Effects of light-level geolocators on return rates and inter-annual territory fidelity of Prairie Warblers Setophaga discolor

In a single-year study, attaching geolocators to male Prairie Warblers Setophaga discolor, one of the smallest species to date to carry geolocators, decreased return rates but had no effect on inter-annual territory fidelity. To examine the effects of light-level geolocators on return rates and inter-annual territory fidelity of adult, male Prairie Warblers migrating to and from their non-breeding grounds. During the 2016 breeding season we captured adult male Prairie Warblers at two sites in northeastern USA and attached geolocators and colour rings to one group of birds, and only colour rings to another group of birds. During the 2017 breeding season, we estimated return rates to the sites and inter-annual territory fidelity (i.e. distance between territory locations from one year to the next) for the birds that returned to the sites. We compared return rates and territory fidelity between birds with and without geolocators. Geolocators significantly decreased return rates of male Prairie Warblers to the study sites by 0.18 (95% credible interval [CI]: −0.37 to 0.01) from 2016 to 2017. Birds that returned exhibited a range of distances between estimated locations of territories in each year (7–2104 m), but the differences in distances between birds with and without geolocators were not statistically significant. Although return rates were lower for Prairie Warblers with geolocators, these effects should not necessarily disqualify their use. Rather, the magnitude of the negative effects on individuals needs to be considered against the benefits and reliability of the information gained for the population or species. We recommend the effects of geolocators and other tracking tags should be studied and reported in conjunction with the main objectives of the study for which the tags are being used.