Small Ranging Area and Less Mobile Pattern of a Long-Distance Migratory Raptor During the Post-Fledging Dependence Period, Determined by Fledging Date and Forest Preferences

The central point of evolutionary ecology is understanding the drivers of variation in life-history strategies among organisms. The post-fledging dependence period is a critical life-history stage in the life cycle of altricial birds, influencing their future long-term survival, fitness, and reproduction. This period represents the transition between fledging and the onset of natal dispersal or migration, when offspring are highly dependent on parental care. In this study, we used GPS-GSM telemetry to analyze for the first time the duration of this phase and the size of the ranging area of Lesser Spotted Eagles during the dependence period, testing the importance of various extrinsic (habitat type, territory, region) and intrinsic (body condition, sex, fledging date, fledging age) factors. Our survey was conducted between 2020 and 2025 in Bulgaria, SoutheastEurope. The mean duration of the post-fledging dependence period (PFDP) in the juveniles was 47 ± 5 days (SD) (n = 14). The age of independence of eagles ranged from 71 to 110 days (median = 95 days). Our results show that the fledging date is the main determinant shaping the dependence period both temporally and spatially. The size of the ranging area (90% home range) of juveniles during the PFDP varied from 0.03 to 16.72 km2, with a mean value of 2.41 km2 ± 4.34 SD (n = 14). Home range was influenced mainly by surrounding habitats (grassland types and forests). In line with our hypothesis, we found that birds are habitat-selective, strongly preferring forests and avoiding human-modified landscapes (infrastructure and agricultural fields) during this early life-history stage. We established a relatively small ranging area inhabited by eagles and a low-mobility pattern with a gradual increase in movements that is highly individual-specific. In general, forests occupied the largest share of the home range of eagles, ranging from 0.38% to 87.45%, with a mean value of 49.47% ± 26.57. The duration of the post-fledging dependence period and the size of the ranging area were constrained by the onset of first migration, coinciding with the beginning of birds’ independence. The home range overlap of eagles varied from 56.35% to 100%, with an average value of 87.94%. The high similarity among ranging areas identified in this study emphasizes the importance and consistency of environmental conditions, such as habitat features, compared to others that would vary yearly. The key aspects (spatial and temporal) of the post-fledging dependence pattern, identified by our study, are important for species conservation and must be integrated into future landscape planning. Our results also contribute to a better understanding of the complex factors shaping raptors’ space-use pattern in this key, a poorly studied life-history trait of raptors, thus enhancing knowledge for similar species of conservation concern.