‘The rapidity with which these changes are happening is a big deal’
*More Summit Voice stories on birds and climate change here.
By Summit Voice
FRISCO — Watching climate change is a little more subtle than just sitting around watching a thermometer, but sometimes even scientists are surprised at just how fast things are changing.
A group of East Coast university researchers probably felt that way as they studied the breeding areas of Carolina and black-capped chickadees. Along a narrow zone in the eastern U.S., the two species interbreed, and that overlap zone is moving northward at 0.7 miles per year — a full-on sprint by geological time standards.
They outlined their findings in a news release last week:
“A lot of the time climate change doesn’t really seem tangible,” said lead author Scott Taylor, a postdoctoral researcher at the Cornell Lab of Ornithology. “But here are these common little backyard birds we all grew up with, and we’re seeing them moving northward on relatively short time scales.” The birds moved so fast the scientists had to add an extra study site partway through their project in order to keep up, he said.
Other recent studies have shown how global warming is affecting migrating songbirds, and how some pelagic birds could actually benefit from changing temperatures, at least for the short term. In the tropics, many bird species are trying move up in elevation, but may not be able to adapt fast enough, according to Duke University scientists. And in the Rockies, long-term studies suggest that migrating hummingbirds are out of synch with key food sources, as plants bloom earlier.
In Pennsylvania, where the chickadee study was conducted, the hybrid zone is just 21 miles across on average. Hybrid chickadees have lower breeding success and survival than either of the pure species. This keeps the contact zone small and well defined, making it a convenient reference point for scientists aiming to track environmental changes.
“Hybridization is kind of a brick wall between these two species,” said Robert Curry, a professor of biology at Villanova University, who led the field component of the study. “Carolina Chickadees can’t blithely disperse north without running into black-caps and creating hybrids. That makes it possible to keep an eye on the hybrid zone and see exactly how the ranges are shifting.”
The researchers drew on field studies, genetic analyses, and crowdsourced bird sightings. The data was matched with winter temperatures observations, and the scientists also closely studied the birds’ DNA to pinpoint the distribution of the two species.
The site that was in the middle of the hybrid zone at the start of the study was almost pure Carolina Chickadees by the end. The next site to the north, which Curry and his students had originally picked as a stronghold of Black-capped Chickadees, had become dominated by hybrids.
Female Carolina Chickadees seem to be leading the charge, Curry said. Field observations show that females move on average about 0.6 mile between where they’re born and where they settle down. That’s about twice as far as males and almost exactly as fast as the hybrid zone is moving.
As a final step, the researchers overlaid temperature records on eBird sightings of hybrid chickadees. They found a very close match: hybrids occurred only in areas where the average winter low temperature was between 14 and 20 degrees Fahrenheit. They also used eBird records to estimate where the hybrid zone had been a decade earlier, and found the same relationship with temperature existed then, too. The only difference was that those temperatures had shifted to the north by about seven miles since 2000.
Chickadees are widespread in North America. The tiny, fluffy birds with bold black-and-white faces are favorite year-round visitors to bird feeders, somehow surviving cold winters despite weighing less than half an ounce.
To the untrained eye the Carolina Chickadee of the southeastern U.S. is almost identical to the more northern Black-capped Chickadee—although the Carolina has a shorter tail, less white on its shoulders, and a song of four notes instead of two notes. Genetic research indicates the two have been distinct species for at least 2.5 million years.
“The rapidity with which these changes are happening is a big deal,” Taylor said. “If we can see it happening with chickadees, which are pretty mobile, we should think more closely about what’s happening to other species. Small mammals, insects, and definitely plants are probably feeling these same pressures—they’re just not as able to move in response.”