Light level geolocators, also known as GLS loggers, are electronic devices intended for tracking the location of wide-ranging animals using ambient light to estimate latitude and longitude. Miniaturized geolocators that can be used on relatively small migratory birds have recently become available, but little is known about the potential harmful effects of geolocators on birds. We examined the possible effects of 1.5-g geolocators (dimensions: 21 x 6.5 x 9 mm) on the breeding success and survival of migratory Lesser Kestrels (Falco naumanni). During the 2007 breeding season, kestrels were fitted with geolocators using two attachment methods (Teflon wing harnesses and darvic bands), and geolocators were removed in 2008 after the birds returned to the breeding grounds. We found no differences in the breeding success of control and tagged pairs during the 2007 breeding season, but tagged pairs had greater fledgling mortality in the following breeding season. Furthermore, nestlings of tagged individuals had higher triglyceride and uric acid concentrations in their blood than control nestlings during the breeding season following tagging. As for return rates, 75% of tagged birds came back to the colony after the nonbreeding period, a proportion similar to that reported in previous studies. Although back-mounts are slightly heavier and require more skill to attach, we recommend their use on small migratory raptors because most leg-mounted geolocators in this study were damaged or rendered useless by dirt obscuring the light sensor
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