Why is this Woodpecker Disguised as Another Woodpecker?

K. J. Zimmer and R. J. Moller
K. J. Zimmer and R. J. Moller / K. J. Zimmer and R. J. Moller

In a lineup, you might have a hard time telling a lineated woodpecker, a robust woodpecker, and a helmeted woodpecker apart. Don’t feel bad, though, because even biologists who study these birds have been duped by their close resemblances to each other and have only recently sorted them out.

Scientists used to think that the trio of South American species were all closely related based on their shared physical characteristics, and placed the lineated and helmeted woodpeckers together in the genus Drocopus. In 2010, though, biologist Mark Robbins began to rethink that when he spotted a helmeted woodpecker in Brazil and heard its call. The bird’s song didn’t sound anything like its supposedly close cousin’s tune, but more like that of another group of birds, the Celeus woodpeckers. 

Biologist Kevin Zimmer had also noted the woodpecker’s identity crisis, so he and Robbins teamed up with Brett Benz, a curator at the American Museum of Natural History, to analyze the species’ genes and settle its place in the woodpecker family tree. Since helmeted woodpeckers are hard to find and capture in the wild, the researchers sampled DNA from taxidermied museum specimens. 

Their work confirmed Robbins’ and Zimmer’s suspicions that, despite appearances, the helmeted woodpecker belongs in Celeus. Genetic analysis done by a separate team came to the same conclusion. As Benz put it in a statement about their work, “The Helmeted Woodpecker is basically a typical Celeus in Dryocopus clothing.”

In answering one question, the researchers raised another: Why is the helmeted woodpecker a dead-ringer for these other, unrelated birds? Mimicry is common among animals, often in the form of a harmless species looking like another, more dangerous one to gain protection from predators—a trick called Batesian mimicry. But the helmeted woodpeckers’ lookalikes aren’t particularly dangerous, and Robbins, Zimmer, and Benz think that something else is going on.

The helmeted woodpecker, they suggest, is engaging in what’s called interspecific social dominance mimicry, or ISDM. This form of mimicry, proposed only a few years ago and so far studied only in birds, is a type of “social parasitism” in which small, subordinate animals visually mimic larger, more aggressive ones to avoid competition with them and get access to food. The two species that the helmeted woodpecker looks so much like are bigger and more dominant, and will drive other birds away from their food sources, trees loaded with ants and other insects. They’re less pushy with birds that are bigger or look like their own species, though, and so the helmeted woodpecker may have evolved a Dryocopus-like feather pattern to get at food without being hassled by them. 

It’s not a perfect disguise, of course. For one thing, the mimic is notably smaller than its models, but birds’ poor perception of size likely keeps that from being a problem, the researchers say. The helmeted woodpecker’s call can also give a clue to its true identity, as Robbins’ experience showed. The team notes, though, that the species calls far less frequently than its Celeus relatives, which may help it maintain its masquerade and avoid being bullied.