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Myriam E. Mermoz, Gustavo J. Fernández
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The Screaming Cowbird (Molothrus rufoaxillaris) is a specialized brood parasite that primarily parasitizes the Bay-winged Cowbird, (Agelaioides badius; Screaming Cowbirds parasitize 80–100% of this species' nests). In contrast, the Shiny Cowbird (M. bonariensis) parasitizes more than 200 hosts. According to the differential reproductive success hypothesis, we expect that Screaming Cowbirds would have a lower reproductive success than Shiny Cowbirds when parasitizing other hosts. We assessed the breeding success of the Screaming Cowbird using an alternative host, the Brown-and-yellow Marshbird (Pseudoleistes virescens). This species is a common host of the Shiny Cowbird with 60–70% of nests parasitized, and is also regularly parasitized by the Screaming Cowbird but with lower frequency (6–20% of the nests). We compared the breeding success of Screaming and Shiny Cowbirds parasitizing this host species. No differences were found in the number of fledglings produced per egg laid between cowbird species. About 8–10% of cowbird eggs produced fledglings. The daily survival rate of Screaming Cowbird eggs was higher than daily survival rates for Shiny Cowbird eggs, but no differences were detected in the nestling daily survival rates. Moreover, we could not detect any difference in the hatching success (nestlings per egg), fledging success (fledglings per nestling), or growth rates of the two parasitic cowbird chicks. Furthermore, breeding success and growth rates of Screaming Cowbird chicks were similar to those previously reported while parasitizing the Bay-winged Cowbird. Our results are not consistent with the differential reproductive success hypothesis proposed as an explanation for the specialized parasitism of Screaming Cowbirds.

Éxito Reproductivo del Parásito de Cría Especialista, Molothrus rufoaxillaris, Parasitando un Hospedador Alternativo

Resumen.Molothrus rufoaxillaris es un parásito de cría especialista que parasita principalmente a Agelaioides badius (80–100% de los nidos son parasitados). Contrariamente, Molothrus bonariensis parasita más de 200 especies. Basándonos en la hipótesis del éxito reproductivo diferencial, esperamos que M. rufoaxillaris tenga un menor éxito reproductivo que M. bonariensis al parasitar a otros hospedadores. En este trabajo evaluamos el éxito reproductivo de M. rufoaxillaris parasitando un hospedador alternativo, Pseudoleistes virescens. Esta especie es un hospedador común de M. bonariensis con 60–70% de los nidos parasitados, y es también regularmente parasitado por M. rufoaxillaris pero con menor frecuencia (6–20% de los nidos). Comparamos el éxito reproductivo de M. rufoaxillaris y M. bonariensis parasitando esta especie. No encontramos diferencias en el número de volantones producidos por huevo puesto entre ambos parásitos. Alrededor del 8–10% de los huevos puestos por los Molothrus produjeron volantones. La tasa de supervivencia diaria de los huevos de M. rufoaxillaris fue más alta que la de huevos de M. bonariensis. Tampoco encontramos diferencias en el éxito de eclosión (pichón por huevo), éxito de emplumamiento (volantón por pichón), ni en las tasas de crecimiento de los pichones de Molothrus. Además, el éxito reproductivo de

Myriam E. Mermoz and Gustavo J. Fernández "BREEDING SUCCESS OF A SPECIALIST BROOD PARASITE, THE SCREAMING COWBIRD, PARASITIZING AN ALTERNATIVE HOST," The Condor 105(1), 63-72, (1 February 2003).[63:BSOSBP]2.0.CO;2
Received: 23 May 2002; Accepted: 1 October 2002; Published: 1 February 2003
breeding success
brood parasitism
Brown-and-yellow Marshbird
host specialization
Screaming Cowbird
Shiny Cowbird
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