Cardinals are known for as "warm-adapted" species, which fly south for winter. Fifty years ago, sightings of these birds in winter in northern states was rare.
Dedicated observers of wild bird populations may have noticed a few changes to the crowd around their backyard feeders in recent years. Researchers with the University of Wisconsin have found that for many northeastern American bird watchers, those populations really are shifting, even in the wintertime.
A recent study published in the journal Global Change Biology examined 20 year’s worth of data on 38 bird species. The scientists found that species once considered rare in the northeast during the cold months are making more regular appearances in the wintertime, demonstrating a northern shift in the migration pattern for many of the country’s birds. Cardinals, Carolina wrens, chipping sparrows, and other species known to fly south for the winter are called "warm-adapted” species, according to lead study author Karine Princé. Fifty years ago, sightings of those types of birds in the winter in northern states were considered extremely rare, as they would usually be flocking south.
Researchers were able to track these changes thanks to the Cornell University Laboratory of Ornithology’s Project FeederWatch. The system has compiled data from thousands of the estimated 53 million Americans who maintain feeding stations near their homes (that figure according to the U.S. Fish and Wildlife Service). FeederWatch participants occasionally count and identify the types of birds present at their feeders from November to early April, and send their data to Cornell for aggregation.
The system isn’t fool-proof, but it’s excellent for mapping major geographic shifts in large populations. The project has its roots in Ontario in the mid-1970s, and is now in its 26th season in its current format, so biologists and others have a deep pool of data from which to draw.
Princé and other researchers examined the populations of those species compared to average winter temperatures and found that the changes in some birds’ habits matched up with warming winters with less snow.
As with any type of trend seen in bird or animal studies though, some of the subjects went their own way and flew south anyway.
"Globally, climate change is pushing species to track their climatic niche northwards,” Princé said. "Nevertheless, southern shifts have been observed for some birds. Changes in climatic conditions are spatially heterogeneous [meaning they vary for different birds in localized areas], and bird species and community responses could also rely on local variation in climate change.”
That local variation could also seem to imply that climate change is not the only factor that could be influencing the northern shift seen from most of the birds—Princé points out that the birds could also be reacting to changes in the landscape like development from humans and along with that development, and more available bird feeders that could provide food when their natural sources disappear for the season. But she’s pretty sure the changes are more major than that, especially since the estimated number of bird feeders in North America has stayed relatively constant since 1991.
"Land use changes and increase in wild feeding may indeed have had an effect in community shifts that we observed,” said Princé. "However, the changes we document in our study are so broad in scope that anything that is occurring at a local level is swamped out by the scale of this analysis.
"Moreover, other studies, in Europe especially, have highlighted similar change occurring in bird communities, which tend to confirm that the increasing CTI trend we found appears to be a broader phenomenon driven by climate change.”
Those shifts could mean bad news for the birds normally present in those northern regions as competition for food sources grows.
"We hypothesized that the community changes we highlighted in our study could result in new species outcompeting some species already present in the assemblage,” Princé said. "We did find some evidence that northerly species are becoming more rare in northern regions, but overall, the larger impact of a community shift was being driven by increasing populations of warm-adapted birds.”
This trend jives with those found in other studies looking at other species, though the study’s authors say that even without that background, birds often provide a good signpost for what’s ahead for the rest of us—in other words, the metaphor of the canary in the coal mine is an apt one.
What could that canary be predicting for the rest of us? It’s hard to say, but climate models predict even warmer winters all over the world in another 100 years, which could prompt movement of new types of birds or other animals into these backyard birds’ ecosystems. The addition of just one new species to an area can drastically affect the balance of those already living there, so a more broadly based shift leaves a lot of uncertainty about the future.
What about you: have you seen changes at your local bird feeder or backyard in recent winters?
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