Begging and cowbirds: brood parasites make hosts scream louder

Avian brood parasites have evolved striking begging ability that often allows them to prevail over the host progeny in competition for parental resources. Host young are therefore selected by brood parasites to evolve behavioral strategies that reduce the cost of parasitism. We tested the prediction that the intensity of nestling begging displays functioning to attract parental care increases across species with the frequency of parasitism by the brown-headed cowbird (Molothrus ater). This was expected because host young should try to prevail over highly competitive parasitic broodmates in scramble interactions, act more selfishly when frequency of parasitism is high because brood parasites often affect more severely host condition than conspecific broodmates, and discount the kin selection costs of subtracting resources to unrelated parasites. Across 31 North American host species, begging loudness positively covaried with parasitism rate in Passerines, and such effect was stronger in species with small compared with large clutches. Begging loudness increased with brood parasitism and nest predation among the most suitable host species. These results held after controlling for concomitant ecological factors and for common ancestry effects. Our results support the hypothesis that avian brood parasitism has played a role in the evolution of begging behavior of host young.