Science

A Woodpecker Disguised as . . . Another Woodpecker?

In this tale of three red-capped woodpeckers from Brazil, one is an impostor.

Every kid knows that to avoid getting bullied, it helps to not stand out. Apparently woodpeckers know this, too: In a paper due to be published in The Auk: Ornithological Advances, researchers reveal that the Helmeted Woodpecker mimics the plumage of other, larger woodpecker species in order to stave off harassment.

To the casual observer, the vulnerable Helmeted Woodpecker looks very similar to two more aggressive species with which it share the forests of Brazil: the Lineated Woodpecker and the Robust Woodpecker. In fact, the Helmeted Woodpecker looks so much like a smaller version of the Lineated Woodpecker that for decades, scientists assumed that the two were closely related. “Those two look so darned similar, with the black back and red crest and lined neck,” says Mark Robbins, one of the authors of the study and a biologist at the University of Kansas Biodiversity Institute.

A Lineated Woodpecker (left) versus a Helmeted Woodpecker (right). The Helmeted has evolved to closely resemble the larger Lineated, even though the two aren't closely related. Photo: David Schenfeld/Flickr Creative Commons (left) and Kevin Zimmer (right)

But over the last few years, researchers began to suspect that the Helmeted Woodpecker wasn’t what it seemed. In 2010, when he was in Brazil for a conference, Robbins went birding and not only spotted a Helmeted Woodpecker, but also heard its call. That’s when he realized that the bird didn’t sound like its supposed relatives at all—instead, it sounded like a genus of drab brown woodpeckers known as Celeus woodpeckers. Robbins contacted two other scientists to ask if they could do a genetic analysis of the Helmeted Woodpecker in order to see if it belongs in the Celeus family tree. The species is so hard to find in the wild that the team had to sample DNA from specimens that were taxidermied in the '60s—but it paid off. The decades-old DNA showed that Robbins’ hunch was right: The Helmeted Woodpecker might not look at all like a Celeus, but “lo and behold,” Robbins says happily, “it was a Celeus.” It was a gratifying discovery for Robbins, but it also raised an important question: How did the Helmeted Woodpecker end up looking like larger, unrelated birds?

The Robust Woodpecker (pictured above) is another one of the Helmeted Woodpecker's role models. Photo: Dario Sanches/Flickr Creative Commons

Robbins and his collaborators think that it’s all about competition for food. Lineated and Robust Woodpeckers will drive smaller birds away from trees containing ants and insect grubs, but are less likely to challenge birds of their own size and species. The Helmeted Woodpecker likely took advantage of the larger birds’ poor perception of size, evolving to have similar feather patterns. Over generations, Helmeted Woodpeckers that looked like their heftier competitors were left alone to feast on their favorite trees, and thus more likely to have several babies and pass on the mimicking traits. This is called “interspecific social dominance mimicry,” or ISDM, and it’s a newly discovered form of imitation that scientists are now finding throughout the world of birds:  Some motmots resemble other motmots, Choco and Chestnut-mandibled Toucans look practically identical, and across North America, birders confuse the Hairy Woodpecker with its smaller mimic, the Downy. Biologists suspect that ISDM may occur in other social animals as well—reef fish, perhaps—but so far, it’s only been confirmed in birds.

Scientists working on this case are confident that ISDM is the reason for the woodpecker’s deception; future field research will allow them to discover just how much mimicry helps the bird. The Helmeted Woodpecker is a secretive species, so there isn’t much published research on its behavior. Worse yet, the Atlantic forest habitat where the woodpecker lives is shrinking from deforestation, and there may not be much time left to study it.

Still, biologists hope that the woodpecker’s trick will give humans another reason to pay attention to the species. “The nice thing is that if you’re preserving habitat, you’re preserving a lot of other species in that area,” says Robbins—in undisturbed forests, the Helmeted Woodpecker uses between three and five square kilometers of territory. If this habitat can be kept intact, at least the bird will have plenty of food—so long as the other woodpeckers don’t wise up to its ruse. 

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