Flying down the food web: Isotope signatures in museum specimens suggest long-term change in diet of a nocturnal aerial insectivore
Aerial insectivores, dietary specialists that rely on flying insects, are exhibiting some of the steepest population declines of any group of birds in North America. One hypothesis for the decline is a change in food availability; however long-term data on insect abundances and avian diet are generally lacking for both breeding and wintering grounds. The eastern whip-poor-will (Antrostomus vociferous), is a rarely studied, nocturnal aerial insectivore that eats primarily lepidopterans (moths) and coleopterans (beetles). We look for evidence of temporal change in the diet of Ontario’s eastern whip-poor-wills using museums specimens collected between 1880 and 2000, and samples from breeding individuals at 3 distinct sites in 2012. We use nitrogen isotope ratios (δ15N), which are known to increase with trophic level and diet quality, of tissues grown on the wintering grounds (claw) and breeding grounds (feathers) to assess changes in the winter and breeding season diet of whip-poor-wills. We compare temporal changes in δ15N of whip-poor-will with δ15N of 3 potential prey insect species (Biston betularia, Phyllophaga anxia, Colymbetes sculptilis) collected from the same region and time period.
We found significant declines in δ15N in both winter and summer tissues of adults and in nestlings over the past 100 years. Nitrogen isotopes of both winter-grown claws and summer-molted feathers did not differ between sexes or breeding sites. Nestlings have lower feather δ15N levels than adults, which is consistent with both lower fractionation due to high growth rates and a lower proportion of high trophic level coleopterans in the nestling diet. None of the insect prey species sampled show any significant temporal trend in δ15N suggesting that the pattern found in bird tissues is not the result of broad-scale changes in nitrogen fertilizer inputs. These results are consistent with the hypothesis that aerial insectivore populations are declining due to changes in abundance of high quality, higher trophic level prey.