Daily Mass Gains among Woodland Migrants at an Inland Stopover Site

Abstract

We investigated fat deposition in transient, nocturnal, long-distance migrants at a wooded stopover site that is not near an ecological barrier (e.g. desert, large water body). The changes in body mass of recaptured birds have traditionally been used as a measure of mass gains at stopover sites. This technique ignores the majority of transients, however, possibly hindering the ability to answer species-level questions regarding stopover mass gain. We compare an analysis of recaptures with a technique that considers all captures and their condition by time of day. Eleven woodland-associated migrant species were analyzed, as well as a resident species (Black-capped Chickadee, Parus atricapillus) for comparison. Based on recapture data alone, our study site appears to be primarily a location of mass loss, rather than one of fat deposition. Conversely, the examination of condition by time of day suggests that individuals of several species have net daily gains greatly exceeding those of recaptured individuals. During autumn, however, several species exhibited net daily losses. Although some of these losses may be related to molt, it seems unlikely that molt is the only contributing factor. Differences among species in mass gains at our site suggest that various fat-deposition patterns (and, thus, migration strategies) may occur among migrants that are not approaching ecological barriers.

Stopover Ecology of Autumn Landbird Migrants in the Boise Foothills of Southwestern Idaho

“…A number of studies have used changes in energetic condition among recaptured individuals to examine the suitability of a site, but recaptured individuals may not be representative of the migrant population as a whole (Winker et al 1992). In particular, birds with less fat are more likely to remain at a stopover site than fatter birds (Moore and Kerlinger 1987, Loria and Moore 1990, Morris 1996, potentially biasing recapture data toward leaner birds.…”

“…Because mass change of recaptured birds may not be representative of the entire migrant population (Winker et al 1992) and adequate sample sizes were available for relatively few species, we also used a regression of energetic condition on time since sunrise for all birds captured to include more species in the analysis. We generated equations for rates of diurnal change in energetic condition from a regression of mass on wing chord for all species with at least 40 individuals captured, excluding recaptures (Winker 1995).…”

“…Due to the similarity of the results, we only present data from the regressions of residuals on time since sunrise. We compared mass gain estimates from recapture data and regression models to determine if the two methods were consistent in assessing whether migrants were gaining or losing mass (Winker et al 1992).…”

A phantom road experiment reveals traffic noise is an invisible source of habitat degradation

“…BCI is a size-adjusted metric of body mass calculated as mass (g)/natural wing chord (mm). Small changes in BCI represent large differences in condition (21). During migration, high body condition signifies birds with the energy stores needed for long migratory flights (15).…”

Modeling the Geography of Migratory Pathways and Stopover Habitats for Neotropical Migratory Birds

“…Spatial analysis of stopovers and pathways can provide a geographic framework for research in stopover ecology: the study of stopover habitat composition, distribution, and relative importance in life-histories of birds (Moore et al 1995). Most stopover ecology research has focused on local habitat use (e.g., Hall 1981, Moore and Kerlinger 1987, Winker et al 1992, Rappole 1995, Yong et al 1998, Kelly et al 1999, with practically no research on broad-scale habitat distribution. We examine this geography using simple models of migratory flights to evaluate the basic regional patterns found today and to encourage others to pursue similar broad-scale migratory research.…”