LAUSR

Tracking small passerines using miniaturised location tags is a rapidly expanding field of study. In a one-year study, we tested whether there were any short- or longer-term effects of fitting geolocators weighing 3% of body mass on male Pied Flycatchers Ficedula hypoleuca. In the deployment year, we compared adult provisioning rates to nestlings, nestling growth and nest success between nesting attempts in which adult males were fitted with a geolocator, with control nests where males had the same capture history but were not tagged. We found no difference between treatments in provisioning effort by males or their associated female two days after geolocator fitting, in terms of nestling growth, subsequent brood reduction or nest success. Return rate, arrival date on territories, timing and breeding parameters were compared between tagged and untagged males in the following breeding season. We found no difference in return rate or arrival date, and no difference in nest timing, fecundity or outcome. Our study suggests that fitting lightweight tags to small passerines need not affect behaviour, breeding or apparent between-year survival. However, tagging new species should still require assessment and comparison with well-matched control cohorts, and it should be recognised that tag effects could vary between years and populations, mediated by environmental conditions. This article is protected by copyright. All rights reserved.