JUVENILE DISPERSAL OF WHITE-TAILED EAGLES IN WESTERN SCOTLAND

In long-lived raptors with delayed maturity, the period between fledging and settlement on a breeding site may take several years and is poorly understood. In our study of a reintroduced population of White-tailed Eagles (Haliaeetus albicilla), we investigated movements during this transient (juvenile dispersal) phase of natal dispersal using records of wing-tagged birds. Maximum juvenile dispersal distance (JDD), as measured from natal (or release) sites to locations recorded prior to birds settling on a breeding site ranged from 18 to 200 km in different individuals. Observation records suggested that (1) the most extensive movements occurred in the first two years after fledging, (2) males initially dispersed further than females, but, in their second year, females tended to be further from their natal site than males, and (3) its males were closer to their natal sites than were Females. Maximum JDD did not differ between wild-bred and released (reintroduced) birds and there was no indication that it was correlated with subsequent recruitment to the breeding population. We also found no evidence that a bird's fledging date, body size, or the brood size ill which it was fledged affected maximum JDD. There was no suggestion that Scottish White-tailed Eagles showed seasonally dependent returns to their natal areas. Maximum JDD during the first two years after fledging and natal dispersal distance (i.e., straight-line distance between natal and first breeding sites) were positively correlated in both females and males. The implication of these results was that, well before reaching maturity, White-tailed Eagles used dispersal movements, at least in part, to assess potential breeding sites, and thus dispersal behaviors were not simply directed toward survival alone.