High rates of cowbird parasitism select for complementary host defenses

M.E. MERMOZ; REBOREDA,JC; FERNÁNDEZ, G. J..

THE CONDOR

COOPER ORNITHOLOGICAL SOC

Lugar: Chicago; Año: 2013 vol. 115 p. 910 - 920

0010-5422

Hosts of brood parasites may have not developed antiparasitic defenses either because they are recently sympatric or because costs of potential defenses outweigh their benefits. We studied antiparasitic defenses of Brown-and-yellow Marshbird (Pseudoleistes virescens) against Shiny Cowbird (Molothrus bonariensis). We evaluated recognition and aggression toward female and male Shiny Cowbirds, estimated rejection rates of cowbird eggs of different colors, and tested the effect of parasite egg size on host rejection. We also observed and video-recorded host nests to estimate host nest attentiveness, frequency of cowbird visits and interactions between marshbirds and cowbirds. When marshbirds were faced with dummy models, they attacked first and more intensively those of female and male cowbirds than those of a control species. Frequency of egg ejection increased as differences between cowbird and spotted host eggs increased (immaculate>intermediate>spotted) and spotted eggs were ejected more frequently when laid before rather than during or after host laying. Marshbirds ejected artificially added immaculate eggs independently of their size. Shiny Cowbirds only visited marshbird nests at the egg stage. Host nest attention was low during egg laying and increased during incubation and after hatching, but aggressiveness against cowbirds was always high. However, nest defense was inefficient as losses due to cowbird egg pecking were high. Ejection of cowbird eggs prevented the cost of lower survival of marshbird nestlings in highly-parasitized nests. As this defense is cost-free, this small benefit would be sufficient to select for the evolutionary maintenance of egg ejection.