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Light-level geolocators, miniature devices used for tracking avian migration over the full annual cycle, are being widely deployed on small migratory passerines. However, the effects of carrying geolocators on the breeding biology of songbirds are unclear, and variable species- and guild-specific conclusions have been drawn regarding their effects on return rates (apparent annual survival). In particular, there is a lack of published information on the effects of geolocators on Nearctic–Neotropical migrant warblers and canopy-dwelling bird species, which limits our ability to determine whether this technology is appropriate for use on species within these groups. During 2014 and 2015, we deployed geolocators on 49 adult male Cerulean Warblers (Setophaga cerulea) in Pennsylvania, Missouri, and Arkansas, USA. We monitored the effects of geolocators across the full annual cycle by comparing apparent within-breeding-season survival (within-season φ), nestling provisioning rates, nest survival, and return rates between geolocator-tagged adult males and color-banded controls. We found no negative effects of geolocators during the breeding season of geolocator deployment, but the return rate of geolocator-tagged birds was lower than that of control birds (16% ± 5% vs. 35% ± 7%). We found no strong evidence that the differential return rate between the 2 groups was influenced by breeding region, body mass, bird age, year of geolocator deployment, or method of attachment. Although finding no effect of geolocators during the breeding season is encouraging, the lower return rate of geolocator-tagged birds warrants further investigation in the field. If further improvements in the design or attachment methods of geolocators are not technologically possible, the potential for increased mortality (or dispersal) of geolocator-tagged birds should be weighed against the potential conservation gains that could be made by identification of critical stopover, wintering, and breeding habitats for populations of interest.

Mixed effects of geolocators on reproduction and survival of Cerulean Warblers, a canopy-dwelling, long-distance migrant