Of the 300 or so species that occur within the Seattle area, 25% (75 species) are birds that migrate through or winter here but breed exclusively or largely in the boreal forests of Canada.

The Red-throated and Common Loons that spend the winter diving for fish at places like West Point and Richmond Beach Park may have been born and raised in a lake within the Mackenzie River watershed of Canada's Northwest Territories. The boreal breeding Bonaparte's Gull, flocks of which can sometimes number in the hundreds in Lake Washington, is the only gull species that nests in trees! Those seen here are thought to have originated in Alberta. The hundreds of Lesser and Greater Scaup that sometimes winter in the area are similarly dependent on the thousands of small wetlands dotted across the boreal zone of Canada and Alaska. Unfortunately scaup populations have declined by an estimated 40% over the last 25 years but researchers don't know why. Many of the birds that use the area in fall migration are on their way even farther south to spend the winter. The flocks of Lesser Yellowlegs feeding in saltmarsh pools here in August, for example, may continue south to bays and wetlands along the Pacific Coast of Mexico or even farther south. The Golden-crowned Sparrow flocks that arrive at backyard feeders every October may have whistled their clear songs from a territory deep in the heart of the Canada's Yukon throughout the summer.

Some species that raised young in the boreal forest have stopped off to feed in the Seattle area on their way to wintering grounds in Central and South America. For example, a Wilson's or Yellow Warbler stopping by in August (some individuals of both species also breed within the state) may have departed Canada in July and be Central or South America by September.