A comparative study of Shiny Cowbird parasitism of two large hosts, the Chalk-browed Mockingbird and the Rufous-bellied Thrush

It is usually accepted that generalist brood parasites should avoid using hosts larger than themselves because host chicks may outcompete parasite chicks for food. We studied the interactions between the Shiny Cowbird (Molothrus bonariensis) and two common hosts larger than the parasite, the Chalk-browed Mockingbird (Mimus saturninus) and the Rufous-bellied Thrush (Turdus rufiventris). For each host we determined (1) frequency and intensity of parasitism during the breeding season, (2) nesting success, egg survival, hatching success, and chick survival in unparasitized and parasitized nests, and (3) antiparasitic defenses. We also determined Shiny Cowbird egg survival, hatching success, and chick survival in both hosts. Parasitism reached 50% in mockingbirds and 66% in thrushes. In both species the main cost of parasitism was egg destruction through punctures. Hatching success, survival of host chicks, and nest survival did not differ between unparasitized and parasitized nests. Both hosts rejected parasitic white-morph eggs but accepted spotted-morph ones, even though they were significantly smaller than host eggs. The proportion of cowbirds fledged per egg laid in successful mockingbird and thrush nests was 0.4 and 0.6, respectively. Considering nest survival, reproductive success of Shiny Cowbirds was 0.15 in mockingbird nests and 0.17 in thrush nests. These values are similar to or higher than cowbird success with smaller hosts. Our results indicate that host quality is not only determined by host-parasite differences in body size, and that other factors, such as host defenses and nest survivorship, should be considered.