The Anomalies: The Acorn Woodpecker

These highly social birds defy the typical two-parent family structure, proving that cooperation can make good evolutionary sense.

Acorn woodpeckers (Melanerpes formicivorus) aren’t exactly shrinking violets. Between their bold plumage and exuberant calls, the birds practically scream for attention. (Famed animator Walter Lantz once claimed that his best-known cartoon creation, Woody Woodpecker, was inspired by a particularly intrusive acorn woodpecker that wouldn’t stop pecking on the roof of his cabin—during his honeymoon!) But these charismatic birds are notable for much more than just their first impression. As their name implies, acorn woodpeckers rely heavily on acorns for sustenance. To make sure this seasonal resource remains available throughout the year, the birds build enormous “granaries” by drilling thousands of holes into their oak-tree homes and stashing a single nut securely inside each hole. Since just one of these holes takes an average of 20 minutes to drill, the birds fiercely defend their granaries, and reuse them from year to year. The largest granary found to date was riddled with individual compartments for more than 50,000 acorns.

Unlike other woodpeckers—or virtually any other birds—acorn woodpeckers live in complex family groups numbering up to 15 adults, all of which work together to raise chicks in a single nest. This surprising discovery led scientists in the 1920s to declare that there was “communism” at play in the species. Biologist Walter Koenig, a professor emeritus at Cornell University, has been studying acorn woodpeckers at the Hastings Natural History Reservation in Carmel Valley, California for more than 40 years to better understand this complex social structure. Every summer, Koenig and his colleagues capture and band hundreds of new woodpecker chicks. Using color-coded leg bands, the researchers are able to identify and follow individual birds and their families year after year. The huge dataset they have amassed over four decades—which now includes genetic data to help the scientists better understand relatedness among individuals—has yielded surprising answers to one of evolutionary biology’s biggest questions: Why cooperate?

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