Low success of shiny cowbird chicks parasitizing rufous-bellied thrushes: chick–chick competition or parental discrimination?

The interaction between parasitic chicks and their hosts can have three outcomes: parasitic chicks might be favoured over the host young, disfavoured, or hosts might behave towards the parasitic chicks as towards their own young. I tested these hypotheses in a study of shiny cowbirds, Molothrus bonariensis, parasitizing rufous-bellied thrushes,Turdus rufiventris . Parasitic chicks were fed significantly less than the host young. I tested whether this was because parasitic chicks were outcompeted by the larger host young, or were disfavoured by the host adults. I videotaped parasitized broods containing a shiny cowbird raised with one host chick and nonparasitized broods containing a large and a small thrush that matched the size difference between the host and the parasitic chick. Parents used different rules for feeding chicks in the parasitized and nonparasitized nests. In nonparasitized nests, parents fed the chick that was positioned highest in the nest. However, in the parasitized nests, host chicks had a significantly greater chance of getting fed if they were higher or at the same height as the cowbird. When the cowbird chick was positioned higher than the thrush chicks, there was no difference between the chicks in the probabilities of their being fed. In addition, even when parents offered food to the cowbird chick, they then removed it from the cowbird's gape to feed their own chick as soon as it started begging. These results suggest that the poor success of shiny cowbird nestlings at nests of rufous-bellied thrushes is not simply due to competition with their larger nestmates, but may also involve parental discrimination. I discuss why hosts with chicks larger than the parasitic chicks may be more likely to evolve discrimination against parasitic chicks.