The Trumpeter Swan (Cygnus buccinator) formerly bred widely across the North American boreal forest, but by early in the 20th century it hovered on the brink of extinction because of overhunting, Since then it has been rigorously protected and managed, and it has bounced back and continues to increase.
But we also have another species of swan in Washington, the arctic-breeding Tundra Swan (Cygnus columbianus). That species has remained more common over the years just because it is much more wide-ranging, but its populations seem more or less stable now, not increasing like those of the Trumpeter.
The two swans tend to forage in single-species flocks, but they also mix from time to time, usually a few of one species associated with a larger flock of the other. How do we tell them apart?
Most Tundra Swans have a small yellow spot between eye and bill, lacking in Trumpeter, but some Tundras lack that spot and look much like Trumpeters. The best way to distinguish these birds is by looking at the base of the bill adjacent to the eye. That area is broad in Trumpeter, so the eye looks like part of the bill, while it is usually distinctly constricted before the eye in Tundra, making the eye stand out.
Finally, the swan species have very different voices. That of the Trumpeter is a low-pitched sound that could be compared with an off-key trumpet. That of the Tundra is a higher-pitched honking, somewhat more musical than that of the Trumpeter, that might be likened to a flock of geese.