Impact of low and high frequencies of brood parasitism by Shiny Cowbirds on Rufous-bellied Thrush reproductive success.

LLAMBÍAS PAULO E; FERRETTI VALENTINA; SACKMANN, PAULA; 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 examined the impact of brood parasitism by Shiny Cowbirds (Molothrus bonariensis) on two nesting populations of Rufous-bellied Trushes (Turdus rufiventris). In population A, the frequency of parasitism was 65.8% (25 of 38 nests) and the number of cowbird eggs per nest was 1.68 ± 0.19, while in population B the values were 21.7% (13 of 60 nests) and 1.27 ± 0.18 respectively. Parasitism reduced Rufous-bellied Trush reproductive success mainly as a consequence of egg losses due to punctures inflicted by Shiny Cowbirds when they visited the nests. Egg punctures were more frequent in parasitized (51.4%) than in non-parasitized nests (10%), and in the population with a high incidence of parasitism (45.9%) that in the one with a low incidence (13.3%). As a result of punctures, the number of host eggs per nest at the end of the incubation stage was lower in parasitized than in non-parasitized nests and in population A than in population B. In addition, parasitized nests had a lower hatchability. There were no effects of parasitism, or intensity of parasitism, on the proportions of host chicks that fledged. Our results indicate that the main impact of Shiny Cowbird parasitism on Rufous–bellied Thrush reproductive success is the puncture of eggs. Egg punctures are positively related to the intensity of parasitism. Egg punctures are present in both parasitized and non-parasitized nests indicating that Shiny Cowbirds puncture eggs when visiting nests in which they do not lay eggs.