The adaptive significance of reoriented migration of chaffinches Fringilla coelebs and bramblings F. montifringilla during autumn in southern Sweden

Abstract

Reoriented autumn migration of chaffinches and bramblings occurs regularly in southernmost Sweden. The reoriented birds fly in a northeasterrly direction from the coast and inland, i.e. approximately opposite to the normal autumn migration direction. The daily peak of reoriented finch migration, as observed at inland sites 20–40 km from the coast, occurs on average 3.5 h later than the early morning departure in the normal migratory direction, and 1 h later than the peak of migration at the coast. According to trapping data the average weight of reoriented migrants and birds interrupting their migration at the coastline is significantly lower than the weight of migrants proceeding in the normal direction, and the proportion of yearlings seems to be larger in the former category. Censuses of flocks of resting finches showed that they mainly forage at stubble fields of summer rape Brassica napus, preferably fields surrounded by wooded vegetation offering shelter from predator attacks. Preferred food and habitats are mostly located inland, 20 km or more from the coast. These findings are consistent with the interpretation that reorientation constitutes an adaptive response by migrants with small fat reserves. When confronted with an ecological barrier, they return to suitable resting sites for restoring the fat reserves before crossing the barrier.