Age and Sex Differences in Eastern Whip-poor-will Ecology (Antrostomus vociferus)

Abstract

Intraspecific variation is increasingly recognized as a key driver of ecological processes, and characterizing variation among demographic groups is critical for understanding species biology and the contributions of individuals to shaping ecosystem-level patterns. Age and sex differences in species biology are prevalent, but female and juvenile groups are underrepresented compared to adult males in ecological studies. Intraspecific variation is especially important to consider in species of conservation concern where the drivers of population decline, or the effectiveness of management interventions, may differ among demographic groups. The Eastern Whip-poor-will (Antrostomus vociferus; hereafter, ‘whip-poor-will’) is a crepuscular aerial insectivore that has experienced acute population declines in Canada. Hypothesized factors driving such declines include habitat loss and insect prey population declines. Whip poor wills are exceptionally well camouflaged and elusive, making them a particularly challenging species to study and contributing to their historical underrepresentation in ecological research. Our objective was to characterize whip-poor-will diet and migration ecology across understudied age and sex classes. We studied a breeding population of whip-poor-wills at a restored habitat in Norfolk County, Ontario. We used radio transmitters paired with the Motus Wildlife Tracking System to explore whip-poor-will migration routes, timing, rate, and barrier navigation in autumn and spring migration. Most birds followed a similar southwesterly route from the breeding grounds to Mexico and Central America, circumnavigating the Gulf of Mexico. We found consistent evidence of differences between adult and juvenile migration but few sex differences. Juvenile birds departed for autumn migration earlier, took more nights than females, and moved at a slower rate than males to reach the Gulf of Mexico coast, ultimately arriving to the coast at the same time as adults. We also found adults were more likely than juveniles to cross an immediate ecological barrier following autumn departure. We suggest age-related differences in autumn migration are in part due to increased migratory efficiency by experienced adults. Though low in sample size, spring migration results trended towards the expected pattern of male-first arrival to the breeding grounds. We provide previously undocumented demographic variation in migrating whip-poor-wills and emphasize high connectivity at the Texas-Mexico Gulf of Mexico coast, highlighting it as a location for habitat protection. On the breeding grounds, we sampled insect assemblages and whip-poor-will diet through fecal DNA metabarcoding. Our objectives were to determine the extent of whip-poor-will prey selection and differences in diet diversity across age and sex groups. We found whip-poor-will diets primarily consisted of moths (Lepidoptera), then beetles (Coleoptera) and flies (Diptera). We found evidence of size selection of moth prey representing an intermediate size range compared to the insects available on the landscape, consistent with optimal foraging theory. Diet diversity also varied between juveniles and adult females from species to family level. We suggest a lack of scarab beetle presence in juvenile diet may be driving group-level differences. We also highlight the importance of insectivore diet studies to better support growing populations in the face of global insect declines. Together, our results provide insights into the biology of vulnerable and understudied demographic groups of an elusive nightjar. We make suggestions for future research and management practices and connect our findings to broader ecological patterns across migratory aerial insectivores.