Abstract
(1) In the course of a study of the social behavior of Azure-winged Magpies Cyanopica cyana in 1984 and 1985 at Azumino, Nagano Prefecture, we found heavy brood parasitism by Common Cuckoos.
(2) A total of 31 nests out of 41 (75.6%), among core groups and 15 nests of 30 (50.0%) in secondary groups were parasitized. Eight nests contained two, and one contained three cuckoo eggs.
(3) The earliest magpie nests, in which the first egg was laid during the middle of May, were parasitized less frequently (11.1%) than later nests (68.8%).
(4) Cuckoo eggs (23.4×17.4 mm, n=13) were smaller than Azure-winged Magpie eggs (27.67×20.16 mm, n=16).
(5) Azure-winged Magpies suffered heavy brood parasitism by Common Cuckoos probably because of the short history of the host-parasite relationship between them, which is as a result of a recent expansion of their breeding areas in Nagano Prefecture.
