Free love and family conflict in cooperative Acorn Woodpeckers
An Acorn Woodpecker granary may supply food for as many as 15 birds.
Many oak trees in central California’s Hastings Reserve bear the scars of thousands of acorn-stuffed holes. Families of Acorn Woodpeckers drill, fill, and feed from these honeycombs of energy, allowing them to stay the winter and pass on territories through dynasties spanning many generations.
Walter Koenig, senior scientist at the Cornell Lab of Ornithology, has been studying this fascinating population since 1974. He notes, “Acorn Woodpeckers exhibit some of the most bizarre social behavior on earth. Mate-sharing, group sex, infanticide, and acorn storing on a monumental scale—it’s all in a day’s work for these clown-faced denizens of the Southwest.”
Acorn Woodpeckers at Hastings breed cooperatively in one of the world’s few polygynandrous mating systems. Families of as many as 15 birds defend a territory with one nest and a granary tree drilled with holes. These birds waste no time courting, both sexes freely sharing mates within social groups. One to four related males form a coalition to nest with up to three females from a different group. Meanwhile, offspring from previous years delay dispersal and become nonbreeding helpers, drilling holes, foraging and storing acorns, guarding the granary, and helping care for young.
Each generation adds to these growing granaries, amassing as many as 30,000 holes, providing a valuable resource for birds remaining at home in the group. Granaries require a lot of time to establish and maintain and are almost always occupied. Such “habitat saturation” makes it difficult for young birds to colonize new territories and start new families. A bird that stays can use its group’s resources until a better opportunity comes along.
Incest avoidance plays an important role in structuring Acorn Woodpecker groups. When all the breeders of one sex die or disappear, a reproductive vacancy opens. Helpers from outside groups compete for open positions, often in coalitions of same-sex siblings. Their wars can last for days, with as many as 50 birds chasing, scolding, and displaying for the chance to join the family and breed. When new breeders of one sex win the battle, helpers of the opposite sex from the original family can inherit breeding status, joining their remaining same-sex parent(s) to mate with the opposite-sex newcomers. This strategic inheritance allows resources to be passed through generations while avoiding consequences of inbreeding.
Securing a breeding position is only the beginning of reproductive competition. Co-breeding females lay eggs slightly out of sync, and it doesn’t pay to go first. If a female finds an egg in the nest before she has laid her own, she will remove and begin to eat it. Soon the entire group, including the female that laid it, will feast on the egg. This abruptly stops once each female has laid at least one egg; then they all settle down to share the tasks of incubation and care.
Group living incurs other costs. Relatively few birds get to breed, competition continues even after winning a reproductive slot, and no one knows for sure whose chick is whose. With all this conflict and parental uncertainty, why live in groups at all?
For Acorn Woodpeckers, the biggest factor seems to be access to granaries. Group life is prevalent in California, where both oaks and woodpeckers are abundant. Acorn Woodpeckers breeding in areas of Arizona where acorn crops are more variable spend their winters in Mexico. In spring, when pairs form, each claims its own territory.
Granary trees in the Hastings Reserve provide valuable inheritable resources in an area where virtually every territory is occupied year-round. Community life may be the best option for young birds, providing food, protection, and the possibility of future reproduction. These trees and the abundant acorns they provide act as ecological constraints, leading Acorn Woodpeckers to develop family dynasties and the complex social behaviors surrounding them.
Carly Hodes is a Bartels Science Illustration Intern.