Late last January, scientists in New Hampshire found something unusual on ice-covered Lake Winnipesaukee: seventeen frozen loons.

Usually, changes in day length and temperature cue the threatened birds to leave in early January for their wintering grounds off the Atlantic coast; they return to the lake about four months later to breed. Biologists think unseasonably warm weather may have disrupted their migratory instincts, prompting them to linger on the lake.

When conditions turned harsh mid-month, the birds were already molting new flying feathers, which usually happens after they migrate. Unable to fly away, they succumbed to the frigid conditions. “It was very unexpected,” says Nick Rodenhouse, an ecologist at Wellesley College. “If warmer winters become more frequent, [loons] could die more often.”

Scientists are unsure how much global warming had to do with the loons’ odd behavior, but they know that climate change will significantly affect birds. Some, like the migrating loons, will lose out; others could benefit.

Among the winners may be residents—birds that stay put year round. At least one study suggests northeastern residents such as the tufted titmouse and northern cardinal might find more food as winter temperatures rise. But scientists predict rising temperatures may alter ecosystems, making habitat inhospitable and food scarce during breeding season. If they’re right, the more than 800 migratory bird species that fly over North America will suffer, and some may ultimately go extinct. Unlike those of resident birds, “the abundance and distributions of migratory birds is expected to contract,” says Rodenhouse.

Blows to migratory bird populations will have broader impacts. For one, the 81 million birders in America would suffer. But there’s more at stake than the loss of a beloved pastime. Entire ecosystems could be affected. Birds returning earlier from winter retreats, coming back in reduced numbers, or not returning at all may throw species interactions out of whack.

“Insects are probably being affected [by climate change] more so than birds, so insects may hatch and be gone before the birds get there,” says Stanford University ecologist Terry Root. “The predator-prey interactions among species can be severely changed.” That, Root says, could leave birds without enough food to survive. In a worst-case scenario, an absence of birds could leave hoards of insects to munch on trees and crops, endangering the resources we depend on and causing enormous economic loss.

Unusual weather can also spell reproductive disaster for migrants. And as the world heats up, extreme weather events will likely occur more often and with greater intensity. “Warmer temperatures may actually be associated with more frequent cold snaps,” says David Winkler, an ornithologist at the Cornell Laboratory of Ornithology.

That could be deadly for tree swallows, the migratory birds Winkler studies, because the birds eat flying insects that only take wing when it’s warm. “We’re going to have more wet springs, more really cold springs,” says Scott Sillett, a wildlife biologist at the Smithsonian’s Migratory Bird Center. “It’s the dramatic fluctuation in weather that’s going to cause larger fluctuations in the reproductive success of our birds.”

Additionally, bird nests might disappear. Many migrant songbirds are picky about their breeding grounds. They need temperatures that fall in narrow ranges and just the right mix of vegetation for nesting and cover. For example, New Hampshire’s black-throated blue warbler thrives in cool areas, where there are few predators and lots of caterpillars to feast on. “As warming progresses, the highest quality habitat could potentially disappear off the mountainside,” says Sillett, who studies warblers in the White Mountains.

Habitat loss isn’t just a concern at summer breeding grounds. Altering the environments where birds winter or stop to refuel can cause their populations to shrink. It’s challenging enough to protect forests where resident birds live; the task is far harder when it comes to migrants because they cover so much ground throughout the year. In addition to warming in the United States, global climate models predict drying in the Caribbean basin, where many migrants winter. “It’s hard to maintain one [habitat] given how many people we have in the world,” says Root. “Now you put warming on top of that. That’s a double whammy.”

It’s too early to tell how severe the consequences of warming will be for birds. But scientists say conserving habitat, preventing further fragmenting of existing territories, and decreasing our greenhouse gas emissions can only help. “You can’t say that [climate change] is going to be overall bad, and it’s not going to be overall good. But overall, there will be more bad than good,” Root says. “We’re going to have overall surprises. And we need to be ready for them.” 

Story by Sarah Parsons. This article originally appeared in "Plenty" in May 2008.

Copyright Environ Press 2008

Lost in migration: The perils of traversing a warming planet
Birds that navigate vast distances, such as loons in New Hampshire, are especially vulnerable to climate change.