Survival probability in a small shorebird decreases with the time an individual carries a tracking device

Effects of tracking devices on survival are generally considered to be
small. However, most studies to date have been conducted over a
time-period of only one year, neglecting the possible accumulation of
negative effects and consequently stronger negative impacts on survival
when the individuals have carried the tracking devices for longer periods.
We studied the effects of geolocators in a closely monitored and
colour-ringed southern dunlin (Calidris alpina schinzii) population
breeding in Finland. Our capture-recapture data spans 2002-2018 and
includes individual histories of 338 colour-ringed breeding adult dunlins
(the term ‘recapture’ includes resightings of colour-ringed and
individually recognizable birds). These data include 53 adults that were
fitted with leg-flag mounted geolocators in 2013-2014. We followed their
fates together with other colour-ringed birds not equipped with
geolocators until 2018. Geolocators were removed within 1-2 years of
attachment or were not removed at all, which allowed us to examine whether
carrying a geolocator reduces survival and whether the reduction in
survival becomes stronger when geolocators are carried for more than one
year. We fit multi-state open population capture-recapture models to the
encounter history data. When assessing geolocator effects, we accounted
for recapture probabilities, time since marking, and sex and year effects
on survival. We found that carrying a geolocator reduced survival, which
contrasts with many studies that examined return rates after one year.
Importantly, survival declined with the time the individual had carried a
geolocator, suggesting that the negative effects accumulate over time.
Hence, the longer monitoring of birds carrying a geolocator may explain
the difference from previous studies. Despite their larger mass, females
tended to be more strongly affected by geolocators than males. Our results
warrant caution in conducting tracking studies and suggest that short-term
studies examining return rates may not reveal all possible effects of
tracking devices on survival.