inhibited in their movements tation (e.g., Hansson 1991, Ims 1995). than are fl ocking species. According to predic- Hypothesis 2: Forest understory species are tions from predation risk theory, birds should less likely to enter open areas than are canopy be more likely to venture into unfamiliar, usu- species. This hypothesis arises from studies of ally open, areas if they are in groups (Lima & extinction, speciation, and colonization patterns Dill 1990, Van Vuren 1998). For example, in of isolated habitat and examples exist from yellow-eyed juncos (Junco phaeonotus), group temperate and tropical forests (Willis 1979, Karr size increases with the distance to cover during 1982, Capparella 1988, Newmark 1991, Bierre- foraging (Caraco et al. 1980). Greater fl ocking gaard et al. 1992). These observations are sup- tendency and larger home range size of black- ported by playback experiments in which canopy capped chickadees might explain their greater species tended to show less inhibition to moving likelihood of entering narrow corridors than across gaps than did understory species (Deroch- white-breasted nuthatches (Sitta carolinensis) ers & Hannon 1997, St. Clair et al. 1999). and hairy woodpeckers (Picoides villosus) (St. Species used to open, exposed microhabitats Clair et al. 1998). In addition, animals tend to be like treetops might be more prepared to avoid most vulnerable to predators when in unfamiliar the predation threats that exist in open areas environments and when alone (Van Vuren 1998). (Derochers & Hannon 1997). There is con- However, this hypothesis is not supported for all siderable overlap between this hypothesis and comparisons. For forest birds moving across hypothesis 1 because some understory species agricultural fi elds, interspecifi c group size only are specialists on that stratum, while canopy affects distances birds move from forest edges species occupy a wider vertical range in forests during winter (Bélisle & Desrochers in press), (Terborgh & Weske 1969). and conspecifi c group size has no infl uence on Hypothesis 3: Tropical species are more forest bird tendency to enter open areas (St. Clair inhibited in their movements than are temper- et al. 1998, Bélisle & Desrochers in press). ate species. Tropical birds are thought to have Hypothesis 5: Non-migratory species are shorter dispersal and colonization distances than more inhibited behaviorally in their movements their temperate counterparts, although direct than are migratory species. This hypothesis evidence is limited (Terborgh 1975). Smaller has been proposed before (e.g., Udvardy 1981, specifi c and subspecifi c range sizes in the trop- Paradis et al. 1998), although there is little ics and distributions that often end at waterways evidence supporting it (Whitcomb et al. 1981, (Mayr 1942, Capparella 1988, Gascon et al. Lynch & Whigham 1984, Böhning-Gaese et al. 2000) are consistent with this assertion. Because 1998). Migrants might exhibit less restrictive of high diversity and population densities, social foraging specialization, allowing use of differ- factors may also act more frequently as barri- ent habitats during breeding and non-breeding 286 Harris & Reed • ANN. ZOOL. FENNICI Vol. 39 seasons, which might result in less inhibition birds. — Wildl. Soc. Bul. 22: 339–347. of movement among habitat types (Greenberg Baker, M. C. 1981: Effective population size in a songbird, some possible implications. — Heredity 46: 1983). 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