Geolocators reveal migration routes, stopover sites, and nonbreeding dispersion in a population of Cerulean Warblers

Cerulean Warblers ( ) are among the fastest declining Nearctic-Neotropical migrant wood-warblers (Parulidae) in North America. Despite ongoing conservation efforts, little is known about their non-breeding distribution. In June 2016-2018, we deployed geolocators ( = 30) on adult male Cerulean Warblers in Indiana, USA, to track annual movements of individuals. Recovered geolocators ( = 4) showed that Cerulean Warblers occurred broadly throughout northern South America. Autumn migration lasted 44-71 days ( 4), whereas spring migration lasted 37-41 days ( 3). The average migration distance was 5268 km. During autumn migration, Cerulean Warblers made 1-4 stopovers (i.e., ≥2 days; 4) and 1-2 stopovers during spring migration ( 3). When crossing the Gulf of Mexico during autumn migration, two birds stopped over after crossing, but not beforehand. Two others navigated through the Caribbean rather than crossing the Gulf of Mexico. During spring migration, one individual stopped after crossing, one individual stopped before crossing, and one individual stopped before and after crossing the Gulf of Mexico. No birds migrated through the Caribbean Islands during spring migration. These results represent novel information describing annual movements of individual Cerulean Warblers and will inform conservation efforts for this declining species.