Caspian is "a size bigger creature" just slightly smaller than American Herring Gull, with much more bigger bill. [Caspian Tern, Nummy Island, Stone Harbor NJ, October 3 2012. The largest of the terns, larger than many gulls. ), but wing span gives a better idea of the size difference (Caspian 50 in., Royal 41 in., Elegant 34 in.). Caspian Terns are larger than Royal and Elegant terns. Legs and feet are black.
Caspian Tern: This large stocky tern has pale gray upperparts and white underparts.

Now I know that > Caspians have a reddish rather than orangish bill and on this point this > tern would have been a Caspian. The trouble is that the Caspian’s bill color can vary, especially with different lighting and depending on the bird’s age. In Alaska, Aleutian Tern has a white forehead. Wings are black-tipped above and black-edged below; tail is deeply forked. The length (bill tip to tail tip) shows some difference (Caspian 21 in., Royal 20 in., Elegant 17 in. One Royal and two Caspian Terns. One field mark is the bill color: orange-red in the Royal, dark red in the Caspian. The breast and face are also white. In North America, it is common along both coasts and locally inland, mainly around large bodies of water. The smaller Elegant Tern … A sleek seabird of warm saltwater coasts, the Royal Tern lives up to its regal name with a tangerine-colored bill and ragged, ink-black crest against crisp white plumage. Noted for its long adolescence, with the young dependent on their parents for many months; even in late winter, many an adult Caspian is trailed by a begging youngster from the previous nesting season. It is white with a light gray mantle and white undersides. The head-to-tail tern sizes according to Sibley are: Caspian 21″, Royal 20″, Sandwich 15″, Gull-billed 14″, Forster’s 13″ Common 12″, Black 9.75″, and Least 9″. Click to enlarge photos.] Terns range in size from the least tern, at 23 cm (9.1 in) in length and weighing 30–45 g (1.1–1.6 oz), to the Caspian tern at 48–56 cm (19–22 in), 500–700 g (18–25 oz).

The large bill is coral-red, the undersides of the primaries are gray, a short white tail is slightly forked, and the legs are black. Aside from a few interior localities in Florida, it is almost never found inland except after hurricanes. Nonbreeding Royal Terns have extensive white on the crown, whereas Caspian Terns are mostly dark on the top of the head with a small area of streaks or white on the forehead. [31] [32] They are longer-billed, lighter-bodied, and more streamlined than gulls, and their long tails and long narrow wings give them an elegance in flight.