BEGGING INTENSITY OF NESTLING BIRDS VARIES WITH SIBLING RELATEDNESS

Begging for food is one of the most conspicuous behaviours performed b y nestling birds. Recent models suggest that the form and intensity of begging evolved not only to communicate nutritional requirements to p arents but also as a mechanism for competing against siblings to obtai n a greater share of parental resources. In an interspecific compariso n of passerine birds, we show that the loudness of nestling begging ca lls increases as the relatedness amongst the members of a brood declin es. Species with high levels of mixed parentage, as well as the brood- parasitic brown-headed cowbird (Molothrus ater), begged louder than th eir closest monogamous and non-parasitic relatives. These results supp ort the hypothesis that sibling relatedness influences begging behavio ur in birds, and suggests that increased intensity of begging can evol ve whenever female promiscuity or brood parasitism lowers the coeffici ent of relatedness among nestmates.