I was out at Tybee Island, Georgia over the weekend and was able to get some good comparison shots of Royal and Caspian Terns. These are the two largest terns in North America (Caspian is the largest tern in the world), and though they are quite different, they can still occasionally be confused. At least they can be confused until you see comparison photos like these!
In the first photo the Royal Tern is on the left and the Caspian on the right. Royal Tern has a bill shaped like a dagger blade while Caspian has a bill like a spear blade, heavy and broad-based. The Caspian is larger and bulkier overall with a blocky head shape. Also notice the white crown of the Royal Tern with the black on the head, limited to a streak behind the eye and a small spot just in front of the eye. In Caspian much of the head is covered with dark streaking, becoming almost solid black in places.
The second photo shows the slimmer body shape of the Royal Tern, well, compared to the larger heads and heavy chests of the Caspian Terns in the background. Caspian Terns have deeper reddish bills with dark smudged tips, while Royal Tern has a bright orange bill lacking any dark markings. A few Royal Terns in the breeding season do show a much more reddish bill, almost as red as Caspian, but the slimmer bill structure should give these birds away.
Cameron Cox currently leads photographic birding tours all over the world. To learn more about these tours or to sign-up for one, please visit the Tropical Birding website.