A new study from the University of Colorado Boulder provides real-time evidence for Darwin’s character displacement theory. Mountain chickadees in Boulder have evolved a distinct song to avoid getting mixed up with their close relatives, the black-capped chickadees. This adaptation highlights how species can diverge in traits like appearance and call to reduce competition and costly hybridization. The findings also shed light on the impact of human activity on wildlife evolution.
Songbird Species Adapt to Coexistence
Mountain chickadees, common in the high-elevation conifer forests of North America’s west coast and Rocky Mountains, are known for their distinctive “bee-bee-bee-bee” song. Their close cousins, the black-capped chickadees, tend to inhabit lower elevations.
In certain regions, such as Colorado’s Boulder County, the habitats of these two chickadee species overlap. This unique living arrangement prompted researchers to investigate whether the birds had evolved different traits to avoid competition or costly hybridization – a phenomenon known as character displacement.
The study, led by researchers at the University of Colorado Boulder, found that mountain chickadees in Boulder have indeed changed their tune compared to their counterparts in California. Instead of the standard four-note song, Boulder’s mountain chickadees now sing with more notes, often five or six. They also frequently include one or two short introductory chirps at the beginning of their song.
Chickadees’ Distinct Songs Help Them Avoid Interbreeding
Previous research has shown that black-capped chickadees are dominant over mountain chickadees when they coexist. The black-capped birds often chase away their mountain cousins if they get too close, and mountain chickadees typically wait for the black-caps to finish eating before approaching feeders.
While the two species can interbreed, the resulting female hybrid offspring are likely to be sterile. This means that hybridization represents a reproductive dead-end for the mountain chickadees.
By singing a different song, the mountain chickadees can better distinguish between friends and foes, and avoid the costly consequences of interbreeding. “There’s a reproductive cost in hybridizing with each other,” said Scott Taylor, one of the study’s senior authors. “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 that the two species are adapted to different elevations, some hybrid chickadees may also struggle to survive the harsh winters in the high mountains.
The researchers believe that the mountain chickadees’ evolving song is a direct response to the increasing presence of black-capped chickadees in their habitat, which has been driven by human activity in the region.
Human Impacts on Wildlife Evolution
A few hundred years ago, mountain chickadees were the primary inhabitants of Boulder’s conifer forests, with likely far fewer black-capped chickadees. However, as settlers moved in and planted ash and maple trees, they created excellent habitats for the black-capped species. As a result, the black-capped population has likely grown much larger, leading to more frequent interactions with the 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, the paper’s first author and a recent graduate of the Department of Ecology and Evolutionary Biology at the University of Colorado Boulder. “For that, I think it’s important to document and understand our impacts on wildlife and how they adapt to co-exist.”
The study provides real-time evidence for Darwin’s character displacement theory, demonstrating how closely related species can evolve distinct traits to reduce competition and the risk of hybridization. As human activity continues to shape the environment, understanding these evolutionary adaptations will be crucial for protecting and managing wildlife populations.
