Black-headed Gulls (Larus ridibundus) were caught on the southern Baltic coast during early phase of their autumn migration between 27 July and 9 August from 1992 to 1995. Due to involvement in breeding, adult Black-headed Gull started primary replacement later than second-year birds, but their molt was slightly faster during the studied two-week period. This suggests that the difference in primary molt score between the age classes may decrease toward the end of the molt period. There were no differences in primary molt between sexes. Adults had larger hole in wing area and they had higher body condition index than second-year birds. This might reflect higher cost of flight of adults due to their lower aerodynamic efficiency of the wing and their tendency to buffer themselves against increased energetic stress. Second-year Black-headed Gulls migrate for longer distances farther than adults on average and this might be a reason of their slower rate of primary replacement.
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Vol. 30 • No. 1