Using GPS-Tracking to Fill Knowledge Gaps in the Full Annual Cycle of an Elusive Aerial Insectivore in Steep Decline
Aaron Skinner, MS
Advisor: Christopher M. Tonra, PhD
Migratory bird populations can be limited by events in disparate parts of the world. Despite that roughly two-thirds of a migrant’s annual cycle is spent migrating and on the winter grounds, these periods are poorly studied, limiting our ability to design effective conservation strategies. An understanding of basic migratory and winter ecology is critical for a full annual cycle approach to the conservation of rapidly declining species. The Eastern whip-poor-will (hereafter, whip-poor-will) is a rapidly declining (-70% from 1966-2016) nightjar, yet data remains elusive for the species outside of the breeding season where they are highly vocal. We extracted data from 52 archival GPS tags from individuals across the Midwestern U.S. to understand large-scale migratory movements and space use on the wintering grounds. We also used satellite imagery and δ13C (measuring habitat moisture) and δ15N (relative trophic level) stable isotope ratios from winter-grown claws to analyze how land use and habitat moisture impact home range size and relative trophic level.
Whip-poor-wills circumvented the Gulf of Mexico, and populations across a large latitudinal gradient came together in eastern Texas in early October, resulting in increasing spatial overlap throughout migration. Migratory connectivity was low (MC =0.22 ± 0.12), with extensive overlap of core wintering areas in southern Mexico and Guatemala. The overlap of wintering areas by individuals across a large latitudinal span suggests that whip-poor-wills are telescopic migrants, although a single line of weak evidence pointed towards a leapfrog migration pattern. We examined predictors of home range size at three spatial scales (broader geographic region, site, and home range), and found that forest fragmentation in the site and the presence of agriculture in the home range were positively related to home range size. These results suggest that both landscape configuration and composition variables within the site and the home range are important in determining space/habitat needs and may be more important than the broad- scale geographic variables we measured. We found no important predictors of δ13C or δ15N stable isotope ratios when examining site-level configuration and composition metrics, along with body size, while controlling for geographic location and ecoregion.
These results imply unique conservation challenges and opportunities for this species. The fragmentation of forest (particularly by agriculture) may force whip-poor- wills to occupy larger home ranges, potentially limiting the carry capacity of winter habitat or reducing body condition as they prepare for spring migration and the breeding season. The high spatiotemporal overlap of geographically dispersed breeding populations during other phases of the annual cycle suggests that non-breeding season conditions regulating populations (both positively and negatively) will affect individuals from across the breeding range. For example, reducing light-pollution during weeks of high spatiotemporal overlap on migration, or conserving forested habitat in the core wintering range would benefit individuals from across the breeding area we studied, whereas the continued conversion of forest to agriculture in this region will negatively impact individuals from across the breeding area we studied. This work fills important knowledge gaps regarding the whip-poor-will annual cycle, and highlights the importance of a full annual cycle approach to conservation of migratory species.