Prospecting movements during the transit phase of immature eagles are driven by age, sex and season.

Background: Dispersal includes three phases: emigration, transit, and immigration. The transit phase, which involves all movements between departure and arrival, is the least understood phase of dispersal. During the transit phase, individuals prospect their environment to gather information about potential breeding sites, thus enhancing their future reproductive success and survival. Studies have revealed a wide inter-individual variability in prospecting behaviours which may result from complex interactions between external and internal factors affecting the costs and benefits of prospecting. Age, sex, and season are expected to strongly influence prospecting behaviours, yet their effects are far from established.

Methodology: We investigated how age, sex, and season interact and influence prospecting movements throughout the transit phase. We analysed telemetry data from 106 immature Golden eagles (Aquila chrysaetos), whose natal dispersal involves a transit phase lasting several years. Using a trajectory segmentation method, we identified the areas sequentially prospected by each individual and we assessed the size, duration of use, and spacing between these areas to uncover spatio-temporal variations in prospecting behaviours.

Results: We confirmed our predictions, revealing strong influences of age, sex, and season, as well as their interactions, on prospecting movements. First, age had a significant effect on prospecting behaviours: individuals displayed a progressive spatial concentration of prospecting, consistent with patterns observed in colonial species. Second, seasonal variations were detected, with peaks of prospection in spring and autumn, likely resulting from the constraints imposed by territorial adult reproduction and weather-related flight conditions. Third we found sexual differences in movement patterns, with females prospecting over a larger spatial range than males, in line with the female-biased dispersal existing in most bird species. The level of inter-sexual differences and seasonal variations in prospecting behaviours differed depending on the age of the individuals.

Conclusions: Our work strongly supports that individuals adjust their prospecting behaviour in response to interacting intrinsic and extrinsic factors, in order to reduce prospecting movement costs while maximising the information gathered to inform their immigration decision.