Cygnus columbianus
Easily confused with Whooper Swan, but yellow on bill restricted to a rounded patch. May sometimes extend towards the bill-tip, but never beyond nostrils. Angle of mouth bold black. Gives a more compact impression than Whooper, due to shorter neck. Head shape noticeably more rounded, and seemingly larger. Juveniles paler grey than Whooper, with the same bill pattern as in adults, but in faint, pale yellow and pink. Flight easy and goose-like.
More vocal than other swans. Both higher pitched and deeper sounds. Birds on the water often gives crooning, crane-like notes with less defined pitch. Tone is less full-bodied in these calls. Flight call a deep, yelping, soft barking sound. Each call short, with a "helpless" quality.
Wikipedia: map (se also Xeno-canto below)
Birdlife ecology
