Stop-over of migrant Blackcaps (Sylvia atricapilla) on autumn passage through the Polish Baltic coast

In this analysis, data on Blackcaps caught on autumn migration at the Operation Baltic stations: Bukowo-Kopań (54°28'N, 16°25'E) in 1984-1999 and Mierzeja Wiślana (54°21'N, 18°19'E) in 1982-1999 (in period: 14 August - 1 November) were used.

Total percentage of stopping-over (retrapped) Blackcaps was 11.6% at Mierzeja Wiślana and 7.9% at Bukowo-Kopań station. On average birds stayed longer at Mierzeja Wiślana (median - 5 days) than at Bukowo-Kopań (median - 4 days). Most probably, the Blackcap stop-over length and frequency depended on the food abundance in the area. Initial weight and fat load of birds retrapped later in the season were on average lower than of those caught only once, what suggests that birds stop-over to replenish their energy deficits. Weight and fat changes were influenced by the stop-over length. Individuals that left early, i.e. 1-2 days after ringing, not only did not gain energy, but also lost the reserves they had on their arrival. As the season progressed, birds tended to stop-over for a shorter time. The density of individuals already present in the studied area did not affect the decision of migrating birds to stop-over.