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.

光感地理定位器（light-level geolocators）是当下广为使用的生物记录工具，其优势在于尺寸小巧、续航持久且成本可控。追踪海鸟的生物学家通常会基于早期虽具创新性，但在分类学、空间尺度与计算能力上存在局限的研究，预设该类定位器的空间精度可达186-202公里。本研究借助新近发展的方法，针对更大规模的多物种研究场景，验证了这一预设的不确定性标准是否成立。

我们通过将光感地理定位器与GPS记录仪（GPS loggers）同步部署，在覆盖5个海鸟物种、11个分布于地中海、东大西洋及南太平洋的繁殖群落的大量个体上开展了野外实地测试，以此评估光感地理定位器的空间精度。首先，我们在未预先知晓GPS追踪路线的前提下，借助FLightR地理定位软件包（FLightR）插值得到地理坐标；同时，我们还使用了另一款软件包probGLS，额外测试了海面温度能否提升路线定位精度。

研究结果显示，光感地理定位器的空间精度低于此前普遍假设的约200公里。其中probGLS的精度表现最优（平均值±标准差（SD）= 304 ± 413 km，有效部署次数n = 185），其输出的不确定性估计范围内涵盖了84.5%的GPS获取纬度与88.8%的GPS获取经度。FLightR的空间精度更低（408 ± 473 km，n = 171），仅有38.6%的GPS获取纬度与27%的GPS获取经度落在其软件包自带的不确定性估计范围内。由GPS位置与日期推导得到的预期暮光间隔是影响精度的最强预测因子：越靠近赤道的日照特征（即时间上更接近二分点，或空间上更贴近赤道）会引入越多的定位误差。个体差异、物种属性与地理定位器型号同样会显著影响精度，而连续两次暮光间的移动距离对精度的影响则取决于所使用的地理定位软件包。

海鸟不同物种间的地理定位精度并不统一，且可能远低于此前的预设值。个体特异性差异与时空动态特征（即随日期与纬度变化的暮光间隔变化幅度更小）意味着研究者在解读光感地理定位器数据时应更加谨慎，且不应使用统一的不确定性估计标准。我们提供了一款可基于光感地理定位器观测得到的暮光间隔来估算定位相对精度的函数。