Lipid content of Nearctic-Neotropical migratory passerines killed during stopovers in a New York City park

摘要

Urban parks often represent the only stopover habitats available to migrating birds encountering expansive metropolitan areas. Green spaces remaining within cities may therefore be valuable to migrants; yet studies of migrants in this context are few. I examined the lipid content of birds killed by window collisions in spring and autumn in a small recreational park in New York City to assess the energetic condition of migratory passerines utilizing an urban habitat as a stopover site. I compared chemically determined fat content (expressed as a lipid index: g fat / g lean dry mass) and visible subcutaneous fat scores between seasons, autumn age classes, and birds grouped by family and foraging guild. Average total body fat (as % of dry mass) was 29.4% in spring and 24.1% in autumn; few lean birds were found in either season. Birds in spring were significantly fatter than in autumn. In spring and autumn, no differences in fat content (i.e., fat scores and lipid indices) were observed between warblers and thrushes. In spring, there were no differences in fat content between warbler foraging guilds, whereas in autumn, ground/understory-foraging warbler species were fatter than warbler species associated with arboreal foraging. In autumn, the fat content of immature birds was comparable to that of adults. It could not be determined whether the high fat content of birds found here was acquired during stopovers in the study site or if birds arrived with substantial fat stores remaining from previous stopovers. The likelihood of each scenario and the value of urban parks to migratory birds are discussed.