Effects of brood size manipulations on sexual attractiveness of offspring in the zebra finch

In a laboratory study the effect of brood size manipulations on the sexual attractiveness of offspring was investigated. Zebra finches, Taeniopygia guttata, were reared in small or large broods. Young were exchanged so that natural siblings from different rearing conditions could be compared. Birds of both sexes reared in small broods were larger and heavier as adults; furthermore, they developed the adult bill colour sooner than birds reared in large broods. Males reared in small broods had a redder beak as adults. Males but not females reared in small broods were more attractive to opposite sex individuals, as measured in choice tests. Males with redder beaks were more attractive to females. No relation was found in females between attractiveness and measured traits. Sexual attractiveness may thus be a potentially important mechanism by which brood size affects fitness. (C) 1996 The Association for the Study of Animal Behaviour.