Effects of geolocators on reproductive performance and annual return rates of a migratory songbird

Our understanding of the annual life-cycle movements of small migratory birds has advanced rapidly with the advent of light-weight geographical positioning devices (i.e., geolocators), yet the effects of geolocators on reproduction and survival have not been adequately quantified. We tested for impacts of attaching a 1-g geolocator (using a harness around the legs and back, anterior to the tail) to adult Tree Swallows (Tachycineta bicolor) on parental feeding behavior, nestling growth and size, fledging success, and return rates between 2011 and 2012. At one breeding site, we compared feeding visits, nestling growth, and nestling size between paired nest boxes where one parent was marked at the 'geolocator' box with a 'control' nest box where neither parent was marked. We detected no differences between geolocator and control nests in either the frequency of feeding visits to nestlings or the amount of time spent at nests. Birds marked with geolocators fed nestlings as frequently as their unmarked mates. Likewise, nestlings raised at geolocator nests grew at similar rates to those at control nests, and had similar structural size and body mass at fledging. At three widely-separated sites across the Tree Swallow breeding range in Canada, we also found that fledging success was similar for geolocator and control nests. Although we found no evidence for short-term negative impacts of geolocators, the return rates of geolocator-marked Tree Swallows tended to be significantly lower than those of unmarked control birds. Thus, we found little evidence for short-term impacts of geolocators on reproduction but our study does suggest that long-term impacts of geolocators could be manifested in terms of lower survival, higher emigration rates, or lower breeding propensity.