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The adult annual survival rate of breeding Blue Tits Parus caeruleus decreased from 0.485 to 0.376 after a pair of Sparrowhawks Accipiter nisus started to breed in the study plot in 1990, whereas in two neighbouring plots, survival rates did not change. This decrease was likely caused by the Sparrowhawk predation mainly in spring. Although the size of the Blue Tit breeding population did not decrease, there remained fewer nonbreeding floaters, and as a result, replacement polygyny became more common.