Differential spatial migration programmes are both sex and age specific for migratory great cormorants

Differential migration strategies in the same population provide an opportunity to investigate the development and constraints of the migratory programme. We used data from birds ringed from the large Danish breeding population of Great Cormorants, Phalacrocorax carbo sinensis, to investigate age- and sex-specific differences in migratory patterns, which may inform our understanding of the ontogeny of partial migration schedules or protandric advantages of males arriving at breeding grounds in advance of females. We tested spatial and temporal variation in migration strategy from migration distance, orientation and by region. We observed clear partial migration strategies in the combined data, and by group, following post-breeding dispersal. We found differential migration patterns by age, with migratory adults departing earlier than birds on their first migration. There was a tendency for a greater concentration of adults utilising sites in central Europe, while a higher than expected proportion of young birds occupied the periphery of the wintering range. Re-encounters of birds of known sex during autumn and winter indicated that males remained closer to the breeding site than females, with sex-specific regional segregation. While our results did not support the migration pattern of young birds being governed by an innate control of both direction and distance, it may be explained in part by intra-specific competition driving the resultant non-breeding distribution. This social dynamic may be extended to the differential migration patterns between the sexes. Furthermore, the sex-specific wintering distributions may explain the development of protandry within the partial migration system.