Confusing Sterna Terns

Forster's Terns

Forster’s Terns. Photographs by Mike Miller (click to see the larger version).

Mike Miller photographed these Sterna terns on Montrose Beach in late May. Both birds are Forster’s Terns and I think both are in their third calendar year, that is, they were born in 2011. The top bird has white underparts, a broad white area between the lower edge of the dark cap and bill, and a tail that extends past the tips of the folded primaries. These are Forster’s field marks. The bill also looks stout and the legs look long to me.

The second bird has a gray tail and a white outer web to the outer pair of tail feathers. These are also Forster’s field marks. Except for the dark outer 5 or 6 primaries both birds look like adults, but adult Forster’s Terns in late May should still have silvery white primaries, so that’s why I think they are in their third calendar year. Immature terns molt their primaries earlier than adults and when tern primaries become worn they darken. The dark primaries of these birds are really the only clue that they aren’t fully mature.