Host eviction vs. host tolerance by avian brood parasites

KACELNIK, A.; GLOAG, R.; REBOREDA, J.C.

Campos de Jordao, Brasil

Congreso; XXV International Ornithological Congress; 2010

International Ornithological Committee

Some brood parasitic birds evict or destroy all host eggs and/or chicks from their adopted nests, while others either don’t at all or only do it partially. It has been suggested that for one parasite, the brown-headed cowbird Molothrus ater, tolerance of host chicks may be adaptive, since parasite growth rates were steeper in nests where some host offspring remained than in those where the parasite chick was raised alone. In another partial tolerant, the shiny cowbird M. bonariensis, growth rates of parasites as a function of number of host’s offspring were found to be a function of relative size between parasite and host: additional chicks of smaller host species were beneficial to the parasite while chicks of larger host species were not. We model this on the basis of a tradeoff between two known effects of begging: the induction of a delayed increase in parental provisioning that benefits all brood members, and the increase in the proportion of total provisioning instantaneously captured by the begging chick. We assume that both functions are non-linear, and that the effectiveness of begging in both respects differs between host and parasite chicks as a function of their relative body mass. For some parameter combinations the model predicts that the parasite’s maximal intake occurs at either extreme (namely no host left or as many as possible), while for others intake is maximal at intermediate brood sizes. The model formalizes previous verbal arguments and provides a predictive tool to compare dyads of parasite-host species.