EXTRA-PAIR PATERNITY IN MONOGAMOUS TREE SWALLOWS

Multilocus DNA fingerprinting of offspring of monogamous pairs of tree swallows, Tachycineta bicolor, revealed that 50% (8/16) of broods contained young sired by extra-pair males, totalling 38% (33/86) of all offspring analysed. No young resulted from intraspecific brood parasitism. The distribution of extra-pair paternity showed a bimodal pattern; either none or many illegitimate young per brood. Thirty-three per cent of illegitimate offspring were sired by three to five nearest male neighbours. Females copulated frequently with their own mate at their own box, but they were never seen copulating with other males. Hence, there was a striking contrast between the rate of extra-pair copulation and the rate of extra-pair fertilization. There was no relationship between the pair copulation rate and the proportion of illegitimate young in a brood, suggesting that males cannot use the number of copulations performed with their mate as a reliable cue to their share of paternity. Males provisioned their young at nearly the same rate as did females, and males with illegitimate offspring in their nest did not reduce provisioning effort compared to males with only legitimate offspring. Hence, females did not suffer any apparent costs of promiscuity. It is argued that females control the sperm transfer success of copulations. Because females do not receive anything other than sperm from extra-pair copulation, the promiscuous behaviour is probably driven by some genetic factors enhancing female fitness. Although the data presented seem consistent with female choice for male genetic quality, there were no detectable phenotypic characteristics of males with high fertilization success. © 1993 The Association for the Study of Animal Behaviour. All rights reserved.