Tracking the migration of a nocturnal aerial insectivore in the Americas

Migratory connectivity and timing for an at-risk Canadian landbird, Eastern Whip-poor-will (Antrostomus vociferus), from two geographically distant breeding areas

Determining the year-round spatial distributions of at-risk avian migratory species is critical for effective conservation. High-precision tracking enables the identification of distant breeding and nonbreeding areas and their connectivity, as well as migratory routes and associated threats. We GPS-tracked two groups of Eastern Whip-poor-wills ( Antrostomus vociferus (A. Wilson, 1812)) that breed near the northern edge of their range, in Manitoba and northwestern Ontario (“west”), and in southern Ontario (“east”), Canada. The western-breeding birds were also ∼5° of latitude farther north than the eastern birds. We aimed to determine the degree of spatiotemporal overlap between the two groups during fall migration and at tropical wintering sites. We found that western-breeding birds departed earlier on migration than eastern-breeding birds, but we did not detect a difference in arrival timing to wintering sites. The two breeding groups retained spatial structure during migration, until all routes converged to circumnavigate the Gulf of Mexico. Western-breeding birds overwintered at sites farther south than eastern-breeding birds, consistent with a leapfrog pattern of migration. Quantifying the strength of migratory connectivity in at-risk species can be a first step toward defining breeding populations and informing customized conservation strategies throughout the annual cycle.

Widely distributed breeding populations of Canada warbler (Cardellina canadensis) converge on migration through Central America

Background

To effectively conserve migratory species, the entire range encompassed by their annual life cycle needs to be considered. Most research on Nearctic-Neotropical migratory birds has focused on the breeding grounds resulting in a general lack of knowledge regarding the wintering and migratory periods. The Canada Warbler (Cardellina canadensis) has declined by 71% from 1970 to 2012, at a rate of 2.9% per year, and is listed as Threatened in Canada. As with most Nearctic-Neotropical migrants, conservation efforts outside the breeding range are limited by a poor understanding of migration routes and the connectivity between specific breeding and wintering populations.

Results

To determine migratory routes of multiple breeding populations of Canada Warblers, we directly-tracked individuals using light-level geolocators deployed at four sites across the breeding range, spanning approximately 43 degrees in longitude (Alberta, Manitoba and Québec, Canada, and New Hampshire, USA). Twenty-five geolocators with usable data were recovered from three sites and were analyzed using FlightR to determine fall migration routes (n = 18) and individual wintering sites (n = 25). Individuals from all breeding populations took a western fall migration route at the Gulf of Mexico; with 77.8% of birds funnelling into a narrow geographic space along the western side of the Gulf of Mexico (97°W-99°W). We found no evidence for population-specific, parallel migration routes. Most individuals (72%) overwintered in Colombia. The remaining individuals overwintered in Venezuela.

Conclusions

Our results demonstrate convergence of migratory routes around a migration barrier for individuals originating from widely distributed breeding areas. Further, we suggest the potential importance of habitat around the Gulf of Mexico during migration and Andean forest in Colombia as overwintering habitat for this threatened species. Future research should be directed at understanding how these areas are used by Canada Warblers.

Remote estimation of overwintering home ranges in an elusive, migratory nocturnal bird

Due to a long running research bias toward the breeding season, there are major gaps in knowledge on the basic nonbreeding ecology of many species, preventing a full-annual cycle focus in ecology and conservation. Exacerbating this problem is the fact that many species are extremely difficult to detect outside of breeding. Here, we demonstrate a partial solution to this problem by using archival GPS tags to examine the overwintering ecology of a migratory nocturnal bird, the eastern whip-poor-will (Antrostomus vociferous). We deployed tags on 21 individuals and were able to recover 11 (52%) one year later. Tags collected high precision (approx. 10 m) points throughout the nonbreeding period. With continuous time movement models, we used these data to estimate overwintering home ranges. All individuals exhibited at least one bounded home range during this phase of the annual cycle, three of eleven had two wintering locations, and home range area ranged from 0.50 to 10.85 ha. All overwintering home ranges contained closed-canopy forest land cover (42%-100%), and no other land cover type represented >40% of any home range. We found some evidence, with caveats, that total edge within the landscape surrounding the home range was negatively related to home range area. The prevalence of contiguous closed-canopy forest cover in overwintering home ranges contrasts with apparent breeding habitat preferences, which includes clear-cuts and other, more open, habitats. This study is the first to reveal key aspects of overwintering space use in this species by using archival GPS to overcome both logistical and methodological limitations. Expanded use of such technology is critical to gathering basic ecological and distributional data, necessary for achieving a more complete understanding of full-annual cycles of animal populations.

First full annual cycle tracking of a declining aerial insectivorous bird, the Common Nighthawk (Chordeiles minor), identifies migration routes, nonbreeding habitat, and breeding site fidelity

Over one third of North American bird species are in decline, and for many species, we still lack fundamental biogeographic information such as migration routes and nonbreeding areas. Identifying causes of declines is limited because tracking many species throughout their annual cycle with high precision and accuracy is challenging. Common Nighthawks (Chordeiles minor (J.R. Forster, 1771)) have declined throughout much of their range and have yet to have their migratory and nonbreeding areas identified and characterized. We tracked Common Nighthawks by deploying a new 3.5 g Pinpoint GPS-Argos tag on adult males. Seven of 10 (70%) tags uploaded locations, providing the first data on migration, nonbreeding habitat, and annual site fidelity to breeding areas. Birds used similar loop migration routes and overwintered in Brazil’s Cerrado and Amazon regions. Nonbreeding season roosting home ranges were 148.22 ± 121 ha (mean ± SE) and included forest, grassland, and cropland. Breeding home-range fidelity was high; all tracked birds returned to within 1.27 ± 0.27 km of original capture locations. Our study is the first tracking of Common Nighthawks throughout their full annual cycle. Continued miniaturization of tracking technology, like the GPS-Argos transmitters used, is critical for identifying the causes of population declines of previously enigmatic migratory species.

Nightjars may adjust breeding phenology to compensate for mismatches between moths and moonlight

Phenology match-mismatch usually refers to the extent of an organism's ability to match reproduction with peaks in food availability, but when mismatch occurs, it may indicate a response to another selective pressure. We assess the value of matching reproductive timing to multiple selective pressures for a migratory lunarphilic aerial insectivore bird, the whip-poor-will (Antrostomus vociferus). We hypothesize that a whip-poor-will's response to shifts in local phenology may be constrained by long annual migrations and a foraging mode that is dependent on both benign weather and the availability of moonlight. To test this, we monitored daily nest survival and overall reproductive success relative to food availability and moon phase in the northern part of whip-poor-will's breeding range. We found that moth abundance, and potentially temperature and moonlight, may all have a positive influence on daily chick survival rates and that the lowest chick survival rates for the period between hatching and fledging occurred when hatch was mismatched with both moths and moonlight. However, rather than breeding too late for peak moth abundance, the average first brood hatch date actually preceded the peak moth abundance and occurred during a period with slightly higher available moonlight than the period of peak food abundance. As a result, a low individual survival rate was partially compensated for by initiating more nesting attempts. This suggests that nightjars were able to adjust their breeding phenology in such a way that the costs of mismatch with food supply were at least partially balanced by a longer breeding season.