For the Birds Radio Program: Acorn Woodpecker: Cooperative Breeding
Acorn Woodpecker This week I’m talking about cooperative breeding in birds. Some cooperation happens only in the form of helpers—usually the young from previous years—assisting their parents in raising new broods of young. The Acorn Woodpecker population living in California is another cooperative breeder, but the basic unit of their society is not a monogamous pair—it’s a group of these adorable, clown-faced woodpeckers all working together to establish what is called a granary—usually an oak tree that is riddled with holes, each holding an acorn. An individual granary tree may have up to 50,000 holes. Each hole is designed to hold one stored acorn or, occasionally, an almond, walnut, hazelnut, pecan, or piñon pine nut—rarely inedible items are stored, too, such as stones. In the 1930s, Acorn Woodpeckers stored a total of 62,264 acorns in the door and window casings of an unused house in the Sierra Nevada, and in a single season, they cached 485 pounds of acorns in a wooden water tank near Flagstaff, AZ. As cached acorns slowly dry up and lose their nutritional value, they are discarded and a fresh acorn placed in the hole. Not all Acorn Woodpeckers establish enormous granaries—many in Central America don’t use granaries at all—but those in major oak-areas of California make huge ones. The group of woodpeckers collecting and storing these acorns includes a breeding unit and helpers. Some populations in Arizona are almost completely monogamous, but overall most population breeding units include 1–7 male cobreeders competing for matings with 1–3 joint-nesting females who lay their eggs in the same nest cavity. Each female may mate with any or all of the males, and each male may mate with any or all of the females, so some of the four to six chicks in a single nest are not siblings. Females not quite ready to lay their own eggs usually destroy eggs in the nest, but once all the females are laying, the eggs and hatchlings are cared for by the entire group. The Acorn Woodpecker breeding system is unique in the world of birds. Considering all the multiple possibilities for pairings, there is surprisingly little mating—virtually none, in fact—between related birds. New breeders are not accepted into a unit until all the birds of one sex have died. Then the only helpers allowed to become breeders are those that immigrated to the group from elsewhere. So the daughter of any of the males will never be allowed to become a breeder while any of her potential fathers still live, and she must either wait until they all die or emigrate to another colony to have a chance at breeding herself. And the same thing is true for the son of any of the females. Some young birds remain with their family unit as helpers for as long as five years before their generation can take over. Meanwhile, all the birds spend much of the year building up and maintaining their granaries. During years when acorn production is low, they depend on stored acorns for many of their calories, though during every year they eat a wide variety of insects and other foods as well as those acorns. This is the species that made the news earlier this year when people in a wealthy California community got permits to kill the woodpeckers digging into their houses—based on just how many holes an industrious group may dig while storing acorns, you can’t help but sympathize with the homeowners, though it shouldn’t take a rocket scientist to figure out a way they could have protected their homes without killing the woodpeckers. Acorn Woodpeckers are as interesting as they are beautiful, and had property rights in California long before the first humans arrived on the scene. The settlement allowed scientists to take into captivity some of the woodpeckers to do studies on how best to keep them from digging into houses. With luck these studies will help us learn to coexist with this extraordinary and worthy bird.