A few people have asked me to post the responses I got about distinguishing Common 
from Forster's terns. The Cape May website sent by the Wests has a photo quiz with good 
comparisons. With the permission of my sources, here's the info:

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Go to this web site, will tell you everything you want to know about the 
difference between Common and Forster's Tern.

http://www.birdcapemay.org/tigrina/quiz_707.shtml

Winger West

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Commons have distinctly shorter legs than Forster's.  Many commons will show 
a gray bar in the wing when perched.  Leg color is redder.  If you have one 
with a black mask, and it wraps back around the neck, it's a Common.  In 
good light, an adult Common should be grayish on the body.

Hope that helps.

Kurt Schwarz

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Mary, I don't know if anyone has responded to you yet, but the
identification challenge here is one you can master!  If you don't
have a copy of the Sibley Guide, you should get one, since it often
covers all of the interesting ID points between close species.

To answer your question about which is more common, both FOTE and
COTE are, well, common right now...grin.  COTE is here in abundant
numbers into mid-September, at which time the numbers begin to dwindle
slowly until they are pretty much all gone by the end of October.
FOTE is abundant through mid-October then tapering to smaller and
smaller numbers.  They are often seen throughout the winter if it
stays warm enough (at the jetties in Ocean City, for example).

Now, telling them apart...so much depends on whether you are looking
at a juvenile or at an adult bird.  FOTE are relatively longer-legged
and their legs are bright red relative to the darker legs of COTE
(I've never found this particular field mark very reliable).

FOTE has shorter wings than COTE.  In a perched bird, their tail
generally is as long as or longer than their folded primaries.  COTE,
on the other hand has primaries extending well beyond the tip of the
tail while the bird is seated.

The dark carpal bar is often a good way to pick out COTE from a
distance.

Now, keep in mind that a lot of these field marks aren't useful with
juvenile birds.

I could go on and on but you really should pick up a Sibley and study
the two birds carefully then go out in the field and spend time examining
them!

Best of luck and I hope this helps.

Norm Saunders
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Mary LaMarca
Silver Spring, MD