The contribution of flight capability to the post-fledging dependence period of golden eagles

The period prior to an individual emigrating from its natal site and initiating dispersal is important for developing the skills that are ultimately required for surviving natal dispersal. Using a novel method to quantify the early movements of 35 juvenile golden eagles fitted with satellite transmitters, we hypothesised that variation in golden eagles' post-fledging dependence period (PFDP) was determined by variation in how quickly movement skills were acquired in order to become independent and disperse. Twenty nine young eagles exhibited an initial increase in mobility (ontogenic phase') levelling off after a median of 68 d, followed by a period of maintained mobility (maintained phase') that lasted a median of 99 d (range 24-176). Eagles that developed their mobility more quickly during the ontogenic phase had a correspondingly shorter ontogenic phase. Despite this, most of the inter-individual variation in the length of the PFDP resulted from variation in the length of the maintained phase'. In general, females (the larger sex) developed more slowly and had a longer ontogenic phase. Males exhibited dispersal strategies around two modes with some dispersing early (mode 1: 89.5 d) and others late (mode 2: 220 d). In contrast females dispersed around a unimodal distribution of timing (mode 167.5 d). Apart from six individuals (mostly males) which dispersed with no discernible maintained phase, most offspring remained in the parental home range after they were fully mobile, even those that developed mobility quickly, suggesting that the PFDP in golden eagles is not simply a function of physical capacity for independence, but also a period when young eagles decide to remain, and are tolerated by parents, in their parental home range before dispersing. We suggest that delaying dispersal may be a beneficial strategy for young raptors facing a competitive environment after PFDP.