The zone of overlap between two popular, closely related backyard birds is moving northward at a rate that matches warming winter temperatures, according to a study. In a narrow strip that runs across the eastern U.S., Carolina Chickadees from the south meet and interbreed with Black-capped Chickadees from the north. The new study finds that this hybrid zone has moved northward at a rate of 0.7 mile per year over the last decade. That’s fast enough that the researchers had to add an extra study site partway through their project in order to keep up.
