Trumpeter Swan on left and Tundra Swan on right photographed by Kent Nickell in Iowa. Middle swan photographed by Jeff Slovin in Natick. For more detailed info on telling the two species apart check out the excellent article by Rod Hug at Swans. One interesting feature in addition to the flatter culmen (upper portion of bill) of the Trumpeter and the prominent extension of the black lores into the eye on the Trumeter is that the forehead is pretty much in line with the upper mandible whereas on the Tundra the rounded forehead extends above the line of the upper mandible. Also, the nostril is about mid-way between gape and bill's tip in the tundra. It should be relatively nearer to gape in a trumpeter.