Polygyny in a socially monogamous monomorphic species: The Rufous-bellied Thrush.

FERRETTI VALENTINA; LLAMBÍAS PAULO E; CASSARÁ, CAROLINA; FALIBENA, AGUSTINA; SOMOZA, GUSTAVO; REBOREDA, JUAN C.

St. John's, Newfoundland

Congreso; 118th Stated meeting of the American Ornithologists' Union in conjunction with the 19th Annual Meeting of the Society of Canadian Ornithologists' Union; 2000

We studied the mating system of an apparent socially monogamous, monomorphic species, the Rufous–bellied Thrush, Turdus rufiventris. The study was conducted at a suburban area 50 km NW of Buenos Aires, Argentina during the 1998 breeding season (September-December). We followed 18 nests from the building stage until either the chicks fledged or the nest failed. In these nests we monitored the behavior of the adults attending the nest during the incubation and nestling periods using focal sampling and video recording techniques. All adults were morphologically, behaviorally and/or genetically sexed and were banded. We observed biparental care in 10 nests while in the other cases there was either only one female attending the nest (four cases) or one female which shared the male with a neighbor female (four cases involving two polygynous males). In the nests attended by only one female we could not assess whether the male was predated or they were other undetected cases of polygyny. Each polygynous male built both nests (1 and 3 m apart from each other) and shared feeding with both females but only one of the females fed the fledglings of these males. The sex ratio of the population under study was not biased towards females. We postulate that heterogeneity in the quality of nesting territories could be the cause of these cases of polygyny.