S41.1: Movement of songbirds in fragmented forests: Can we 'scale up' from behaviour to explain occupancy patterns in the landscape?

1 Centre de Recherche en biologie forestière, Faculté de foresterie et de géomatique, Université Laval, Sainte-Foy, Québec, G1K 7P4, Canada, fax 418 656 3551, e-mail Andre.Desrochers@sbf.ulaval.ca; ²Department of Biological Sciences, University of Alberta, Edmonton, Alberta, T6G 2E9, Canada, e-mail Sue.Hannon@ualberta.ca

INTRODUCTION

The importance of explicitly recognising scale in ecological studies has been emphasised numerous times (e.g. Addicott et al. 1987; Wiens 1989; Kotliar & Wiens 1990). Ecological systems are organised hierarchically and processes acting at one scale may or may not explain phenomena at other scales (Allen & Starr 1982; Wiens 1989). Behavioural and landscape ecology operate on different spatial scales and thus historically scientists in these disciplines have operated independently. Recently, Lima & Zollner (1996) and others emphasised the importance of behavioural phenomena (e.g. movement and habitat selection) in explaining population dynamics and patterns of occupancy of species across patchy landscapes and called for the development of a 'behavioural ecology of ecological landscapes'. The potential link between behaviour and landscape processes is but one illustration of the importance of behavioural ecology in solving conservation problems, as highlighted in a recent book (Clemmons & Buchholz 1997).

One of the conservation problems that affects birds most pervasively is forest fragmentation (e.g. Wilcove et al. 1986; Saunders et al. 1991; Andrén 1994; Askins 1995, but see Andrén 1994; Fahrig 1997; Bender et al. 1998). Although birds are more vagile than other vertebrate taxa, several studies have shown that landscape-level fragmentation of forests may constrain the ability of birds to explore and colonise forest patches (e.g. Villard et al. 1995). Studying the effects of forest fragmentation on forest songbirds thus provides an excellent opportunity for linking behavioural and landscape perspectives. However, very few studies have addressed the relationship between forest patch occupancy and concomitant dispersal and exploration behaviours, even though these may be important for the acquisition of suitable territories (Brewer & Harrison 1975; Morton 1992; Matthysen & Currie 1996; Morton 1997), nesting success (Badyaev et al. 1996) and survival outside the breeding season.

Studies of the impacts of forest fragmentation on birds have typically been conducted at the patch and landscape scales and have focused on descriptions of patterns of occupancy and/or breeding success in patches of varying sizes and isolation. More recently, investigators have focused on movements of marked individuals across fragmented landscapes, especially during dispersal or the post-fledging period, in order to explain occupancy patterns and investigate metapopulation dynamics. The major weakness of these studies is that the pathway or trajectory of movement has not been measured, only the start and end points. Thus, relative 'permeability' of landscape elements to movement can only be inferred. Patch occupancy patterns by some species may be determined by the cost of exploratory movements (discussed by Bernstein et al. 1991; Plissner & Gowaty 1996). For other species, however, exploration of landscapes, fragmented or not, may be less costly and they may use the degree of forest fragmentation as an indicator of habitat quality (i.e. level of nest predation (e.g. Wilcove 1985; Robinson et al. 1995) or food availability (Burke & Nol 1998)).

In order for behavioural ecologists to make a contribution to explaining effects of forest fragmentation, we must first understand the responses of species at smaller spatial scales (i.e. their perceptual range, propensity to cross gaps or to use corridors, relative movement rates across different habitat types, responses to barriers and so on; see Wiens 1992 for a conceptual model). Second we must determine whether such responses help predict patterns at the landscape scale. If so, it should be possible to predict the cumulative effect of a large number of short-term, small-scale movement decisions on bird occurrence in forest fragments. Use of detailed conceptual models (e.g. Wiens 1992) and individual-based, spatially explicit models (e.g. Schippers et al. 1996; Tyler & Hargrove 1997) will aid us choosing which behaviours to measure and allow us to test predictions empirically in the field.

