A benefit of parasite egg tolerance by a host of the shiny cowbird

GLOAG, R.; FIORINI, V.D.; REBOREDA, J.C.; KACELNIK, A.

Bloomington, Indiana

Congreso; Joint Meetining of the Animal Behavior Society & International Ethological Conference; 2011

Even when parasites are host-tolerant, their eggs impose a cost of incubation and nestling rearing to the host, but while some hosts remove foreign eggs others don’t. We investigate the hypothesis that the presence of parasite eggs reduces host egg loss in subsequent parasitic attacks to a sufficient degree to compensate for the extra parental effort. Chalk-browed mockingbirds Mimus saturninus are parasitized by the host-tolerant shiny cowbird Molothrus bonariensis, which punctures one or more eggs when visiting host nests. Mockingbirds struggle to prevent puncturing and laying, but seldom remove parasite eggs once laid. We filmed cowbirds visiting mockingbird nests manipulated to contain one mockingbird egg plus zero, one or three cowbird eggs. We found that mockingbird eggs were considerably more likely to survive the larger the number of cowbird eggs in the nest. A model of the trade-off between egg loss and cost of rearing shows that given the intensity of this dilution effect, for moderate rearing costs the presence of parasite eggs confers a net advantage to the host, and may explain why egg tolerance is evolutionarily stable.