High pre-fledging but low post-fledging success of shiny cowbirds parasitizing bay-winged cowbirds

DE MÁRSICO, M.C.; REBOREDA, J.C.

Campos de Jordao, Brasil

Congreso; XXV International Ornithological Congress; 2010

International Ornithological Committee

The shiny cowbird (Molothrus bonariensis) is one of the most generalist brood parasites at the species level. However, shiny cowbirds only rarely parasitizes the bay-winged cowbird (Agelaioides badius), a species sympatric with many of their primary hosts. The low frequency of parasitism in bay-winged cowbird nests cannot be explained by the presence of effective antiparasitic defences because bay-winged cowbirds neither reject parasite eggs nor show differential aggression toward shiny cowbird females. We tested the hypothesis that shiny cowbirds do not use bay-winged cowbirds because the parasite’s reproductive success is lower than in other frequently parasitized hosts. The study was conducted in the Province of Buenos Aires, Argentina, between 2003 and 2009. We artificially parasitized bay-winged cowbird nests with shiny cowbird eggs or newly hatched chicks and followed their fate throughout the host nesting cycle. Our results showed that shiny cowbird egg survival, hatching success and chick survival were similarly high in bay-winged cowbird nests and in two primary hosts of the same study area, the chalk-browed mockingbird (Mimus saturninus) and the house wren (Troglodytes aedon). Shiny cowbird chicks in bay-winged cowbird nests grew faster and reached a higher asymptotic weight than host chicks, and growth parameters where comparable to those observed in the primary hosts. However, shiny cowbird fledglings did not receive further parental care from bay-winged cowbirds once they left the nest, indicating that mimicry between parasite and host young is critical for the parasite’s post-fledging survival. Low post-fledging survival may explain why shiny cowbirds rarely parasitize bay-winged cowbirds.