“Habitat saturation results in joint-nesting female coalitions in a social bird”
Sahas Barve, Walter D. Koenig, Joseph Haydock, and Eric L. Walters (June 2019)
Sisters unite! Acorn woodpecker females joint nest in saturated habitats influencing fitness in both males and females
What makes female acorn woodpeckers lay their eggs in the nest of their mother or sister?
Acorn woodpeckers live in close-knit family groups and have one of the most complex breeding systems of any bird. In about 20% of family groups, up to three related females may lay eggs in a nest together and raise the chicks cooperatively with one or more related males, a behavior known as joint nesting or cooperative polyandry and known in only 0.2% of all bird species. Based on demographic data collected over 35 years (1982-2016) at the Hastings Natural History Reservation in central coastal California, the authors quantified the costs and benefits of joint nesting to attempt to explain why some woodpecker females exhibit this rare behavior. They found that the incidence of joint nesting was more common in years when the population was high, all the breeding territories were occupied, and opportunities for a female to nest on her own very unlikely. Although forming joint nests reduces the number of offspring each female can produce compared to when she nests alone, such females make the “best of a bad situation” by nesting jointly with their mother or sister rather than not nesting at all. Additionally, females that decide to nest jointly do so in groups where there are two or more breeder males, thus increasing the number of caregivers and, hence, the total number of chicks that females can successfully raise. Years of population boom may have therefore been an important mechanism driving the evolution of such highly social behaviors like joint nesting in acorn woodpeckers.
Joint nesting by females and cooperative polyandry—cooperatively breeding groups with a male-biased breeder sex ratio—are little-understood, rare breeding systems. We tested alternative hypotheses of factors potentially driving these phenomena in a population of joint-nesting acorn woodpeckers (Melanerpes formicivorus). During periods of high population density and, thus low independent breeding opportunities, acorn woodpecker females formed joint-nesting coalitions with close kin. Coalitions were typically associated with groups with a male bias. We found strong evidence for both inter- and intra-sexual conflict, as joint nesting conferred a fitness benefit to some males, a significant fitness cost to females, and no gain in per capita reproductive output for either sex. Such conflict, particularly the cost to females, may be an important reason why joint nesting is rare among cooperatively breeding taxa.