In this paper we review empirical studies that have looked at dispersal as a species-specific or an individual-based process, but with varying degrees of precision. (For a review of theoretical models, see Lima and Zollner (1996)). Based on this diversity of approaches, we assess ways to better predict the patterns of the occurrence of birds in fragmented landscapes. We have integrated this disparate literature by grouping the studies into three methodological categories and by evaluating the strengths and weaknesses of each. The first category of studies infer dispersal limitations from species occupancy of forest patches. The second category of studies measures endpoints or points along the paths of marked individuals' movements between forested patches. The third category of studies describe trajectories of individuals across landscapes with various degrees of forest fragmentation.

Our focus is on the third category which we illustrate with some of our own studies. Finally, we highlight the unexplored potential of behavioural data as a tool to predict patterns of occupancy on the landscape.

Inferring constraints on dispersal from patch occupancy

Because birds are relatively vagile organisms, they may perceive a fragmented landscape as more connected than species with poorer dispersal abilities. In particular, the large-scale seasonal movements of migratory birds, invite one to question whether gaps between patches on the order of hundreds of metres would deter bird movement across landscapes. Migration and dispersal, however, are very different processes with different motivations and costs/benefits to movement (Villard et al. 1995). Perceptions of landscape connectivity to a bird likely differ seasonally, due partly to its reproductive state. One way of inferring a lack of landscape connectivity (thwarted dispersal) to birds is to describe their pattern of occupancy in patches which vary in their isolation and connectedness across fragmented landscapes.

Occupancy of isolated patches

There has been a plethora of studies examining impacts of forest fragmentation on bird communities. Here, we discuss those studies which have explicitly examined the effect of patch isolation on the probability of colonisation or occupancy of the patch and have limited our discussion to studies done in forest. Van Dorp & Opdam (1987) and Opdam (1991) point out the difficulties in comparing studies that have used different measures of isolation, done in different regions which vary in the type of landscape and matrix, the size and isolation range of forest patches and the regional abundance of the species. With this in mind, Opdam (1991) reviewed these studies and noted that in most cases, area of forest patch explained a higher amount of the variation in bird occupancy patterns than did isolation. In general, results within a region tended to be species-specific and studies which combined all species together may have missed significant differences in species responses (e.g. compare Helliwell 1976 with McCollin 1993). Opdam et al. (1984 and 1985) found that variables measuring isolation significantly improved species-area regressions for several mature forest species. In a more extensive study, van Dorp & Opdam (1987) found that forest patch occupancy of six of 32 species was negatively affected by increasing interpatch distance and positively affected by amount of forest in the vicinity of the forest patch. More detailed studies on the sedentary European Nuthatch Sitta europaea and the Spotted Woodpecker Dendrocopos medius found that patch occupancy or colonisation rates were correlated with distance to other occupied patches or large patches of suitable forest (Petterson 1985; Verboom et al. 1991). Several studies focusing primarily on passerines in the eastern United States reported that 'interior' forest species, 'area-sensitive' and/or neotropical migrants were less abundant or were absent in isolated forest patches (e.g. Whitcomb et al. 1981; Lynch & Whigham 1984; Askins & Philbrick 1987; Askins et al. 1987; Robbins et al. 1989). In studies involving non-passerines, Spruce Grouse Dendragapus canadensis were less likely to occupy forest patches that were smaller and further away from occupied patches (Fritz 1979) and Red-cockaded Woodpeckers Picoides borealis were less likely to colonise isolated clusters of suitable nest trees (Thomlinson 1995).

A major difficulty with interpreting these inferential studies is separating the effects of isolation and correlated factors such as patch size and regional forest cover which in turn influence regional abundance of a species (e.g. van Dorp & Opdam 1987; Bellamy et al. 1996; Table 1). Isolated patches are often in areas with lower forest cover and these patches may have increased edge effects that could reduce local forest suitability (Dunning et al. 1995). Andrén (1994) and Fahrig (1997) found that regional forest cover may be more important than fragmentation per se in explaining species distribution across landscapes. Villard et al. (1995) attempted to separate the effects of area, isolation, vegetation structure of the fragments, breeding philopatry and a combination of these factors on the incidence of four species (Wood Thrush Hylocichla mustelina, Black-and-white Warbler Mniotilta varia, Ovenbird Seiurus aurocapilla and Scarlet Tanager Piranga olivacea) in forest patches in an agricultural area in Ontario. Isolation alone appeared to be an important predictor of extinction and recolonisation of a patch only for the Black-and-white Warbler, but including isolation in combination models improved the fit for three of the four species.

