Data from: Assessing costs of carrying geolocators using feather corticosterone in two species of aerial insectivore

Despite benefits of using light-sensitive geolocators to track animal
movements and describe patterns of migratory connectivity, concerns have
been raised about negative effects of these devices, particularly in small
species of aerial insectivore. Geolocators may act as handicaps that
increase energetic expenditure, which could explain reported effects of
geolocators on survival. We tested this ‘Energetic Expenditure Hypothesis’
in 12 populations of tree swallows (Tachycineta bicolor) and barn swallows
(Hirundo rustica) from North America and Europe, using measurements of
corticosterone from feathers (CORTf) grown after deployment of geolocators
as a measure of physiology relevant to energetics. Contrary to
predictions, neither among- (both species) nor within-individual (tree
swallows only) levels of CORTf differed with respect to instrumentation.
Thus, to the extent that CORTf reflects energetic expenditure, geolocators
apparently were not a strong handicap for birds that returned
post-deployment. While this physiological evidence suggests that
information about migration obtained from returning geolocator-equipped
swallows is unbiased with regard to levels of stress, we cannot discount
the possibility that corticosterone played a role in reported effects of
geolocators on survival in birds, and suggest that future studies relate
corticosterone to antecedent factors, such as reproductive history, and to
downstream fitness costs.