Low success of shiny cowbird chicks parasitising rufous-bellied thrushes, chick competition or parental discrimination?

LICHTENSTEIN, G

ANIMAL BEHAVIOUR

Academic Press

Lugar: Londres; Año: 2001 vol. 61 p. 401 - 413

0003-3472

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 parasitising 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. Video tapes were taken of parasitised broods containing a shiny cowbird raised with one host chick; and non-parasitised broods containing a big 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 parasitised and non-parasitised nests. In non-parasitised nests parents fed the chick that reached the highest. However, in the parasitised nests, the 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 the nest 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.