Aerial insectivore birds, those that capture prey while in flight themselves, are the avian guild experiencing the steepest population declines in Canada. Although we lack long-term data on insect abundances, one potential cause of these declines could be a change in prey availability. Furthermore, some nocturnal insectivores, like nightjars, face the additional challenge of only foraging during twilight periods, or when adequate moonlight is available. In this thesis, I take a variety of approaches to test and inform predictions associated with possible drivers of population decline in a threatened nightjar, Eastern Whip-poor-will (Antrostomus vociferous). Whip-poor-will prefer open-canopy, or fragmented, forests that allow moonlight to penetrate the canopy, so I first tested whether an increase in forest area appeared to explain their population decline. In contrast to expectation, whip-poor-will presence was positively associated with forest area at a regional scale, and only a delayed effect of urban area explained disappearances. At a more local scale, however, whip-poor-will abundance was positively related to both presence of open-canopy forests, and insect abundance. At this local scale, insect abundance also influenced daily survival rates of chicks, and productivity was higher when hatching coincided with peaks in insect abundance. Next, I tracked migration using light-logging geolocation tags to identify the wintering range of individuals from across the northern portion of the breeding range. I found evidence of migratory stopover in the southeastern United States and wintering locations in Mexico and Central America, suggesting that both regions are potentially important for this population. Finally, I tested for diet change over the past century using nitrogen isotope ratios of museum specimen tissues. Whip-poor-will isotope profiles were consistent with a gradual shift to feeding on lower trophic-level prey for both breeding season grown feathers and winter grown claws. All of these results are consistent with the hypothesis that whip-poor-will populations are declining due to changes in prey abundance, but I caution that habitat and climatic conditions at locations used throughout the annual cycle could also be contributing to these population declines.
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