Southern birds are moving north. But is this a sign of climate change? I often am asked whether we are seeing an effect on Maine birds because of global warming. It’s a complicated question, because many species routinely colonize new areas.
We certainly are seeing birds now that we didn’t see in Maine half a century ago. The northern cardinal, tufted titmouse, red-bellied woodpecker and turkey vulture continue to expand their ranges. It would be easy to think these historically southern birds find Maine’s warming climate more hospitable. It is more likely the proliferation of bird feeding and the suburbanization of habitat are the chief contributing factors. For the vulture, some credit must go to the Interstate Highway System, which has created a paved buffet table of road kill across northern New England.
Birds normally expand their ranges, and they have been doing so since the last ice age. Instead of decipher why some species move north, it’s also appropriate to determine why other species don’t. Natural barriers prevent expansion in most cases. The Rocky Mountains confine western birds to the Pacific slope. The Great Plains and Mississippi River block east-west movement. Birds can be limited by a dependence on specific food supplies or by vulnerability to new predators and competitors in a new territory.
Birds fly. Some range expansion happens when they overshoot their migration routes and find suitable habitat in new places. Birds adapt. Over a surprisingly brief period, crossbills develop bigger bills when they move into coniferous forests with bigger cones. Climate is not the only reason range change happens.
But it’s a big one.
During September, National Audubon released a landmark report documenting 314 bird species — virtually half of all North American birds — imperiled by shrinking and shifting ranges. Nearly 50 of those species are familiar birds in Maine. Declines in bird populations can be because of changes here on their breeding grounds, in the tropics on their wintering grounds or both.
Before anyone panics, some of those Maine birds are not in grave peril. I chuckle to see herring gulls on the list. Yes, they are declining, but they’ve got a long way to fall before anyone gets worried. On the other hand, some of our most beautiful warblers are slowly disappearing. Blackburnian, black-throated blue and Canada warblers are less common than they once were.
The Audubon report is a modeled projection of what to expect from a warmer planet in the future, but scientists are actually watching it happen in real time. The black-capped chickadee is the Maine State Bird. Its coastal range stretches southward, barely into New Jersey. Here, it briefly overlaps with the Carolina chickadee. The two are similar in appearance and behavior, but the call of the diminutive Carolina chickadee is faster and higher, and the song is different.
Black-capped chickadees are adapted to colder weather, while Carolina chickadees prefer a warmer climate. The area where the two species overlap is very narrow, only about 21 miles across. Some hybridization occurs within the crossover zone, and scientists have been tracking it for 15 years. They’ve observed that the overlap zone is moving northward at a rate exceeding half a mile per year. Because chickadees tend to stay where they were raised, that’s a rapid change. In this case, temperature alone is the apparent cause. Milder winters are allowing the Carolina chickadees to move northward, and warmer summers slowly are forcing the black-capped chickadees out of New Jersey and Pennsylvania.
A rising temperature does more than just make birds uncomfortable. It alters habitat and food supplies. Two years ago, the Gulf of Maine experienced a significant period of warmer water. Warm-water fish moved in as our native cold-water species retreated. Puffin chicks were unable to swallow the substitutes, and our nesting colonies suffered a major population crash.
Maine’s forests were once more coniferous than they are now. Large stands of spruce were decimated by spruce budworm several decades ago. As the forest healed, warmer weather encouraged stands of hardwood to take over. Evening grosbeaks, Cape May and Tennessee warblers, and a host of other northern species declined with the change in habitat.
For those of us who spend time in the woods, the change is obvious. Southern birds are increasing, northern birds are decreasing. Some of it is related to climate change; some of it isn’t. Change is a natural phenomenon — except when it isn’t.
Bob Duchesne serves as a Maine Audubon trustee and vice president of its Penobscot Valley Chapter.