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Mountain chickadees in Boulder have evolved a different tune to avoid getting mixed up with their cousins, according to published Oct. 9 in the Journal of Evolutionary Biology. 

The results provide real-time evidence for one of Charles Darwin’s famous theories and shed light on how pressure from human activity can impact wildlife’s evolution. 

 

Top: A mountain chickadee. (Credit: ) Bottom: A black-capped chickadee. (Credit: )

Mountain chickadees, common in the high-elevation conifer forests on North America’s west coast and in the Rocky Mountains, are vocal animals. They constantly whistle a chirpy “bee-bee-bee-bee” song to attract mates or defend their territories. 

Black-capped chickadees are their close relatives. The two birds look very similar, except that mountain chickadees have a pair of white eyebrow-like stripes above their eyes. Black-capped chickadees tend to live at lower elevations, and in certain regions, such as Colorado’s Boulder County, the two birds’ habitats overlap. 

Scott Taylor, one of the paper’s senior authors and associate professor in the Department of Ecology and Evolutionary Biology, wondered whether this unique living arrangement would push the chickadees to evolve different traits. His team’s hypothesis is based on Darwin’s “character displacement” theory, which suggests that closely related species with overlapping habitats tend to diverge in traits—such as appearance and call—to reduce competition or costly hybridization between different species. 

. These closely related birds on the Galápagos Islands evolved different beak shapes and sizes from one another to specialize in eating different types of seeds, reducing competition. 

In Boulder, where two species of chickadee coexist, researchers wondered if the birds had started humming a different tune. 

“In birds, song is an important characteristic for individuals to recognize each other,” said Olivia Taylor, the paper’s first author and a recent graduate of the Department of Ecology and Evolutionary Biology.

Olivia Taylor

To explore this, the team collaborated with researchers at the University of Western Ontario and Cornell University, who recorded more than 2,000 chickadee songs. They sampled black-capped and mountain chickadees in Boulder, where the two species coexist. They also followed populations of mountain chickadees in California and black-capped chickadees in New York, where each species lives by themselves. 

They found that Boulder’s mountain chickadees sing differently compared to those in California. Instead of the common four-note “bee-bee-bee-bee” song, Boulder’s mountain chickadees whistle with more notes. For instance, many of their songs have five to six notes, which is significantly longer than the two-note songs of black-capped chickadees living in the same area.
 
Boulder’s mountain chickadees are also more likely to include one or two introductory notes—short chirps at the beginning of a song—compared with the Californian mountain chickadees and their black-capped cousins.  

“Our ears couldn’t really pick up these longer introductory notes, and we only noticed that after we recorded and analyzed the songs. But the birds’ hearing is so much better than ours, so they can certainly tell them apart,” said Scott Taylor, who has led the at CU Boulder’s Mountain Research Station for six years. 

Previous research shows black-capped chickadees are dominant over mountain chickadees when they coexist. Black-capped chickadees often chase mountain chickadees away if they get too close, and mountain chickadees usually wait for black-capped chickadees to finish eating and leave before they approach feeders. 

While the two species can breed with each other, female hybrid offspring produced from a black-capped chickadee and a mountain chickadee are likely sterile.

Singing a different song can help mountain chickadees distinguish between friends or foes and avoid interbreeding, the authors said. 

“There’s a reproductive cost in hybridizing with each other. From an evolutionary perspective, sterile females are a dead-end in reproduction. And maybe the hybrid males also suffer some physiological costs we don’t yet know about. Given the two species are adapted to different elevations, some hybrids may struggle to survive cold winters in the high mountains,” Scott Taylor said.   

A few hundred years ago, mountain chickadees inhabited Boulder’s conifer forests alongside likely far fewer black-capped chickadees than today. As settlers moved in and planted ash and maple trees, they also created excellent habitats for black-capped chickadees. As a result, the black-capped population is likely much larger now, and the birds are interacting more frequently with local mountain chickadees.

“It’s very interesting to see how these species are responding to what is ultimately a human-introduced pressure,” said Olivia Taylor. “For that, I think it’s important to document and understand our impacts on wildlife and how they adapt to co-exist.”