No effect of geolocators on apparent return rates of a declining Neotropical migrant, the Canada Warbler ( Cardellina canadensis ).

Canada Warblers ( Cardellina canadensis ) are small Neotropical migrants whose populations are declining across most of their range. Understanding factors limiting Canada Warbler populations requires knowledge of population ecology across the full annual cycle, including migratory pathways and over-winter locations. Light-level geolocator tags have offered unprecedented insight into migratory ecology for many species, but previous studies suggest that geolocators may influence apparent return rates. We sought to determine if geolocators influence apparent return rates of adult male Canada Warblers breeding in West Virginia, USA. In 2020, we deployed geolocators on 32 birds and color banded an additional 78 birds without geolocators. The following year, 13 of 32 (40.6%) geolocator birds and 37 of 78 (47.4%) color-banded birds were detected with no significant difference in apparent return rates between groups (χ² = 0.19, p = 0.66). Although further evaluation of additional groups will be valuable, the lack of significant effect on adult male Canada Warblers suggests that the slightly lower return rate does not preclude the use of geolocators as a tool to assess the migration ecology of this small songbird of conservation concern.