GPS tracking reveals range size and movement patterns in immature eagles ( Clanga clanga and C. clanga x pomarina ) with delayed maturation

Abstract The immature period is an important stage in the life histories of animals, influencing their subsequent development and future experiences. This stage is particularly pronounced in long-maturing raptors. In this study, we examined individual ranging areas at breeding and wintering grounds of the vulnerable Clanga clanga (Greater Spotted Eagle) and their hybrids with C. pomarina (Lesser Spotted Eagle) from their primary European Union populations in Poland and Estonia. We analyzed GPS tracking data from 26 individuals (including 11 hybrids) tagged between 2008 and 2022. Our results revealed a reduction in individual ranging areas at both breeding and wintering grounds as the birds matured. With increasing age, C. clanga arrived earlier at future breeding sites, likely to synchronize with breeding adults and enhance their ability to compete for territories and mates. As they aged, immature eagles also extended their stay at breeding grounds. Some individuals limited their ranging areas as early as their 4th or 5th calendar year, indicating the onset of site fidelity, whereas others continued wide-ranging movements into their 7th year. Males, in particular, exhibited philopatry and frequently visited nests of established pairs within their natal population prior to their own recruitment as breeders. Immatures were first observed at nests between their 3rd and 7th calendar years, but confirmed breeding typically occurred later, between their 8th and 10th calendar years. Eight individuals (7 males) were found breeding at an average distance of 8.1 ± 7.3 km from their natal nests. No behavioral differences were observed between C. clanga and their hybrids. Our findings illustrate the slow maturation and prolonged preparation for breeding in this species and its hybrids. These results may inform population dynamics modeling and aid in the conservation of these vulnerable species.