Individual year-round movement patterns of the GPS-tracked Grey Herons (Ardea cinerea) in Central Europe: a multi-year case study.

BACKGROUND: The year-round migratory behaviour of Grey Herons (Ardea cinerea) breeding in Central Europe is recognized; however, individual consistency in their movement patterns remains poorly known. In this study, we present movements of three adult Grey Herons breeding in Northern Poland and Eastern German global positioning system (GPS)-tracked between 2012 and 2020. METHODS: We analyzed annual movement trajectories, focusing on migration distances, fidelity to breeding and wintering grounds, and the use of stop-over sites. We performed an Analysis of Variance from Summary Data (ANOVA) on summary data to compare migratory parameters across different Eurasian populations and age groups. RESULTS: We identified four main phases in the annual cycle of all studied individuals: breeding, wintering, spring and autumn migration. Our data reveal high inter-individual variation in migratory strategies. Although two individuals changed their wintering sites from year to year, all studied ones exhibited strict fidelity to the same breeding grounds across consecutive seasons. Migration distances ranged from 175 to 1,758 km; notably, the individual breeding in Eastern Germany covered the shortest distance (mean ± SD: 221 ± 51.8 km). We observed substantial inter-individual variation in winter site fidelity, migration distances, and stop-over numbers within this Central European Grey Heron population. The herons selected the shortest migration paths during spring, but not during autumn. While the broad migratory characteristics-specifically the number of stop-overs (3.0 ± 2.8), migration distance (1,220 ± 580.4 km), and migration duration (9 ± 9 days)-closely match findings from other populations, the tracked herons in our study made fewer stop-overs during autumn migration compared to the values reported in the literature. CONCLUSIONS: We observed inter-individual variations in the annual movement patterns of the Grey Heron, encompassing the timing of the four annual cycle phases, the range of distances covered annually and the birds' fidelity to areas used as breeding or wintering grounds. Our report provides the first long-term, high-resolution insights into the year-round movements of GPS-tracked Grey Herons from Central Europe, emphasizing that specific individuals employ diverse and flexible migratory strategies, even within the same breeding region.