Data from: An experimental evaluation of the effects of geolocator design and attachment method on between-year survival on whinchats Saxicola rubetra

Data from location logging tags have revolutionised our understanding of
migration ecology, but methods of tagging that do not compromise survival
need to be identified. We compared resighting rates for 156
geolocator-tagged and 316 colour ringed-only whinchats on their African
wintering grounds after migration to and from eastern Europe in two
separate years. We experimentally varied both light stalk length (0, 5 and
10 mm) and harness material (elastic or non-elastic nylon braid tied on,
leg-loop ‘Rappole’ harnesses) in the second year using a reasonably
balanced design (all tags in the first year used an elastic harness and 10
mm light stalk). Tags weighed 0.63 g (0.01 SE), representing 4.1% of
average body mass. There was no overall significant reduction in
between-year resighting rate (our proxy for survival) comparing tagged and
untagged birds in either year. When comparing within tagged birds,
however, using a tied harness significantly reduced resighting rate by 53%
on average compared to using an elastic harness (in all models), but stalk
length effects were not statistically significant in any model considered.
There was no strong evidence that the fit (relative tightness) or added
tag mass affected survival, although tied tags were fitted more tightly
later in the study, and birds fitted with tied tags later may have had
lower survival. Overall, on a precautionary principle, deploying tags with
non-elastic tied harnesses should be avoided because the necessary fit, so
as not to reduce survival, is time-consuming to achieve and does not
necessarily improve with experience. Geolocator tags of the recommended
percentage of body mass fitted with elastic leg-loop harnesses and with
short light stalks can be used without survival effects in small
long-distance migrant birds.