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

Abstract

1. Light-level geolocators are popular bio-logging tools, with advantageous sizes, longevity and affordability. Biologists tracking seabirds often presume geolocator spatial accuracies between 186 and 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.2. 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.3. Geolocator spatial accuracy was lower than the ~200 km often assumed. prob-GLS 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 23.7% 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.4. Geolocation accuracy is not uniform among seabird species and can be considerably lower than assumed. Individual idiosyncrasies and spatiotemporal dynamics

Individual repeatability of avian migration phenology: A systematic review and meta‐analysis

“…This may be different, however, when estimating spatial repeatability due to the different spatial resolutions and measurement errors of each method (see Dingemanse et al, 2022). For example, geolocators can have large errors around location estimates (Halpin et al, 2021; Phillips et al, 2004), which may underestimate repeatability due to uncertainty when a bird reaches an exact location. Nonetheless, it is important to note the costs and limitations associated with each tracking method that is likely to be a constraint of the study system.…”

“…on a minute or hourly basis) and spatial accuracy (within ~150 m and 10 m, respectively) but until recently their mass restricted them to species of larger body size (Hobson et al, 2019). In contrast, GLS have low power requirements, allowing the devices to be considerably lighter (<1 g; Bridge et al, 2011), and are relatively cheap but provide only two locations per day with varying levels of spatial inaccuracy (Halpin et al, 2021; Phillips et al, 2004).…”

Interspecific variation in non-breeding aggregation: a multi-colony tracking study of two sympatric seabirds

“…We have identified a location of aggregation in razorbills in the central North Sea, which is not clearly evident from at-sea densities of auks in winter based on survey data from 1980−2018 (Waggitt et al 2020). Although it is well-known that there are high levels of error associated with locations derived from geolocators, our methods of processing the data followed current best practice for minimising that error (Halpin et al 2021). Further, we extracted 50% kernel density contours from relatively high sample sizes of individuals from each colony, so we believe that our findings are reliable.…”

“…Both species had high quality light curves throughout the non-breeding season, with the majority of transitions clearly identifiable. We then used 'probGLS' (Merkel et al 2016) to determine locations, a method shown to reduce the generally large error associated with geolocation (Phillips et al 2004, Halpin et al 2021. This method incorporates remotely-sensed environmental data, such SST, with light, activity and saltwater temperature recorded by the logger.…”

Across atoms to crossing continents: application of similarity measures to biological location data

“…during periods close to the spring and autumnal equinox and near the equator. 46 More generally, the variability in migratory schedules among individual birds creates deviations that infer problems in the comparison among flight paths. Long-distance migratory birds typically undertake sequences of nightly flights interspersed by extended multi-day stopovers to replenish energy reserves.…”