Density Dependent Refueling of Migratory Songbirds During Stopover Within an Urbanizing Coastal Landscape

Emily B Cohen,Frank R Moore,Jill M Lafleur

doi:10.3389/fevo.2022.837790

Abstract

Refueling performance is the primary currency of a successful migration as birds must maintain energy stores to achieve an optimal travel schedule. Migrating birds can anticipate heightened energy demand, not to mention increased uncertainty that energy demands will be satisfied, especially within an urbanizing landscape following long-distance flights. We tested the expectation that refueling performance of songbirds is reduced as densities increase at stopover sites in an urbanizing coastline of the Gulf of Mexico. We measured the density of migrating birds, their refueling performance, and arthropod abundance in two large tracts of contiguous forest paired with two small isolated patches embedded within residential settings throughout spring migration over the course of 2 years. Refueling performance declined with increasing migrant densities, even though the overall daily densities of birds stopping in these landscapes were relatively low and arthropod densities were low throughout. Habitat patch size alone did not account for differences in refueling performance, but smaller habitat patches more often concentrated migrants in higher densities where they experienced reduced refueling performance. We found support for density-dependent refueling performance during spring migration through a region where overall passage and stopover densities are low; suggesting that larger contiguous forest tracks within urban landscapes provide higher quality habitat for refueling and that effect is likely even more pronounced in landscapes within higher density migratory corridors. The nutritional challenges encountered during migration influence the overall pace of migration and changes in access to food resources due to increasing urbanization may ultimately impact optimal travel schedules.

Highlights

How migratory birds are responding to urbanizing landscapes during passage has received surprisingly little attention, despite the fact that long-distance, intercontinental migratory birds are unlikely to be able to avoid urbanizing landscapes during passage (Rodewald and Matthews, 2005; Matthews and Rodewald, 2010; Seewagen et al, 2010, 2011)
Migrating birds stopped in higher densities at the smaller sites as compared to the larger (0.54 ± 0.06 birds ha−1 in small and 0.35 ± 0.04 birds ha−1 in large, χ2 = 512.95, p < 0.001, n = 651) (Figure 2) and this difference was reflected in netting capture rates that were higher at the small sites (Supplementary Table 2)
Migrants occurred in marginally higher densities on afternoon surveys, as compared to the morning

Summary

Introduction

How migratory birds are responding to urbanizing landscapes during passage has received surprisingly little attention, despite the fact that long-distance, intercontinental migratory birds are unlikely to be able to avoid urbanizing landscapes during passage (Rodewald and Matthews, 2005; Matthews and Rodewald, 2010; Seewagen et al, 2010, 2011). Suitable stopover habitat in urban areas may be highly fragmented into small patches (McKinney, 2002) wherein migrants may experience elevated competitor density with negative consequences for meeting energy demands (Moore and Yong, 1991). These conditions are magnified when habitat is within an urbanizing landscape along the edge of an ecological barrier or imbedded within inhospitable terrain (Shochat et al, 2002; Cohen et al, 2017). Large forested patches may provide more suitable habitat than smaller isolated patches for most long-distance migrants, en route migrant-habitat associations within fragmented, urbanizing landscapes are poorly understood, during initial stopover when birds are energetically constrained and opportunity for habitat selection is limited

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