Natal and breeding dispersal in a reintroduced population of White‐tailed Eagles Haliaeetus albicilla

Abstract

Capsule Natal dispersal distance was significantly shorter in males than in females. Aim To examine the correlates of variation in dispersal in a reintroduced population of White‐tailed Eagles Haliaeetus albicilla in western Scotland. Methods Observations of natal (or release) sites and subsequent breeding sites of individually marked birds were used to calculate natal dispersal distance (NDD; the distance between natal (or release) site and first breeding site) and breeding dispersal distance, which is the distance moved by adults between sites of reproduction. Results NDD was significantly shorter in males than in females. Male NDD did not change over the 25+‐year study as the population expanded. Female NDD appeared to increase over time. Brood size at fledging and fledging date were not associated with NDD in either sex. There was no evidence of an association between parent and offspring NDD. Breeding dispersal was uncommon and involved moves to neighbouring territories when it did occur. Conclusions In White‐tailed Eagles, like the vast majority of animals, most movements affecting gene flow and demography result from natal dispersal. Our finding that NDD was lower in males than in females was consistent with the hypothesis that males compete for resources in order to attract females, and hence there is a greater selective advantage for males to stay closer to their natal sites, whereas females choose between the available resources of different males and so can disperse further. The apparent increase in female NDD over time was probably because, when first reintroduced, all birds shared the same ‘natal’ (release) site and female NDD was initially constrained to follow male NDD (since males settle at breeding sites earlier than females). Later, however, greater female NDD could be expressed in an expanded population with greater range of natal and breeding sites. There was little evidence that NDD of White‐tailed Eagles in western Scotland is being affected by increasing population density, suggesting that carrying capacity is far from being realized.