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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.

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.

Galápagos finches are a classic example. 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.

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