Birds of a feather may flock together, but that doesn’t mean they necessarily share close genetic ties. Taxonomists first classified birds into groups primarily based on appearance, relying heavily on the coloration of feathers to determine relationships among species. But research led by Museum Curatorial Associate Brett Benz and published this September in The Auk: Ornithological Advances shows that this historical reliance on feather coloration has confounded avian taxonomy and led ornithologists to overlook an intriguing case of visual mimicry.
New genetic analyses confirm that a pair of highly similar-looking South American woodpecker species once thought to be closely related are actually only distant cousins. By copying the appearance of a larger, socially dominant woodpecker species, the subordinate mimic species reduces the aggression that it receives from other potential competitors, enhancing its access to food resources.
“Most people are familiar with the idea of mimicry in the context of warning or ‘aposematic’ coloration, in which a harmless species resembles the appearance of a poisonous one as a form of predator avoidance,” said Benz. “But within birds, a largely overlooked type of mimicry instead serves to reduce aggressive attack from competitors. Subtle differences in weight and size can confer a huge advantage in a competitive skirmish, just like in a boxing match. By deceptively mimicking the appearance of a larger, socially dominant species, the smaller bird is able to avoid attack.”
In the new study, Benz and his colleagues, Mark Robbins of the University of Kansas Biodiversity Institute and Kevin Zimmer of the Los Angeles County Museum of Natural History, document a striking case of interspecific social dominance mimicry (ISDM) in the helmeted woodpecker (Dryocopus galeatus), a rare and little-known species found in the Atlantic Forest of Brazil, Paraguay, and Argentina.
The bird’s call and behavior tipped off the researchers that this species may have been misclassified. When Robbins encountered a helmeted woodpecker during a trip to Brazil in 2010, he was stunned that its vocalizations sounded nothing like other Dryocopus species in the region. Co-author Kevin Zimmer had also noted the helmeted woodpecker’s behavior was unlike that of other Dryocopus euncountered during his 20 years of field research in Brazil.
The shy and little-known helmeted woodpecker shares the red crest, black back, and barred underside of two larger woodpeckers—Dryocopus lineatus and Campephilus robustus—all of which occupy the same habitat and share similar food preferences. Though it had been previously classified in the genus Dryocopus due to its remarkable similarities in appearance with Dryocopus lineatus, genetic analysis by Benz and his colleagues confirms that the helmeted woodpecker is actually not closely related to other Dryocopus woodpeckers at all and belongs in a different genus, Celeus.
After examining specimens in the AMNH collection, Benz concluded “The helmeted woodpecker is basically a typical Celeus in Dryocopus clothing.”
Benz’s work also hearkens back to that of Lester Short, a former curator in the Museum’s Department of Ornithology. In a 1982 monograph on woodpeckers, Short noted that the helmeted woodpecker had features that were intermediate between Dryocopus and Celeus, and “could be placed in either genus.”
New Yorkers need not travel any farther than Central Park to see an example of ISDM, as the unrelated but incredibly similar in appearance hairy and downy woodpeckers (Picoides villosus and Picoides pubescens, respectively) have been eliciting double takes from birders for years.