WITH a wary eye on the clouds, I hurried through a few last outdoor chores at my old farmhouse before the rain and snow arrived. But even in my haste, one faint sound stopped me - braying whoops high overhead, the telltale knell of autumn's final retreat. All but hidden in the clouds, a flock of tundra swans was riding the storm front, aiming for the sheltered coves of the Chesapeake more than 100 miles to the south.
Here in eastern Pennsylvania, the migratory web binds up threads that originate far beyond these gentle hills. Peregrine falcons born in Greenland chase ducks that hatched in Manitoba. Long-billed dowitchers from the Northwest Territories leapfrog to the mid-Atlantic states, while blackpoll warblers from across Canada funnel through on their way to Amazonia.
But that hemispheric dance, that most compelling of all natural phenomena, now carries darker undertones. As the deadly H5N1 strain of avian flu marches across the Old World, those of us who marvel at migratory birds wonder whether - or perhaps simply when - one of them will carry the disease to this hemisphere.
The virulent form of the flu has not yet been found in the Western Hemisphere, but some Americans are still panicking. Birders on the Internet trade anecdotes of people refusing to hang their sunflower-seed feeders, and the Cornell Lab of Ornithology had to issue a press release saying, in effect, it's still safe to go bird-watching. When a domestic duck in British Columbia was found last week to be infected with a mild and widespread form of bird flu, the United States responded by imposing an interim ban on all poultry imports from that province.
Overreaction? Of course. But as I cock an ear to the swans, I feel some unease mixed with my awe. These swans have come so very far, some perhaps flying from Alaska or even Siberia. And Alaska is H5N1's logical entry point into the Western Hemisphere. While we have understandably focused on the danger to humans, the flu's impact on North American birds could be disastrous.
Last summer, I spent a week on the flat, waterlogged tundra at Old Chevak, an abandoned Cup'ik Eskimo village in Yukon Delta National Wildlife Refuge, not far from the Bering Sea. The birdlife was astonishing in both its diversity and abundance; hiking across the spongy landscape in hip boots, I rarely walked more than a few yards without flushing a nesting duck, loon, swan, shorebird, sparrow or tern. They come here from regions as far-flung as the Philippines, Amazonia, New Zealand, tropical Africa and Tierra del Fuego. That includes places where the deadly flu has already been found - like Java, where the slender songbird known as the yellow wagtail winters, or Vietnam and China, whose coastlines are important staging grounds for migrant shorebirds.
If and when the virulent flu enters Alaska in the bodies of Asian migratory birds and spreads among the breeding population, it will then be carried heaven knows where. While the large-scale risk to humans is still theoretical, H5N1 has already proven deadly to many species of wild birds. In May, a single outbreak in China killed up to a tenth of the world's bar-headed geese, and last month a United Nations task force identified three dozen species of rare Eurasian birds at particular risk from the flu. Here in North America, where emerging diseases like West Nile virus are already exacting a heavy toll on some birds, the damage from this new pathogen could be even greater.
The task force also correctly noted that we shouldn't scapegoat migratory birds for a problem of our own making. H5N1 is a product of intensive poultry production, especially in regions like Southeast Asia with scanty farm hygiene and large live-bird markets, which create a hothouse environment for influenza viruses and a transmission route to people. The biggest risk to this country comes not from a bird crossing the Bering Strait, but from an infected human boarding a jet.
Will that realization stop officials and the public here from eventually making the kind of counterproductive demands we've already heard in Asia, for the mass culling of migratory birds or the destruction of wetlands and other habitats? Or will it draw attention to measures that cut to the root cause of this problem, like better monitoring and oversight of global poultry production, and curbing the worldwide (and often illegal) trade in wild birds, a step the European Union has already taken?
As the sound of the swans faded, I could only hope - for the sake of the birds, and ourselves - that we choose the latter course.
Scott Weidensaul is the author, most recently, of "Return to Wild America."