CHARLESTON, W.Va. -- What a difference a thousand miles can make.

Along the U.S.-Canada border, people and wildlife pass relatively freely. Most wildlife crossings go unnoticed. Wolves, bears and moose have no need for borders.

But every few years a visitor from the north captures the birding world's attention. Snowy owls, residents of the arctic tundra, have been reported in Pennsylvania and Ohio this winter. Just a few weeks ago I read that at least 400 were known to have reached Wisconsin. Popularized by the Harry Potter movies, snowy owls are large and conspicuous. They wander south when lemming populations crash.

Lemmings are small, mouselike rodents and an important food item for snowy owls. Though lemming populations crash every four years, snowy owls visit unpredictably. They eat other prey, too, such as snowshoe hares, grouse, songbirds and small ducks, so they are not tied exclusively to lemming populations.

In fact, when lemming numbers boom, so too does snowy owl nest success. But when lemming populations inevitably crash, competition for food intensifies for the now larger owl population. Older owls usually hold their ground; younger individuals face a choice: Move south or starve. At least that's one interpretation of the facts.

Fortunately, there's an easy way to age snowy owls in the field. Snowy owls stand almost 2 feet tall, have a 4-foot wingspan and bright yellow eyes. Adult males are mostly white, and adult females show some black barring. But first-year birds are heavily marked by black horizontal bars. They appear noticeably darker than adults.

So keep binoculars in the car, and check out any large, pale, "earless" owls you might see perched on fence posts, and buildings near hayfields, grasslands and airports. If the birds are mostly white, they are probably adult snowy owls. If they seem dirty or the black barring is evident, they are juveniles.

Along the U.S.-Mexico border, it's a different story. Walls and fences stretch intermittently along the nearly 2,000-mile international border to keep illegal immigrants out of the U.S. Based on news reports, they are not terribly effective. Long, sophisticated tunnels seem easy to build. And a 2009 government report revealed that there had been 3,363 breaches of the fence just through May of that year.

But under the heading of "unintended consequences," border fences are proving to be very effective at disrupting the movements of wildlife. The current issue of The Wildlife Professional, a publication of The Wildlife Society, reports that endangered species such as jaguars, ocelots, Sonoran pronghorn and many smaller, less-glamorous species are being disturbed. Birds are less affected, though the movement of ground-dwelling species such as quail can be impeded.

In the name of homeland security, we seem to be doing our best to destroy wildlife populations along the southern border. In 1996, the Illegal Immigration Reform and Immigrant Responsibility Act required portions of the fence to be cleared of all vegetation within 50 feet of either side of the fence. So even if animals try to find a way through the fence, they must do so without any cover.

In 2005, the Real ID Act authorized the waiver of laws that might delay construction of barriers along the California border. Consequently, the Endangered Species Act, the National Environmental Policy Act, and the Migratory Bird Treaty Act have all been ignored in the name of national security.

And since passage of the Secure Fence Act in 2006, more than 30 federal laws, including the Clean Water Act and the Wilderness Act have been waived.

The impact of waivers of environmental laws is still being studied, but it is clear that border wildlife populations suffer. In one study using cameras and radio collars, bobcats influenced by fences moved their territories and experienced more collisions with highway traffic.

Biologists fear that species with dwindling U.S. populations will suffer as access to Mexican populations disappears. Long-term survival of U.S. populations of larger species such as jaguars and ocelots is in doubt. Fences restrict movement and gene flow and induce stress.

Snowy owls and northern feeder birds such as pine siskins, purple finches, and red-breasted nuthatches thrill birders when food shortages drive them southward across the border. Except for hummingbirds, I doubt the same will ever be said about Mexican species wandering north.

Send questions and comments to Dr. Scott Shalaway, RD 5, Cameron, WV 26033 or via my website, http://scottshalaway.googlepages.com.