Another inferential approach has been to compare occupancy rates of fragments that are joined by corridors to those that are completely isolated. Beier & Noss (unpublished manuscript) have thoroughly reviewed studies of this type and note that the interpretation of these studies is unclear due to the confounding effects of corridor presence and overall forest cover, patch size, distance to source populations, distance away from urban centres etc. In addition, many of these studies are unreplicated ( e.g. MacClintock et al. 1977; see Nicholls & Margules (1991) for a discussion on experimental designs involving corridors). Two studies have attempted to control for some of these problems. Dunning et al. (1995) chose study sites which differed mainly in the presence or absence of corridors: they found that corridor presence facilitated colonisation of clearcuts by Bachman's Sparrow Aimophila aestivalis. Schmiegelow et al. (1997) experimentally set up isolated and connected reserves that were matched for forest type and found that reserves connected by corridors did not have higher abundance of neotropical migrants and resident songbird species than isolated reserves two years after the forest was fragmented.

Movement through fragmented landscapes by known individuals

The use of radio telemetry to monitor bird movements circumvents some of the problems inherent in the resighting technique above (Table 1). By using telemetry, mortality of birds during dispersal and in different habitat types can be determined and the distance moved and time taken to disperse quantified. For example, Miller et al. (1997) used telemetry to monitor juvenile dispersal in Spotted Owls Strix occidentalis caurina. By taking telemetry fixes daily they discovered that juveniles chose closed canopy forest over open-canopy forest and suffered higher mortality when they used clearcuts and open areas with young saplings. Owls did not choose to move in continuous forest and their dispersal distance did not vary with respect to levels of fragmentation. Nonetheless, clearcuts, agricultural areas and urban centres appeared to be barriers to dispersal. Overall, using telemetry improves the resolution of dispersal studies and still contributes greatly to the field, but the method has drawbacks: it is expensive, labour-intensive, and the detailed trajectory of birds is usually not known (Table 1).

Measuring flow and trajectories of individuals

Given the shortcomings of inferential methods above, how can we improve significantly our understanding of bird dispersal in fragmented landscapes? Greater resolution of bird dispersal in fragmented forests can be provided by direct measures of movements of individuals, such as measuring their flow through particular landscape elements, by mapping their actual trajectories, and by making direct measures of movement decisions of birds encountering contrasting landscape elements. These approaches have clear advantages over the inferential methods because they can pinpoint the exact locations of movement and allow decision rules to be quantified for future use in dispersal models at the landscape scale. This section is necessarily brief, due to the paucity of studies of this nature, but it demonstrates the potential of flow and, particularly, trajectory studies.

Flow studies

Passive trajectory studies

Following the path or trajectory of birds as they move through fragmented landscapes will provide evidence of avoidance or utilisation of various landscape elements. Until recently, detailed measures of trajectories have not been possible. However, recent technological advances should open the way to exciting developments in bird movement analysis. We review two such advances below: differentially-corrected Global Positioning Systems (GPS; e.g. Rempel et al. 1995) and harmonic radar (Mascanzoni & Wallin 1986; Roland et al. 1996). With GPS receivers, we obtained detailed trajectories of birds in the post-fledging period by following them visually and recording their locations. Trajectories were then plotted on a high-resolution computerised map, ready for analysis (Fig 1). High-quality GPS receivers can work very well even under forested cover (D'Eon 1995) and provide locations with a 5 meter accuracy after data correction by specialised software (Hurn 1993). Of course, the ideal situation would be to place GPS receivers on the birds themselves, either coupled with a transmitter (to find individuals and download GPS data remotely using a modem) or with the possibility to recapture the bird and download the GPS data directly. GPS-transmitter packs have only begun to be used for mammals (Edenius 1997; Moen et al. 1997; Rempel & Rodgers 1997); they are still too heavy for songbirds but we should expect lighter circuitry to be available in the coming few years. In the meantime, it is possible to follow easily detectable species within ca. 10 m while holding a GPS receiver to obtain useful trajectory data, if care is taken not to disturb the bird (Fig. 1).

