Reproductive success of the specialist brood parasite Screaming Cowbird in an alternative host, the Chopi Blackbird

DI GIACOMO, A.G.; REBOREDA, J.C.

AUK

AMER ORNITHOLOGISTS UNION

Año: 2015 vol. 132 p. 16 - 24

0004-8038

The Screaming Cowbird (Molothrus rufoaxillaris) is the most specialized brood parasitic cowbird and it relies almost entirely on the Bay-winged Cowbird (Agelaioides badius) as host. Recently, Screaming Cowbirds have expanded their range towards areas where Bay-winged Cowbirds are absent and they are exploiting the Chopi Blackbird (Gnorimopsar chopi). Interactions between Screaming Cowbirds and Chopi Blackbirds are largely unexplored as is the reproductive success of the parasite in this host. All three species (Screaming Cowbirds, Chopi Blackbirds and Bay-winged Cowbirds) co-exist in the NE of Argentina, rendering an ideal system to explore interactions between a specialist brood parasite and an alternative host and to compare the reproductive success of the parasite in its main and in an alternative host. Screaming Cowbirds parasitized both hosts throughout their breeding seasons (Chopi Blackbirds: mid October-mid January; Bay-winged Cowbirds: mid November mid March). Frequency of parasitism was lower in Chopi Blackbirds than in Bay-winged Cowbirds (46% vs. 74%). Nest survival was higher in Chopi Blackbirds than in Bay-winged Cowbirds (37% vs. 15%). In successful nests, survival of Screaming Cowbirds eggs and chicks were high and relatively similar in both hosts (Chopi Blackbirds: 99% and 90%; Bay-winged Cowbirds: 93% and 93%), but hatchability was lower in Chopi Blackbirds than in Bay-winged Cowbirds (52% vs. 92%). Considering: 1) nest survival, and 2) egg survival, hatchability and chick survival in successful nests, the reproductive success of Screaming Cowbirds (i.e. proportion of eggs that resulted in fledglings) was 0.17 in Chopi Blackbirds and 0.12 in Bay-winged Cowbirds. Our results indicate that the Chopi Blackbird is a frequent host of the Screaming Cowbird and parasitism of this alternative host may help to explain the range expansion of this parasite in areas of Brazil where the Bay-winged Cowbird is absent.