We studied the breeding biology of the Baywing (Agelaioides badius), a shared host of Screaming (Molothrus rufoaxillaris) and Shiny (M. bonariensis) cowbirds. We monitored 193 nests from December 2002 to March 2007 in the Province of Buenos Aires, Argentina. Baywings used a wide variety of nesting sites, mainly old nests of furnarids. Their breeding season lasted from late November to February and was closely matched by that of Screaming Cowbirds. The breeding season for Shiny Cowbirds started in late September but overlapped that of Baywings. Frequency and intensity of Screaming Cowbird parasitism were 93% and 5 eggs per parasitized nest, while for Shiny Cowbirds they were 16% and 1.4 eggs. Host clutch size was 4.0 ± 0.1 eggs and did not vary with time of breeding. Weight at hatching and age of maximum growth were similar for host and Screaming Cowbird nestlings. Shiny Cowbird nestlings had higher weight at hatching and lower age of maximum growth than the other two species. Screaming and Shiny cowbird nestlings had higher growth rates and asymptotic weights than host nestlings. Sex-specific growth curves of Screaming Cowbirds indicated males had higher growth rate and asymptotic weight than females. Only 19% of the nests produced fledglings. Host egg survival, hatching success, and nestling survival were 0.92, 0.88, and 0.94, respectively. Excluding nest failures, hosts fledged 0.78 chicks per egg laid. Baywings were smaller than Screaming and Shiny cowbirds, and experienced a high frequency and intensity of parasitism. However, the effect of parasitism on host hatching success and chick survival was low and comparable to that observed in larger hosts.
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