PHILADELPHIA — 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 backyard 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. They initially focused on three main areas: a large wetlands in Chester County’s East Nantmeal, Pa.; Nolde Forest, near Reading, Pa.; and Hawk Mountain Sanctuary, farther north in Berks County, Pa.
They cut sewer pipe into short lengths, crafted the pieces to look like dead trees, and fashioned a cavity and entrance in the top.
Chickadees normally nest in dead trees, Curry said, “but if you have to find their nest out there in the woods in some random tree, you can waste an awful lot of time.”
The chickadees accepted the pipes. Ultimately, Curry and his students put out 450 fake nests.
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, the Chester County, Pa., population was all Carolinas. Nolde Forest had a mix of Carolinas and hybrids. At Hawk Mountain, blackcaps dominated.
But as the research continued over the years, that changed. Today, almost half of Hawk Mountain’s chickadees are hybrids. And if there’s a chickadee at your feeder in southeastern Pennsylvania or southern New Jersey — unless it’s an “irruption,” an unusual year in which blackcaps move southward — the bird is likely a Carolina.
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, Tuscarora State Forest in Schuylkill County, Pa., 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 backyard birds we all grew up with, and we’re seeing them moving northward on relatively short timescales.”
Curry was struck by “how strong the climate signal was,” he said. “I didn’t think we’d be able to show it as quickly and clearly as we did.”
Keith Russell, outreach coordinator for Pennsylvania Audubon, said that Audubon research also has shown how winter distributions of many North American birds were shifting and that the chickadee research was another important piece of evidence.
If birds are moving north, other things likely are as well, from parasites to diseases and organisms that affect agriculture.
“There are probably endless ways in which small changes in climate can change the natural world (and) impact humans significantly,” Russell said.
What Curry and his colleagues documented has already been noted anecdotally by birders.
Russell has researched historic local bird records for a book project, and they show a similar shift.
In the mid-1800s, Carolina chickadees were virtually unknown in Philadelphia, and blackcaps showed up only in winter. In the 1940s, Carolinas began to breed in small numbers. In 1965, there was a population explosion, “and thereafter, the Carolina chickadee was a common resident species,” he said, and blackcaps had become rare winter visitors.