Title
Growth and provisioning of shiny cowbird and house wren host nestlings
Author(s)
Kattan G.H.
Published
1996
Publisher
Journal of Field Ornithology
Abstract
The brood-parasitic Shiny Cowbird (Molothrus bonariensis) parasitizes more than 200 species of birds throughout its range, many of which are small passerines. Growth rates of cowbird nestlings are expected to vary according to factors such as host size, total number of young in the nest, and rates of food delivery by the host. Here I report growth and provisioning rates of cowbird and host nestlings in House Wren (Troglodytes aedon) nests in the Cauca Valley, Colombia. Cowbirds raised singly in wren nests had high growth rates and a short nestling period. Their mass increased 10-12-fold, reaching an asymptote at age 9-10 d and fledging at 12-14 d of age. Cowbirds fledged at 65-75% of adult mass and remained small until their first molt, after which they reached adult mass. Being raised in mixed broods with wrens did not affect growth rates of cowbirds. Wrens, in contrast, increased in mass 10-fold and reached asymptotic mass, equal to adult mass, at the age of 10-11 d. They remained in the nest for an additional 7-8 d before fledging. At 10 d of age, mass of wrens in parasitized nests was significantly lower than in unparasitized nests. The mass and provisioning rates of a single cowbird were equivalent to those of a modal brood of three wrens. The combination of rapid growth and early fledging resulted in a short nestling period for cowbirds, which together with a short incubation period, form part of a suite of adaptations that allow cowbirds to outcompete their foster nestmates.
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PUB12983