I always come across the Terns overhead and in constant motion or at a distance you can't really ID at, so I just log as Common/Forster's Tern with Ebird. I was at Frank Lake yesterday and managed to catch them on the ground and not too spooked so I could get close enough to try for ID pictures. I believe I have 3 species here. The first I believe are Common as the tail does not seen to extend beyond the wing tips and a Grayish body. The second I think are the Forster's species as the tail appears to extend past the wing tips and the body has a more whitish appearance. The 3rd species is the Black Tern. Can members let me know if I am right on these species?
John, I'm not the best on these terns and given most of my experience is of them in flight and not perched. I think your reasoning is pretty solid and feel the possible commons have darker wing tips and smaller legs than the second photo of possible Forster's having paler wing tips, longer tails and longer legs. Legs can be relative to posture and how the belly feathers are fluffed or held tight to the body which influences the amount of leg visible. The 'Forster's photo' is a good example as the leftmost bird is tight and alert showing longer legs than the more relaxed preening bird to its immediate right.
I would submit your sightings as you feel fit with the supporting photos and see what the reviewer for the area has to say as they'll have some good insight. Thanks for sharing and good luck. Nice little Black Tern by the way. Maybe I'll see my first one this year. Any that pass by the area will be going through shortly.
Gord Fraser Valley Birding Administrator eBird Regional Editor (Fraser Valley)
Thanks Gord for your opinion. This seems to be a difficult subject to the majority of people I met. Even at Frank Lake asking local birders for their opinion and the usually reply was they couldn't tell what species they were either