Harmonic radar (Mascanzoni & Wallin 1986; Roland et al. 1996) also holds promise for the detailed study of bird movement. This technology involves tags that absorb a radar signal from a transmitter, which is re-radiated at a different wavelength, which can be picked up by a Yagi antenna (commonly used for radio-tracking). Tags, which consist of wire and a diode with no internal power supply, are small, light (< 5 mg), and inexpensive. Also, conventional radio-tracking receivers can be modified easily and at a small cost to send and receive signals. In their study, Roland et al. (1996) were able to track butterflies and moths at up to 50 m from the receiver, over periods of up to 48 h. The main shortcoming of harmonic radar is its short range (approximately 100 m), which limits the kinds of studies that can rely on this method. No studies of birds have yet taken advantage of harmonic radar, although it has obvious relevance to small-scale movement analysis.

Despite technological and conceptual advances, obtaining detailed movement data in a landscape context will remain labour-intensive. Furthermore, more sophisticated radio-telemetry will not eliminate all its shortcomings. But more importantly, passive monitoring of birds moving over large areas will not control for a variety of ways in which habitat type and movement may be confounded (Table 1). For example, birds found moving along forest edges (e.g. Fig. 1) may be selecting the edge as prime foraging habitat or they may 'bounce' on an impermeable boundary. Also, however detailed, passive trajectory studies will require assumptions about the motivation of movement (e.g. dispersal or foraging). Resolving the effects of habitat elements and motivation on movement behaviour thus requires a much finer-scale understanding of individual decision-making.

To understand motivations behind bird movements, we will first need to characterise movement patterns. For this purpose, many analytical procedures already exist. Simple descriptive statistics such as step length, turning angle, and directionality indices (reviewed by Bell 1991; see also Claussen et al. 1997), have often been used to describe the searching paths of ground foraging birds (e.g. Smith 1974; Zach & Falls 1976). Methods used to compare movement patterns among species or under different environmental conditions, include the modelling of the observed movement patterns as passive diffusion (e.g. Kareiva 1983; Turchin 1991) or correlated random walk (e.g. Kareiva & Shigesada 1983; Bovet & Benhamou 1988; Crist et al. 1992) models whose input parameter values are then compared across treatments. Yet another method used to compare movement patterns, whose results are often difficult to interpret however, is fractal analysis. This method calculates the fractal dimension of movement paths in order to describe and compare movement patterns across spatial scales (e.g. Johnson et al. 1992; Wiens et al. 1995). Fractal analysis is considered to be a way of extrapolating small scale movement patterns to larger scales, and ultimately infer their influence on population processes at the landscape level (Wiens et al. 1995; but see Turchin 1996). These advanced methods have been mostly used with insects and are yet to be used with birds.

Experimental trajectory studies

In both flow and trajectory studies, a variety of ecological, behavioural and social components of bird movement need to be addressed. Although we found no evidence that variation in weather variables influenced the tendency of birds to cross gaps in forest cover (St. Clair et al. unpublished manuscript), severe weather and experimental stimuli themselves may influence the responses of foraging birds to predation risk and it is likely that this effect extends to movement decisions. Similarly, conspecific attraction, measured by the number of individuals detected during trials, had no effect on movement decisions in our study, but clearly influences the behaviour of animals in other contexts (e.g. Beauchamp et al. 1997). Beyond these factors, we need to determine the nature of movements under study: territorial residents may have decision rules very different from those of dispersing birds. Also, needed are better and species-specific measures of perceptual range (i.e. the distance at which an individual can detect a given landscape element; Zollner & Lima 1997), in order to improve models of landscape exploration.

