Communal Parental Care by Monogamous Magpie Hosts of Fledgling Great Spotted Cuckoos

Fledglings of the brood-parasitic Great Spotted Cuckoo (Clamator glandarius) often formed groups with other fledgling cuckoos. Group size ranged from one to five fledglings that originated from one to four different host nests. Each group of cuckoo fledglings was attended by a group of Black-billed magpies (Pica pica). Frequently, a fledgling group was attended by more magpies than ones involved in nestling care. The feeding rate of fledgling cuckoos increased with the number of cuckoos per group and number of adult magpies attending the group. Flocking behavior presumably was advantageous for cuckoo fledglings because each fledgling in larger groups received more food.