Caspian Tern: As large as a big gull, the Caspian Tern is the largest tern in the world. Its large coral red bill makes it one of the most easily identified terns throughout its worldwide range.
Elegant Tern: A medium-sized tern with a shaggy crest and a long, slightly drooping orange bill, the Elegant Tern is seen along the Pacific Coast. After nesting in very southern California and Mexico, it wanders northward to northern California, or even to British Columbia.
Forster’s Tern: One of several medium-sized terns that are similar in appearance, the Forster’s Tern breeds primarily in marshes and winters along the coasts. The comma-shaped black ear patch in winter plumage is distinctive, but some other plumages are very confusing.
Common Tern: A graceful, black-and-white waterbird, the Common Tern is the most widespread tern in North America. It can be seen plunging from the air into water to catch small fish along rivers, lakes, and oceans.
Excerpts from: AllAboutBirds.org