Shiny Cowbird parasitism of a low quality host: effect of host traits on a parasite's reproductive success

ASTIÉ, ANDREA ALEJANDRA; REBOREDA JUAN CARLOS

JOURNAL OF FIELD ORNITHOLOGY

Wiley-Blackwell

Año: 2009 vol. 80 p. 224 - 233

0273-8570

The reproductive success of parasitic cowbirds (Molothrus spp.) can vary greatly among hosts and some of the characteristics that affect it are the extent of synchrony between parasitism and host laying, the length of the incubation periods of parasite and host, and the ability of host to incubate and rear young. We studied the reproductive success of the Shiny Cowbird (Molothrus bonariensis) in nests of the Creamy-bellied Thrush (Turdus amaurochalinus). Shiny Cowbirds frequently parasitized Creamy-bellied Thrush nests (60%) but cowbird reproductive success in this host was quite low. Shiny Cowbirds synchronized parasitism with host laying in 79 % (N = 47 nests) of the cases, and approximately 80 % of the parasitic eggs (N = 28) hatched within one day of the hatching of the first host egg. More than 91 % of the eggs (N = 26) survived until the end of the incubation, but only 60 % of them (N = 70 nests) hatched and 52 % of young (N = 42 nests) survived. The proportion of Shiny Cowbirds eggs that fledge in successful Creamy-bellied Thrush nests was 0.28. This value is the lowest observed for hosts of this parasite. When we considered loss by predation or desertion only 3 % of cowbird eggs produced fledglings. Failure to hatch was not associated with number of eggs present at the nest but with asynchrony between parasitism and host laying. The survival of cowbird young was negatively associated with the number of host chicks. Although Shiny Cowbird success in Creamy-bellied Thrush nests was very low, this host was heavily parasitized. We suggest that high frequency of parasitism could be the result of diffuse selection or because Shiny Cowbird chicks could have a better performance in Creamy-bellied Thrush nests in years or areas where they do not suffer brood reduction.