|Illustration by the geezer hisself|
There are two species of the bird. The Carolina Chickadee lived in the Southern U. S. as far west as Texas. The Black Capped Chickadee lives in the northeastern US, into Canada and all the way west to Alaska. There is a narrow "ribbon" of area where the two cousins mix, Philadelphia was not in the mix region in 1999.
To learn more the Professor and his students live trapped a lot of birds, banded them and kept track of when, where and with whom they interacted. That was the way we now know how the weather affects the little birds.
In a few years after starting the study both species showed up in Philadelphia, now they both live further and further north in Pennsylvania. The warm weather version is becoming more common in areas they never lived in before. Global Warming has come to the bird world, at least that's what I gather from reading about it.
Follow this link: http://www.pressherald.com/news/What_chickadees_are_telling_us_about_climate_change.html