Every spring, the southern Yukon is treated to the spectacle of tens of thousands of swans passing through on the way to breeding grounds further north in the Yukon and in Alaska. This year a couple of special visitors were part of the crowd.

They showed up at M'Clintock Bay, south of Whitehorse, an important stop for the swans. Ice leaves the bay early, and the relatively shallow water provides plenty of plant life for the birds to feed on.

More than 3,000 of the swans that pass through M'Clintock Bay each year are Trumpeter Swans, North America's largest waterfowl. That's about a fifth of the world population of Trumpeters, says Jukka Jantunen, who keeps track of the swans for the Canadian Wildlife Service.

Jantunen has been monitoring the spring migration at M'Clintock Bay for the past three years. Besides the Trumpeters, he says, as many as 12,000 Tundra Swans use the area.

"That gives you an idea of how important the flyway is," he says.

However, it's hard to tell how long each bird stays. They're not individually marked, Jantunen explains, and one big white bird looks pretty much like another.

This year, however, a couple of Trumpeters looked a bit different from the rest. In the third week of April, Jantunen spotted a pair of Trumpeters sporting distinctive red collars with identification codes printed on them in white.

Their arrival wasn't a surprise for Jantunen. He'd been watching and listening for the swans all spring. The red collars contain tiny radio transmitters broadcasting on specific and separate frequencies, and Jantunen has been scanning the airwaves for their signals since the migration began.

The M'Clintock Bay pair are two of 27 Trumpeters captured and banded in Washington and southern British Columbia last December. The 27 red collars contained 27 radio transmitters, each tuned to an individual frequency so that the birds can be identified even if they can't be seen.

All the swans were healthy at the time of banding, but they come from areas where large numbers of Trumpeters have been dying of lead poisoning, killed by consuming lead shot and lead sinkers left over from human outdoor activity. They were banded in an attempt to determine where they are picking up the fatal lead.

The two that arrived at M'Clintock Bay appeared healthy. However, three of the original group of 27 have already died of lead poisoning and a fourth is in recovery, says Jantunen. That's about 10 percent of a small sample, he adds. Expanded to the whole population that winters in the affected area, that would mean about 1500 Trumpeters are dying of lead poisoning each winter.

"That's a lot in a population that was endangered not too many years ago."

Although the purpose of the banding is to track lead poisoning, a side-effect has been to provide the first solid information about how swans use the M'Clintock area.

Jantunen first spotted the banded swans on April 21, a Sunday. They left the area the following Friday morning, April 26, after spending five full days feeding and resting at M'Clintock Bay. Although Jantunen watched for them and listened for their radio signals near open parts of the Yukon River where swans are often sighted, they don't appear to have strayed from the bay.

Five days is a long stopover for birds racing to breed in the short northern summer. It's an indication of the importance of M'Clintock Bay in the migration, Jantunen says.

In fact, birds that don't face the pressure to breed stay even longer, he adds. Every year a few non-breeding and young adult swans linger at M'Clintock Bay until late May or early June.

The peak of the migration has passed now and most of the breeding swans are leaving. The red-collared pair are probably settling in to a summer's domesticity somewhere in the central Yukon or Alaska.

However, you can still glimpse the last stages of the migration from the viewing platform at Swan Haven on Marsh Lake.

For more information about Trumpeter Swans, contact the Canadian Wildlife Service, Environment Canada, Yukon Region, at (867) 393-6700, or the Yukon Government's Wildlife Viewing Program at (867) 667-5331.