Spatial behaviour of Spanish Imperial Eagle Aquila adalberti juveniles during the dependence period revealed by high-resolution GPS tracking data

Abstract

During the post-fledging dependence period, young raptors are particularly vulnerable as they are still developing flying, hunting and social skills, and collecting spatial environmental information required for emancipation. We studied the use of space during this critical period by the highly threatened Spanish Imperial Eagle Aquila adalberti, using high temporal and spatial resolution tracking devices. Ten juveniles from four different nests were tracked throughout their dependence period with GPS/GSM tags. This period lasted until a median age of 137 days (93–153). Juveniles remained within a stable home range with a median of 232 ha, but when 90–100 days old began making exploratory movements that became more frequent and extensive with the approach of emancipation. These movements had a median of 22.2 km and were usually completed within the same day, but this exploratory behaviour proved to be quite plastic, as one movement was 1291 km long and lasted 4.5 days. Moreover, three individuals that were apparently forced to emancipate particularly early seem to have virtually skipped the exploratory phase. The length of the movements varied among years within the same nest, and those of females were about twice those of males. Most movements were towards the NW, and the outgoing flight was usually against the wind. This behaviour may keep animals from flying so far from the nest that they are then unable to return. The high spatial and time resolution provided by GPS tracking resulted in data that highlight the conservation importance of the nest site until well after fledging. Measures to minimize nest disturbance should thus be extended until early October. Moreover, data show that so much of the activity of the still inexperienced juveniles is concentrated in the small pre-emancipation range that is critical to protecting its habitats and eliminating all anthropogenic sources of mortality.