Eastern Whip-poor-wills have larger nonbreeding home ranges in areas with more agriculture and forest fragmentation

Abstract

Migratory bird populations can be limited by events in disparate parts of the world. Birds wintering in tropical regions are facing rapid habitat loss, climate change, and intensive agricultural regimes, potentially contributing to population declines. However, an understanding of basic nonbreeding ecology of species, such as habitat and space use, is critical for determining if this is the case. Populations of the nocturnal/crepuscular Eastern Whip-poor-will (Antrostomus vociferus) have declined by 70% since the 1960’s, yet data on the species are sparse outside of the breeding season. We extracted data from 41 archival GPS tags deployed on whip-poor-wills and estimated nonbreeding home ranges and land covers used. We used satellite imagery and stable carbon and nitrogen isotope values from claws grown during the nonbreeding season to analyze how land cover and habitat moisture impacted home range size and relative trophic level. Forest was by far the most prevalent land cover used by whip-poor-wills, occurring in all home ranges and accounting for >80% of diurnal roosting points. We found that less forest, the presence of agriculture, and more edge (irrespective of land cover) were associated with larger home ranges. Stable isotope values differed by broadscale ecoregion but not local land cover characteristics in our study, indicating that regional idiosyncrasies or broadscale processes can be more important in determining stable isotope ratios. Our findings suggest that the loss, fragmentation, and replacement of forest by agriculture in the core of the whip-poor-will’s nonbreeding range may represent a threat to the species, as they rely heavily upon forest, and appear to alter space use in response to changes in forest cover.

Winter GPS tagging reveals home ranges during the breeding season for a boreal-nesting migrant songbird, the Golden-crowned Sparrow

“…We filtered points conservatively in line with manufacturer recommendations and previous studies (that found points with high HDOP and 3 satellites should be down weighted [ 50 ]) and expect a high accuracy for our locations. However, further studies could improve the accuracy of home range estimates and determine how space use changes throughout the breeding season by collecting more points on the breeding grounds as tracking technology improves and/or through individual tag calibration to assess tag-specific error that can be incorporated into modeling [ 50 , 71 ]. More generally, our data align with other studies that show how visual and auditory observations of breeding birds have the potential to miss more subtle movements outside of territories [ 15 , 16 , 19 ].…”

“…We did not find the proportion of shrubland within the home range to be an important predictor for home range size, but the maximum size of 95% KDEs with 100% shrub cover was only 34.8 ha, which is slightly smaller than the mean 95% KDE size across all birds (44.1 ha). This could suggest that shrub habitat is of higher quality so that less area is needed for a breeding pair [ 49 , 50 ], or that habitat quality correlates with population density [ 78 ] which restricts the size of an area that can be defended. Due to the limited sample size (18 KDEs), this requires further study and would benefit from fitness estimates in the field.…”

“…Land cover can be an important driver of home range size, with less suitable habitat potentially resulting in larger home ranges [ 49 , 50 ]. At each home range, we determined land cover type at breeding grounds using the 2020 North America Land Change Monitoring System map [ 51 ].…”

Assessing an age-old ecogeographical rule in nightjars across the full annual cycle

“…We calculated each variable as the mean value within a buffer that corresponded to the average home range size (rounded up to 1 significant figure) for each species in the corresponding season (whip-poor-will: breeding 500 m, winter 100 m; nighthawk: breeding 5000 m, winter 1000 m; European nightjar: 500 m, 100 m). See the following references for whippoor-will (Wilson, 2003;Tonra et al, 2019;Bakermans et al, 2022;Skinner et al, 2022a), nighthawk (Ng et al, 2018), and European nightjar (Sharps et al, 2015;Mitchell et al, 2020). We used only the dates when individuals were thought to be stationary on the breeding or wintering grounds (breeding: May to September; winter: October to April).…”