Wetter and more forested nonbreeding areas result in later departures but a faster spring migration in <i>Vermivora chrysoptera</i> (Golden-winged Warbler)

ABSTRACT Declines in migratory landbird populations require targeted research to identify the population-limiting factors acting throughout the annual cycle. Tracking studies provide data on migration timing and speed, which have been used to describe carryover effects of nonbreeding habitat use. To explore how variation in precipitation and forest cover among nonbreeding sites influenced departure dates and migration speed in Vermivora chrysoptera (Golden-winged Warbler), we fitted 69 birds with radio transmitters in Costa Rica and tracked them via the Motus Wildlife Tracking System. Capture sites covered a broad gradient of precipitation and forest cover and were separated by &amp;lt;100 km. Warblers departed Costa Rica during a 33-day window (26 March to 27 April, n = 46), and 23 birds were subsequently detected at Motus stations in North America, primarily in a narrow migration pathway through the Midwestern United States. Individual variation in departure date was explained by adults departing before immatures, males before females, and birds occupying drier (lower annual precipitation) and less forested sites departing 10 days earlier, on average, than those in wetter and more forested sites. Following departure, later-departing birds migrated faster allowing them to reach latitudes between 36° and 44° in fewer days and thereby catch up to and, in 28% of paired birds, overtake the earlier-departing bird. Our findings contrast with the general expectation that earlier departures lead to earlier arrivals along migration routes, instead providing evidence for alternative migratory strategies covering a spectrum of outcomes from earlier departures leading to earlier arrivals through to later departures leading to earlier arrivals. We present hypotheses for why these alternative strategies might exist within an optimal migration theory framework. Future studies in Costa Rica should examine additional proxies for habitat quality, to corroborate whether faster migration speeds from wetter more forested habitats are linked to higher habitat quality.