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STOPOVER HABITAT USE BY SPRING MIGRANT LANDBIRDS: THE ROLES OF HABITAT STRUCTURE, LEAF DEVELOPMENT, AND FOOD AVAILABILITY
Auk, The, Jul 2007 by Rodewald, Paul G, Brittingham, Margaret C
Patterns of habitat use.-We documented strong year-to-year differences in transient abundance during spring migration. In particular, suburban forests were heavily used by mature-forest-breeding migrants in 1997, whereas forest-agricultural edges were used far more heavily in 1998 and 1999. During fall migration at the same study sites, migrants also exhibited year-to-year variation in their use of stopover habitats (Rodewald and Brittingham 2004). Very few stopover studies have addressed year-to-year variation in habitat use. Our findings also are consistent with Winker et al. (1992), who reported that Swainson's Thrushes in Minnesota exhibited interannual variation in stopover habitat use. However, Bairlein (1983) found that patterns of fall habitat use were consistent among years during a seven-year migration study in southwestern Germany. Year-to-year variation in habitat use by migrating landbirds is interesting, because it may be a response to interannual variation in plant phenology.
We examined leaf development to elucidate potential proximate cues used in habitat selection and to better understand temporal and spatial variation in habitat use. An association between migrant arrival in spring, budburst, and emergence of lepidoptera is often mentioned (e.g., Kendeigh 1979, Graber and Graber 1983) and is expected because larvae of many lepidoptera are most numerous at early stages of leaf development, when leaf quality is higher (Feeny 1970, Futuyma and Gould 1979). In our study, mean canopy leaf-development at sites showed little relationship to habitat-use patterns of migrant landbirds within weekly intervals, a pattern similar to that reported for Ohio forests (Rodewald and Matthews 2005). Leaf development still may serve as a cue in habitat selection at smaller spatial scales (e.g., observations of warblers indicated higher use of trees at early stages of leaf development than nearby trees with undeveloped or more mature leaves; P. G. Rodewald unpubl. data). In addition, annual differences in the progression of leaf development were consistent with year-to-year variation in the use of stopover habitats. In the cold spring of 1997, canopy leaves emerged late in all habitats and seemed to strongly concentrate early-migrant landbirds into suburban forests where migrants foraged heavily in understory vegetation, the only area across habitats where woody plants had experienced budburst (P. G. Rodewald and M. C. Brittingham unpubl. data). By contrast, warmer weather in spring 1998 and 1999 caused leaf development to begin about one week earlier than in 1997, resulting in fewer differences among habitats in migrant abundance, leaf development, and possibly arthropod abundance.
Landscape features (e.g., habitat availability) may have contributed to some of the differences we observed in transients' use of forest habitats. Although we did not measure habitat availability, if this resulted in high transient use of suburban forests, then this pattern should have been annually detected. We found, however, that habitat-use patterns of spring transients differed strongly by year, similar to our findings from fall migration at the same sites (Rodewald and Brittingham 2004). Because annual differences in transient abundance among habitats were consistent with year-to-year variation in leaf emergence, it seems that habitat selection by transients was active rather than passive. Further, with the exception of suburban forests, sites were uniformly distributed within the overall study area, such that annual differences in landbird migration routes (if such differences existed) should not have caused annual differences in patterns of habitat use.
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