Trumpeter Swans Rebound, with an Assist from Global Warming

Image: U.S. Fish and Wildlife

ANCHORAGE, Alaska - Outside Alaska's largest city, where wildlife is more common than pigeons, locals bearing field glasses turn out every year to watch blazingly white trumpeter swans stop to feed on their way south for the winter.

The swans, famed for their French horn call and immortalized by author E.B. White, were nearly hunted to extinction in much of the United States and Canada by the late 1800s for their meat, feathers, down and quills.

Now, North America's largest wild fowl may be one of the few good-news stories of global warming - at least for the short term.

Trumpeters, which reach 38 pounds with an 8-foot wingspan, need a long summer to raise young to a size where they can keep up with the flock on the thousand-plus-mile journey to ice-free ponds in British Columbia, Washington and Oregon.

A warming climate is helping, expanding the swans' summer range northward into habitat never before used in their ancestral boreal forest, allowing populations to flourish, according to new study by Alaska scientists.

As spring arrives earlier in Alaska and winter comes later, the season lengthens for breeding, hatching and cygnet rearing, all advantageous to the birds, which need at the minimum 145 ice-free days, according to the study, led by Joshua Schmidt of the National Park Service and published in the December issue of the journal Wildlife Biology. In comparison, the tundra swan, a smaller and more-abundant species distinguished by a yellow dot above its black bill, needs about 100 days.

Trumpeter swans breed from central interior Alaska to the southeast coast and north to the Brooks Range in forested ponds and lakes. Since the mid-20th century, the population has recovered to roughly 25,000 swans. Prohibitions against hunting and attention to protection and food supply in winter feeding grounds have aided the surge. Small remnant populations of non-migratory trumpeters live year round in isolated spots of Montana, Wyoming, Idaho, Minnesota and Wisconsin, including Yellowstone National Park and Red Rock Lakes National Wildlife Refuge, the setting for White's affectionate portrayal of the resourceful, voiceless Louis in "Trumpet of the Swan."

Two years ago, a team of Alaska scientists confirmed that trumpeter pairs have been steadily increasing since at least the 1960s. The team found that swans have benefited from higher temperatures associated with warming atmosphere and oceans. There are fewer trumpeter swans in northern Alaska than in the southern half. But those in the northern population, between the Alaska and Brooks ranges and including the Yukon River basin, are increasing faster than in the southern portion, the researchers found.

New research by the National Park Service team has found the swans have expanded their range northward since 1968 into areas never before used as breeding habitat - even before hunting killed off much of the population. That shift, researchers concluded, is likely linked to the rising temperatures of the past hundred years.

The Arctic has a history of amplifying global temperature changes, warming or cooling faster than the rest of the planet. Recent trends are no exception: While the globe on average has warmed 0.6 degrees Celsius compared to historic norms, the Arctic has jumped 1.5 degrees Celsius, with some areas approaching 3 degrees Celsius warmer. Higher spring temperatures are causing the earlier snowmelt, and the timing of that melt influences the warming of the atmosphere.

Reduction of the snow cover in the region over those days, researchers found, magnified the heating effect three times.
"We found a direct link between temperature and the occupancy of breeding trumpeter swans in Alaska," said Schmidt, a wildlife biologist and data manager for the Park Service's Central Alaska Network. "In warmer periods, there are more pairs observed occupying the summer breeding habitat than in colder periods." With rising temperatures, the swans are gaining more habitat than they are losing, he added. They can now use thousands of acres for breeding that in colder eras were inaccessible.

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