Intraspecific adoption and foster feeding of fledglings in the North Island robin

Intraspecific foster feeding and adoption has rarely been observed in birds, with the exception of waterfowl. In this study, I document for the first time the existence of intraspecific foster feeding and adoption of fledglings by adult passerines with their own young. During a three-year study of the North Island robin (Petroica longipes), a species with very low levels of extra-pair paternity, eight fledglings (4% of the fledglings in the population the study years) were fed by adults other than their parents, with four of these being adopted. In cases of foster feeding and adoption in this species: 1) adopted fledglings came from parents with lower feeding rates than the population average; 2) the territories of adopting/foster parents were of higher quality than the fledglings original parents’ territories; 3) in all cases the adopting/foster-feeding parent was a male; 4) adoption of a fledgling was associated with a very low survival of the foster parent’s own fledglings; 5) the original parents of the adopted fledglings had a higher reproductive output than the general population. Although the frequency of adoption/foster-feeding was very low, the findings suggest that both the original parents and the fledglings may have a fitness payoff from this behaviour with the fledgling most likely being the active participant. Foster parents appear to be victims of this phenomenon, with it reducing their life-time reproductive output. This begs the question of why selection does not act to improve recognition of one’s own young. One possibility is that males are trapped in a situation where better discrimination of offspring is more costly than any benefits it may bring. To better understand the complex phenomenon and possible adaptive explanations for adoption, the perspective of all participants, juveniles, original parents and the foster parents, need to be considered.