Anim. Migr. 2020; 7: 19–26 https://doi.org/10.1515/ami-2020-0003 Received December 6, 2019; accepted February 24, 2020 Abstract: Cerulean Warblers (Setophaga cerulea) 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 (n = 30) on adult male Cerulean Warblers in Indiana, USA, to track annual movements of individuals. Recovered geolocators (n = 4) showed that Cerulean Warblers occurred broadly throughout northern South America. Autumn migration lasted 44-71 days (n = 4), whereas spring migration lasted 37-41 days (n = 3). The average migration distance was 5268 km. During autumn migration, Cerulean Warblers made 1-4 stopovers (i.e., ≥2 days; n = 4) and 1-2 stopovers during spring migration (n = 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. Keywords: full annual cycle, migratory connectivity, Setophaga cerulea 1 Introduction Identifying the geographic distribution and assortment of migratory individuals during migration and nonbreeding periods is critical for understanding population trends [1]. Populations may be limited by factors occurring outside the breeding period including along population-specific migratory routes [2] or at nonbreeding areas [3]. Until recently, there has been a dearth of information on the nonbreeding dispersion and migratory behaviors of most species of New World wood-warblers (Parulidae). Most parulids migrate considerable distances between Nearctic breeding grounds and Neotropical nonbreeding grounds [4], and many of these species are declining [5]. The recent miniaturization of tracking technology (light-level geolocators) has made tracking small migratory species (< 10 g) possible [6-8]. Cerulean Warblers (Setophaga cerulea) are small (~9 g), Nearctic-Neotropical migratory songbirds that breed in mature deciduous forests of the eastern United States and adjacent southern Canada. Individuals overwinter in northern South America along the eastern slopes of the northern Andes at elevations of ~500- 2000 m [9]. Cerulean Warblers are among the fastest declining Nearctic-Neotropical migrant wood-warbler species in North America (~3% per year) [9] with declines purportedly linked to extensive habitat loss on both breeding and nonbreeding grounds [9, 10]. However, because Cerulean Warbler research, conservation, and management are focused on the breeding grounds, there is currently little information on the dispersion of Cerulean Warbler populations outside of the breeding period (i.e., stopovers during autumn and spring migrations, and the nonbreeding period), which may be hindering conservation efforts if limiting factors are experienced away from the breeding grounds [3]. Determining where populations of Cerulean Warblers overwinter, the routes they use to migrate between breeding and nonbreeding *Corresponding author: Kamal Islam, Ball State University, Muncie, IN United States, E-mail: kislam@bsu.edu Clayton D Delancey, Garrett J MacDonald, Alexander R Sharp, Bran- don M Connare, Ball State University, Muncie, IN United States Gunnar R Kramer, University of Toledo, Toledo, OH United States Clayton D Delancey, Kamal Islam*, Gunnar R Kramer, Garrett J MacDonald, Alexander R Sharp, Brandon M Connare Geolocators reveal migration routes, stopover sites, and nonbreeding dispersion in a population of Cerulean Warblers Communication Open Access. © 2020 Kamal Islam et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 Public License.