We are posting this on behalf of the Missouri eBird team and the MBRC. 

Common Tern is not common in Missouri.  It is easy to misidentify, especially in spring and because of plumage variability in Forster's Tern, which is much more widespread and regular here. Due to frequent misidentifications, eBird has now set the filter for Common Tern at zero in spring, statewide; this means that all eBird submissions will need photos and/or clear written details in order to be validated. Here is an outline that we hope will help.

I. SPRING ADULTS IN FLIGHT:  Breeding-plumage Forster's is distinctive, with a full black cap, pure white underparts, long tail streamers, orange- to red-based bill, and primaries that are silvery-white above with no dark markings. At very close range, you may be able to see that the tail streamers are dark on the inner web, and that the tail is pale gray, leaving a white rump area between the pale gray back and tail. (These features may be visible even when perched.)

Common Tern has a full black cap with light gray underparts that can be hard to discern in flight, especially at poor angles or in shadow. The outer primaries are somewhat grayer than on Forster's, typically bordered by a dark streak or wedge that cuts across the wing at about mid-primaries. At very close range, you may be able to see that the tail is white (not pale gray, thus no rump contrast) with dark outer webs (not inner) to the outer tail feathers.

II. SPRING ADULTS PERCHED: With a good look, perched birds can be easier. (1) The gray underparts of a Common (vs. white in Forster's) may be easier to discern. (2) The longer tail streamers of Forster's usually project beyond the tail; those of Common do not. (3) The primaries of Forster's, even when folded, still look pale gray or whitish; those of Common tend to look darker gray, though not strikingly so. (4) The legs of Forster's are notably longer than those of Common, a difference easily seen if the birds are close together and similarly posed. (If you see a tern that has really super-short legs, you may have the first state record of Arctic!).

III. UNRELIABLE FEATURES: (1) It is true that a Forster's bill averages heavier and more orange than a Common's, but this is just a "soft" indicator and should not be the basis of identification. (2) The underwing pattern is pretty similar in both. (3) Forget about any differences in silhouette or flight behavior; they are too subtle to be helpful.

IV. ADDED COMPLICATIONS: The most troublesome problem is that sub-adult Forster's look different from adults. 

(1) A first-summer (one-year-old) Forster's will have dark outer primaries. Fortunately, these birds usually have the immature/non-breeding Forster's face pattern, with a white crown and bold black cheek patch. 

(2) BUT we also get second-summer (two-year-old) Forster's that still have the dark gray primaries along with a partial or full black cap and orange/red-based bill. Their tails may be shorter too.  These birds, we suspect, are the source of many Common Tern records. They can be separated by their white underparts, the absence of a dark wedge bordering the gray primaries, and (if perched) their long legs. Obviously, good photographs will help. 

(3) Note also that there is such a thing as a first-summer Common Tern, but this will resemble a fall bird, with a wrap-around dark "shawl" on the head, a dark shoulder bar, and a dark secondary bar. They have white underparts, while second-summer Common is more like adult and is getting gray.

The points mentioned above for adults are covered well by several of the standard more-thorough field guides: Sibley, National Geographic, Stokes. But none of these illustrates the second-summer Forster's that can be so annoyingly similar to Common. This is shown only in specialty guides like Terns of Europe and North America, by Olsen and Larsson.

The main takeaway? Look at terns very carefully, use multiple features in making your identification, take the best notes you can, with photos if possible, and use "Common/Forster's" in eBird whenever you are uncertain. 

If you wish, feel free to write us for our opinions -- or guesses.

Bill Rowe and Josh Uffman

St. Louis

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