Active facilitation of helper dispersal by parents and siblings in the cooperatively breeding acorn woodpecker

Abstract

Offspring that delay dispersal in cooperatively breeding species have been hypothesized to gain direct fitness benefits via parental facilitation—being passively tolerated on their natal territory by their parents—thereby enjoying enhanced survival and increased probability of acquiring a breeding position in the population. Here we describe active facilitation in the acorn woodpecker (Melanerpes formicivorus) by parents and siblings assisting the dispersal of helpers in their social group. Helpers in this species compete for reproductive opportunities in “power struggles” that take place when all breeders of one sex die or disappear, creating a reproductive vacancy. Individuals compete at power struggles in coalitions of relatives, in which larger coalitions are more likely to be victorious. Based on observations of banded birds, we found that an estimated 26% of individuals competing as part of a winning coalition at a power struggle returned to their home territory at its conclusion, suggesting that they were facilitating the dispersal of kin (generally parents or siblings) that stayed to become breeders on the new territory. In at least one group, sibling facilitation was reciprocated; that is, a bird that was helped at a power struggle by a sibling joined that same sibling as part of a coalition at a subsequent power struggle. Dispersal facilitation is a novel means by which parents can nepotistically enhance the direct fitness of offspring and siblings can enhance each other’s inclusive fitness in this highly social species. Significance statement Parental facilitation—being passively tolerated on the natal territory—may provide significant direct fitness benefits to helpers in cooperatively breeding species. We describe active facilitation of helper dispersal in the acorn woodpecker, where helpers compete in coalitions for reproductive opportunities at “power struggles” following the death or disappearance of all breeders of one sex. About one-fourth of individuals—including both parents and siblings—competing at power struggles were apparently facilitators who assisted related helpers by participating in the power struggle but then returned to their home territory rather than stay to breed on the new territory. In at least one group, dispersal facilitation was reciprocated; that is, a bird that was helped at a power struggle by a relative later joined that same relative as part of a coalition at a subsequent power struggle. Active dispersal facilitation by parents and siblings is an important, previously unrecognized, form of nepotistic behavior in this highly social species.