Birders who have been keeping a tally of their feeder visitors over the past half-century may have noticed an unusual trend: some species that once strictly summered here, including tufted titmice, northern cardinals, mourning doves, and evening grosbeaks, are now spending more if not all of their winters here, too.
According to Kent McFarland of the Vermont Institute of Natural Science (VINS), these proverbial “canaries in the coal mine” are expanding their winter ranges northward because of changes in food, habitat, and climate.
In the first Vermont Breeding Bird Atlas (BBA), published in 1985, for example, there were just a handful of tufted titmouse accounts in winter in the Connecticut valley. “Maybe northern Windsor County was the farthest northern record,” McFarland says. The revision of the Vermont BBA, soon to be published, now includes winter records of the tufted titmouse all over the state.
Cardinals too were once rare in New England winters, says McFarland. New studies indicate that the cardinal depends on stored fat to survive in cold climates, and that winter temperatures are thus the limiting factor in determining its northern range limits.
Similar observations in the United Kingdom led to a study in which researchers discovered that from 1970 to 1990, the northern range boundaries of what were considered to be southerly species shifted northward by 19 kilometers, on average.
There are three schools of thought on why this range expansion is happening. First, more people are feeding birds in winter, and the birds are following the feeders. It is estimated that a third of households in North America each distribute 60 pounds of bird food every year. “Birds can hang out if they have the food supply,” says McFarland.
Another explanation is habitat change, which benefits some species while harming others. One example is the wild turkey, whose range has significantly increased because of successful management and the increased availability of the mixed habitat it likes.
Of greater concern is our warming climate. “Birds shift ranges according to availability of habitat, whether it be in the form of food resources, nesting habitat, or decreases in competition with similar species, within physiological limits,” says Rosalind Renfrew, director of the Vermont BBA project at VINS. “Climate change can affect all of these factors.”
Bolstering the warming theory is increased evidence of migratory birds coming north and laying eggs earlier than before. A recent analysis of the Cornell Nest Record Program, which includes hundreds of thousands of nest records from the past 50 years, found that red-winged blackbirds and bluebirds have significantly earlier egg-laying times than they did a half-century ago, a change attributed to warming temperatures over the same period of time.
A report by the Climate Change Research Center at the University of New Hampshire found that, “Over the past 100 years, and especially the last 30 years, all of the climate change indicators for the region reveal a warming trend.” The average annual temperature of the Northeast, according the report, has increased 1.8˚F since 1899, 1.4˚F of that warming happening in the past 30 years. Winter temperatures have increased more greatly than temperatures of any other season – an impressive average of 4.4˚F over the past 30 years.
A 1988 study showed that range boundaries of 50 songbirds are dictated by average minimum January temperatures, and ongoing research shows that some of these species’ ranges have significantly shifted in a direct relationship to the warming climate.
Some bird species will benefit from a warming climate, while others will decline. According to a recent Forest Service/University of Maine study, more than half of all birds in the East will decline as a result of climate change, while 20 percent will increase, because of changes in available habitat. The same study predicts the ranges occupied by about 40 percent of bird species will decrease, while those of about 20 percent will increase.
With shifting ranges, too, comes a whole series of shifting relationships between bird species once separated geographically. Diseases may spread, such as the conjunctivitis carried by non-native house finches that can infect other birds, including siskins and goldfinches. Competition may also occur, such as that between cavity nesters like the chickadee, a common winter resident, and the tufted titmouse, newly on the scene, for nesting sites.
“Nobody has looked at interspecific conflicts yet,” says McFarland, but he’s quick to add that it’s most important to look at new species arrivals as an indicator that something is going on in the greater ecosystem. “Maybe a few birds are not a big deal,” he says; “However, there may be all kinds of cascading effects in the ecosystem we don’t know about yet. They are warning flags that tell us, whoa, there are some changes going on in our environment.”