About 15 years ago, Villanova University biology professor Robert Curry was looking for a project that would allow his students to investigate something interesting without much travel.
He found it in a cheeky little bird with a black cap, familiar to anyone with a back-yard feeder: the chickadee.
His idea was to catch a lot of birds (with special nets), band them to identify individuals, and keep track of all they did — who was nesting with whom and where, how many offspring they had, where the young went when they set out on their own.
Little did Curry know how quickly this creature, weighing less than two quarters, would provide clear evidence of birds moving northward — at quite a clip — in association with climate change.
Curry focused on two species: the Carolina chickadee and its more northerly relative, the black-capped chickadee. They look similar and are closely related, but genetic research indicates the two have been distinct for 2.5 million years.
The birds were good candidates for his project, since they don't migrate seasonally. At the time, Carolina chickadees existed only in the southern half of the Eastern United States, west into Texas. Black-capped chickadees inhabited northern North America, up into Canada and all the way across to Alaska.
The two ranges overlapped in a ribbon of habitat about 21 miles wide. Part of it crossed southeastern Pennsylvania.
Within that swath, the Carolinas and the blackcaps interbred, producing hybrids. One of the zone's telltale signs: Hybrid chicks were less likely to hatch and survive.
Curry wanted his students to pin down those boundaries, focusing on three areas. Curry's students put out 450 fake nests to make it easier to track the birds.
As they captured birds, they also took blood samples, in part to determine which were hybrids or even backcrosses — a bird whose parents were a hybrid and a nonhybrid.
When the project began in 1998, one area had all Carolinas. Another had a mix of Carolinas and hybrids. At the third, blackcaps dominated.
But as the research continued over the years, that changed, with half the chickadees at the third location now hybrids.
Curry's collaborators in the project, Cornell University and the Cornell Lab of Ornithology in New York, analyzed nearly 200 blood samples the students collected. The scientists also collated citizen bird sightings reported to online database eBird, and studied temperature records.
One climate-change variable shifting in this region has been the average minimum temperature in winter, this year's deep freeze notwithstanding.
The dividing line between the two chickadee species turned out to be about 17 degrees. In other words, Carolinas won't live where it's colder; blackcaps will.
The Cornell researchers realized that the warming temperatures and the Carolina chickadees were moving north in sync — at an average of 0.7 miles a year.
The pace was so fast that in 2006, Curry added another site to keep ahead of the Carolinas' push north.
"A lot of the time, climate change doesn't really seem tangible," said Scott Taylor of the Cornell Lab, the lead author on the group's recent paper in the journal Current Biology. "But here are these common little back-yard birds we all grew up with, and we're seeing them moving northward on relatively short time scales."