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Everyone:

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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