Plasma Metabolites and Mass Changes of Migratory Landbirds Indicate Adequate Stopover Refueling in a Heavily Urbanized Landscape

Large concentrations of migrating landbirds in cities have been well documented, but the refueling conditions urban stopover sites provide are almost entirely unknown. We compared plasma triglyceride (indicator of mass gain) and B-OH-butyrate (indicator of mass loss) concentrations in landbirds in three New York City forests to those of conspecifics in two less disturbed, non-urban forests outside the city to evaluate the quality of urban stopover habitats. We quantified diurnal mass gains with regressions of body mass and capture time and measured arthropod biomass in leaf litter to assess food abundance for ground-foraging insectivores. Metabolite concentrations in Ovenbirds (Seiurus aurocapilla) at urban and non-urban sites did not differ during spring or autumn. In autumn, triglyceride levels of Swainson's Thrushes (Catharus ustulatus) indicated significantly higher refueling rates at the urban sites. In the Yellow-rumped Warbler (Dendroica coronata), butyrate was lowest out-side the city, suggesting better refueling conditions there, but differences in triglyceride did not suggest a consistent difference between the habitats in refueling rates. Autumn triglyceride and butyrate levels of three additional species did not indicate different rates of refueling within and outside the city. In the city, significant mass-gain rates ranged from 1.0 to 2.5% of total body mass hr-1. At no point during either season was there a consistent difference between habitat types in arthropod biomass. Our results suggest that although the availability of stopover habitats may be low in cities, migrating birds using these sites may refuel at rates comparable to those stopping in less disturbed areas.