Although miniaturized data loggers allow new insights into avian migration, incomplete knowledge of basic patterns persists, especially for nightjars. Using GPS data loggers, this study examined migration ecology of the Eastern whip-poor-will (Antrostomus vociferus), across three migration strategies: flyover, short-stay, and long-stay. We documented migration movements, conducted hotspot analyses, quantified land cover within 1-km and 5-km buffers at used and available locations, and modeled habitat selection during migration. From 2018-2020 we captured breeding whip-poor-wills from three study sites in Massachusetts and programmed GPS tags to collect data during fall and spring migration periods. Across 19 individual males (nine of them with repeated years of data), GPS tags collected 479 locations, where 30% were classified as flyover points, 33% as short-stays, and 37% as long-stay locations. We documented seasonal flexibility in migration duration, routes, and stopover locations among individuals and between years. Analyses identified hotspot clusters in fall and spring migration in the Sierra de Tamaulipas in Mexico. Land cover at used locations differed across location types at the 5-km scale, where closed forest cover increased and crop cover decreased for flyover, short-stay, and long-stay locations, and urban cover was lowest at long-stay locations. Discrete choice modeling indicated that habitat selection by migrating whip-poor-wills differs depending on the scale and migration strategy. For example, at the 5-km scale birds avoided urban cover at long-stay locations and selected closed forest cover at short-stay locations. We suggest that whip-poor-wills may use land cover cues at large spatial scales, like 5-km, to influence rush or stay tactics during migration.

From 2018-2020, we captured breeding whip-poor-wills from three study sites in Massachusetts and programmed GPS tags to collect data during fall and spring migration periods. Across 19 individual males (nine of them with repeated years of data), GPS tags collected 479 locations, where 30% were classified as flyover points, 33% as short-stays, and 37% as long-stay locations.

Data processing

We filtered and retained migration data points when loggers connected to ≥ 4 satellites and points had dilution of precision values < 5 to ensure a 3D fix of the location (Forrest et al. 2022, Bakermans et al. 2022). Using 30-m USGS DEM (digital elevation model; http://ned.usgs.gov) data, we generated the altitude of each point by converting the GPS tags’ altitude to altitude above sea level and then subtracted the local elevation (from the DEM) from the bird’s altitude (A. Korpach, pers. communication). Next, we classified migration points based on altitude and number of points at a single location as either flyover, short-stay, or long-stay. Long-stays were locations with ≥ 2 GPS points within the same vicinity (i.e., < 10 km). Short-stay and flyovers consisted of one GPS point at a single location. We differentiated short-stay versus flyover points by altitude based on the altitudes of birds at long-stay locations (mean = 17 m, range = 121 m). Short-stays were locations with elevations < 100 m (mean = 15 m), and flyover locations had an altitude ≥ 100 m above the ground (mean = 800 m).

Hotspot Analyses

To identify areas of high or low use during migration, we ran an optimized hotspot analysis in ArcGIS 10.8.2 to identify statistically significant spatial clusters of high (hotspot) and low values (coldspot) of migration locations using the Getis-Ord Gi* statistic (Sussman et al. 2019). This tool can “aggregate data, identify an appropriate scale of analysis, and correct for both multiple testing and spatial dependence” (ESRI 2021). 

Land cover classification

We used ArcGIS and quantified land cover types from 2019 data using the 100-m Copernicus Global Land Service layer (Buchhorn et al. 2020). Land cover types were classified as (a) closed forest, (b) open forest, (c) shrubland, (d) herbaceous vegetation (hereafter, grassland), (e) herbaceous wetland, (f) cropland, (g) bare, (h) fresh- or saltwater, and (i) developed land (Buchhorn et al. 2020). Using the geoprocessing features of ArcMap, we quantified land cover at 5-km and 1-km circle at an actual migration location (i.e., used) and random locations (i.e., available).

Habitat selection

We used discrete choice modeling to determine habitat selection of Eastern whip-poor-will during migration. Discrete choice models examine the probability that an individual chooses a location based on a choice set of alternative available locations (Cooper and Millspaugh 1999). Choice sets included one used location based on the GPS fix and ten available locations. We constructed separate models for each type of migration point (i.e., flyover, short-stay, and long-stay) and spatial scale (i.e., 1 km and 5 km) with individual as a random effect. We used package jagsUI (Kellner 2021) with the software JAGS 4.3.1 (Plummer 2003).