SCALING UP BEHAVIOUR

Will new behavioural tools help us 'scale-up' to explain bird occupancy patterns in the landscape? It is premature to fully answer this question here. In the short term, the success of new empirical work on short-term, small-scale trajectories and associated decision rules by dispersing birds will be measured first by its ability to generate testable predictions about landscape occupancy patterns. From our own work, certain predictions can already be made. For example, data on the reluctance of birds to cross gaps (Fig. 2; Desrochers & Hannon 1997) can be used to predict that the species most reluctant to come into the open will respond more negatively to forest patch isolation than others, at least in similar landscapes. We can provide a first, crude assessment of this prediction with data from two of our studies (Fig. 3). Despite the very small sample, Fig. 3 suggests that species reluctant to come into the open may respond more negatively to experimental isolation of their habitat. We are currently obtaining information both on gap avoidance and response to isolation with more species, so the relationship in Fig. 3 (or lack thereof) remains to be confirmed.

CONCLUSION

ACKNOWLEDGEMENTS

We thank Steve Brisson, Isabelle Chouinard, Jean-Michel Roberge, and Stéphane Roy for assistance with the field work. Thomas C. Grubb, Jr. and Jeff Walters provided useful comments on an early version of this text. Work by the authors was supported by the Natural Sciences and Engineering Research Council of Canada through operating grants to A.D. and S.J.H., and a research grant by the FCAR (Québec) to A.D. M.B. was funded by a N.S.E.R.C. scholarship and a P.Q.S.P.B. grant (Québec). C.C.S. was funded by Killam and C.R.B.F. post-doctoral fellowships.

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Table. 2. Homing success (counts in parentheses) of male birds relocated ca. 2 km away from their territory through fragmented (agricultural) and connected/continuous forests near Québec city, Canada, 1997 and 1998. Species included Black-capped Chickadees Poecile atricapillus, Black-throated blue Warblers Dendroica caerulescens and Ovenbirds Seiurus aurocapillus. Treatments were alternated through time within species.

Table 3. Winter abundance of Black-capped Chickadees in forest fragments isolated from the nearest neighboring fragment by greater than or less than 50 m. Chickadee abundance in each fragment was determined by the average number of birds to respond over three census dates to a constant-volume broadcast of a chickadee mobbing calls (audible for approximately 250 m). Data were collected from aspen- (Populus tremuloides) dominated forest fragments in northern Alberta in January through March, 1997.

Fig. 1. Movement paths of late-summer dispersing songbirds in wooded parts of a golf course near Québec city, Canada, August and September 1997. Shaded and clear areas depict open (lawn) and wooded areas, respectively. Letters 's' and 'e' refer to the start and end of paths, respectively. Path I: Black-throated green Warbler Dendroica virens, length: 1680 m, positions: 156, duration: 21 min. Path II: Red-eyed Vireo Vireo olivaceus, length: 5294 m, positions: 1468, duration: 79 min. Path III: Red-eyed Vireo, length: 2439 m, positions: 564, duration: 37 min. Data collected with a GPS receiver. Positions differentially corrected (accuracy ± 5 m) and map digitized from a corrected 1:5000 aerial photo.

Fig. 3. Possible relationship between response to experimental isolation of forest patch and avoidance of open areas by five species of birds. Isolation responses were calculated as percent abundance change in response to two years of isolation by 200 m of clearcutting divided by percent abundance change in continuous sites (Schimegelow et al. 1997; Table 6). Reluctances to cross open areas were calculated by dividing probability of crossing a 50 m tract of forest by probability of crossing a 50 m open area during playback experiments (Desrochers & Hannon 1997). BCCH= Black-capped Chickadee, REVI= Red-eyed Vireo, RBNU=Red-breasted Nuthatch S. canadensis, YRWA=Yellow-rumped Warbler D. coronata. Kinglets are Regulus satrapa (behaviour data) and R. calendula (isolation data) and were pooled because of insufficient data. The line was calculated by linear regression.