Double-tagging scores of seabirds reveals that light-level geolocator accuracy is limited by species idiosyncrasies and equatorial solar profiles

Light-level geolocators are popular bio-logging tools, with advantageous
sizes, longevity, and affordability. Biologists tracking seabirds often
presume geolocator spatial accuracies between 186-202 km from
previously-innovative, yet taxonomically, spatially, and computationally
limited, studies. Using recently developed methods, we investigated
whether assumed uncertainty norms held across a larger-scale, multispecies
study. We field-tested geolocator spatial accuracy by synchronously
deploying these with GPS loggers on scores of seabirds across five species
and 11 Mediterranean Sea, East Atlantic and South Pacific breeding
colonies. We first interpolated geolocations using the geolocation package
FLightR without prior knowledge of GPS tracked routes. We likewise applied
another package, probGLS, additionally testing whether sea-surface
temperatures could improve route accuracy. Geolocator spatial accuracy was
lower than the ~200km often assumed. probGLS produced the best accuracy
(mean ± SD = 304 ± 413 km, n = 185 deployments) with 84.5% of GPS-derived
latitudes and 88.8% of longitudes falling within resulting uncertainty
estimates. FLightR produced lower spatial accuracy (408 ± 473 km, n = 171
deployments) with 38.6% of GPS-derived latitudes and 27% of longitudes
within package-specific uncertainty estimates. Expected inter-twilight
period (from GPS position and date) was the strongest predictor of
accuracy, with increasingly equatorial solar profiles (i.e., closer
temporally to equinoxes and/or spatially to the Equator) inducing more
error. Individuals, species and geolocator model also significantly
affected accuracy, while the impact of distance travelled between
successive twilights depended on the geolocation package. Geolocation
accuracy is not uniform among seabird species and can be considerably
lower than assumed. Individual idiosyncrasies and spatiotemporal dynamics
(i.e., shallower inter-twilight shifts by date and latitude) mean that
practitioners should exercise greater caution in interpreting geolocator
data and avoid universal uncertainty estimates. We provide a function
capable of estimating relative accuracy of positions based on
geolocator-observed inter-twilight period.