# Do GPS collars and coded neckbands tell the same story about year-round movements in geese?

## TL;DR

The study compares GPS tracking and coded neckbands to assess if they provide similar insights into the year-round movements of greylag geese.

## Contribution

The paper provides a direct comparison of GPS and CMR methods for tracking goose movements, evaluating their agreement in space use and migration patterns.

## Key findings

- GPS and CMR methods generally agree on year-round space use estimates for greylag geese.
- Movement metrics differ between methods when CMR captures behavior at a different temporal scale than GPS.
- Range overlap estimates for summer and winter are consistent regardless of tracking method.

## Abstract

GPS telemetry has become the norm for the tracking of large-bodied bird species, whereas management and conservation of populations often rely on low-tech methods such as capture-mark-resighting (CMR). Direct evaluations of the comparability of the respective outcome from these methods remain rare despite being crucial for comparative studies and management decisions. Here, we investigated whether GPS tracking and CMR lead to same conclusions about seasonal migration and year-round space use. We chose greylag geese (Anser anser) as a study species, for which a long record of both coded neckband reports and GPS tracking are available, and whose management relies on CMR data.

Our data set was comprised of neckband reports and GPS tracks collected for birds from five capture sites in Sweden (n = 665 neckband birds; n = 156 GPS collar birds). We evaluated the similarity of movement metrics and year-round space use derived from continuous-time movement models and auto-correlated kernel density estimators. We further quantified overlap of spatial range estimates between tracking methods for the breeding period and the wintering season. We approximated spatial observation bias by contrasting range estimates estimated with and without the use of a debiasing algorithm.

We found that estimates of space use derived from CMR and GPS tracking were in general agreement: average year-round space use for most individuals was similar even if means among tracking methods differed among all individuals per method (CMR: \documentclass[12pt]{minimal}
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\begin{document}$$5.07\times10^5 \mathrm{km}^2$$\end{document}; GPS: \documentclass[12pt]{minimal}
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\begin{document}$$1.76\times10^5 \mathrm{km}^2$$\end{document}), and mean overlap of range estimates for summer and winter did not differ depending on whether the comparison was with the same, or differing tracking methods. Movement metrics differed considerably between methods whenever the CMR data captured behaviour at a different temporal scale than GPS (position & velocity autocorrelation), and else in agreement with GPS tracking (periodicity).

Our study suggests that the historical and current use of coded neckband data for greylag goose management decisions is appropriate regarding space use of migratory greylag geese in Europe. Understanding whether the existing reporter network can capture changes to the migratory behaviour of greylag geese including short-stopping of migration will however require additional in-depth analyses.

The online version contains supplementary material available at 10.1186/s40462-025-00620-y.