The Cliff Swallows are back!

They arrived suddenly, as they always do. One day there's nothing but water flowing under the bridges, and the occasional robin singing from trees along the riverbank.

The next day, squadrons of small birds are swooping under bridge spans from Watson Lake to the Alaska border, chattering in rows on nearby power lines, and zipping in and out of holes in the high cutbanks or neatly constructed nests still clinging to bridge supports and roof peaks.

"Swallows are fantastic aerial acrobats -- a real joy to watch," says Nancy Hughes of the Canadian Wildlife Service in Whitehorse.

They have to be acrobats. They're insectivores, and make a living by eating their way through the hordes of insects that also arrive with the Yukon spring. Outmanoeuvring enough insects to make a meal requires some pretty fancy flying.

Hughes admits that not everyone is as pleased as she is when the swallows arrive. Many people consider their nests a nuisance. But before you knock down a swallow nest, take a moment to consider the Cliff Swallow.

Cliff Swallows come a long way to raise their young in the Yukon, all the way from the southern parts of South America. Apart from the demands the journey places on our hospitality, the long migration brings them under the protection of the Migratory Birds Convention Act, which bans disturbing or destroying the nests and eggs of migratory birds.

There are also less legalistic reasons for being kind to nesting swallows. Mosquitoes, for example. Swallows eat them -- lots of them. A nest of swallows can consume a substantial swarm of mosquitoes over the summer. During mosquito season, a healthy neighbourhood swallow population can mean less itching in the human population.

And there's the entertainment value of swallow-watching. As Hughes says, they are spectacular aerial acrobats. However, they also lead an interesting and entertaining domestic life.

Cliff Swallows build mud nests and prefer to live close together, in colonies. In the past, they built their nests under ledges and in steep banks like those along the Yukon River.

Some swallows still follow the old pattern, but others have adapted spectacularly well to human structures. They build nests under bridges, under the roofs of buildings, in culverts -- anywhere that offers mud for building material, a safe place to attach the nest, and a good supply of insects.

Over the course of a summer, the birds become almost as comfortable with people as they are with human structures, raising their young and going about their daily business in full view of their human neighbours.

However, since some people still object to living quite that close to swallows, there are ways to discourage swallows from nesting on a building.

One approach is to tack heavy nylon mesh or chicken wire from the outer edge of the roof to the wall of the building so that it forms an angle of about 45 degrees. That blocks the birds from their favorite nest place, right at the top of the wall under the eaves.

Another approach is to cover the birds' favoured nesting locations, at the top of the walls in the shelter of the roof, with a smooth material like sheet metal. The swallows will have trouble attaching their nests to the smooth surface and will move on to a more welcoming site.

If you don't mind swallows nesting on your buildings and simply want to keep them away from windows and doors, just protect those areas. And then enjoy your avian neighbours.

"For me, their arrival marks the beginning of summer," says Hughes.

For more information about how to control swallow nesting, go to http://www.agric.gov.ab.ca/agdex/600/685-7.html. For more information about Cliff Swallows, call the Canadian Wildlife Service, Environment Canada, at (867) 393-6700.