Complete adoption of unrelated broods by foster parents is surprisingly common among birds and the behaviour appears to be maladaptive in several species. In a small, marked, southern African population of African Marsh Harriers Circus ranivorus, studied over 4 years, a young male took over the complete parenting (provisioning and defence) of an unrelated brood, and several times fed the brood in preference to the soliciting maternal parent. This female then deserted and deceitfully courted (out of season) a third male, whose provisioned food she took to feed to her own young. Since the adopting male retained the territory and bred successfully for at least two further years, he suffered no expected loss in fitness and hence this case of fostering was neither maladaptive nor apparently altruistic.