Black-capped Chickadees and their Carolina cousins interbreed where their ranges meet in southeastern Pennsylvania. That’s not news. What is noteworthy is the finding that the range of hybridization has moved seven miles north. The Carolina Chickadees are moving north into Black-capped territory. This information comes from an email posted by Scott Taylor of the Fuller Evolutionary Biology Lab at the Cornell Lab of Ornithology. He adds that there is evidence of northward shifts in the northern range limits of other species as well. The implication for me is that warmer temperatures are the driving force behind the range changes.
Recognizing a hybrid is not simple. There is little visible difference. Carolina Chickadees are smaller than Black-capped, have shorter tails, and are less brightly colored. They can be distinguised by voice. Below is a Caroline Chickadee photographed in Louisiana.