As we stand on a rough track in the McIntyre Creek wetland area near Whitehorse, swallows dart past our heads and ravens cruise the warm rising air currents. We can hear bird calls coming from all directions.

"There's a Ruby-crowned Kinglet somewhere over there," says Wendy Nixon, biologist with Environment Canada. "Can you hear that song? That's a Yellow-rumped Warbler. And that's a Golden-crowned Sparrow -- we only see them during migration because they like to nest right up in the subalpine."

Birdsongs are everywhere, but the birds themselves are hard to spot. Many are small and dull colored, visible only as shapes flitting through the tree branches or disappearing into the tall grass. Through binoculars, however, they are more exotic.

The White-crowned Sparrow has a startling set of black and white strips on top of its head. Yellow-rumped Warblers show flashes of bright yellow on their sides and backs when they take flight. The Violet-green Swallows swooping overhead have flashy white patches on their sides and around their eyes, contrasting with variations of iridescent green and violet on their wings and backs.

The Ruby-crowned Kinglet is a tiny olive-colored bird with an iridescent ruby-red crown and weighs only six or seven grams. But it makes up for its lack of size and visibility with the power of its song.

"On a cold morning, that song can carry for over 200 metres," says Nixon.

Birds have a vocal system totally different from mammals, she says. Their songs come from a combination of bone and highly elastic cords surrounded by an air sac, located in the bird's chest. The system allows the sound to resonate, producing an unexpectedly large sound from a tiny bird. In addition, Nixon explains, birds have a flow-through respiratory system so they can continue breathing while they sing.

Spring is the time to hear and see songbirds in the southern Yukon, the Environment Canada biologist says. Partly it's because there are no leaves on the trees and shrubs to hide the birds, but mainly it's because there are simply a lot of birds.

"We've got the entire migratory bird population that has wintered south of us passing through on the way to their nesting sites."

Some of them winter as far south as Central and South America. White-crowned Sparrows winter from southern B.C. to Mexico. The tiny Ruby-crowned Kinglet might travel all the way to Guatemala. Cliff Swallows might go as far as Paraguay and Argentina, well below the equator.

Usually they pass through the southern Yukon in a steady stream in the spring, heading to nesting grounds further north. Occasionally they arrive in spectacular numbers.

In early May this year, thousands of White-crowned Sparrows suddenly appeared, all at once, in the Whitehorse area. Nixon calls it "a migration phenomenon." There was a similar event reported about 20 years ago in the Teslin area, she says.

"In some years the migration front gets to a point and stalls," she explains, "so the birds are everywhere you look, thousands at once."

Within a week or two, most of the White-crowned Sparrows have moved on to their nesting sites, to take advantage of the bounty of emerging insects to feed their young. You will spot lots of them later in June in shrubs along the Dempster Highway and Top of the World Highway, Nixon says.

Each year, once the songbirds have reached their nesting sites, Environment Canada co-ordinates the Breeding Bird Survey. About 16 volunteers head out before dawn to drive set routes that cover as much of the territory as possible, Nixon explains. Each volunteer stops at 50 different points along the way and records everything he or she sees and hears during three minutes of observation, identifying most of the birds just from their songs.

"It's usually freezing cold!" says Nixon, who surveys along the Carcross road. She says she's impressed that the Yukon, with its relatively small human population, can produce enough volunteers with the skill and dedication to carry out the survey. Information from the survey is combined with data gathered across North America and used to determine bird population trends.

For more information about Yukon songbirds or the Breeding Bird Survey, contact Environment Canada in Whitehorse.