Begging for food is one of the most conspicuous behaviours performed by nestling birds. Recent models suggest that the form and intensity of begging evolved not only to communicate nutritional requirements to parents but also as a mechanism for competing against siblings to obtain a greater share of parental resources. In an interspecific comparison of passerine birds, we show that the loudness of nestling begging calls increases as the relatedness amongst the members of a brood declines. Species with high levels of mixed parentage, as well as the brood-parasitic brown-headed cowbird (Molothrus ater), begged louder than their closest monogamous and non-parasitic relatives. These results support the hypothesis that sibling relatedness influences begging behaviour in birds, and suggests that increased intensity of begging can evolve whenever female promiscuity or brood parasitism lowers the coefficient of relatedness among nestmates.