It's a tern (I think), but which one?

I've only recently started birding with any sort of regularity or enthusiasm, and I'm not all that good at it yet. I snapped a couple of pictures of a bird in flight the other day, and I've been trying to figure out what species it is ever since. Despite my lack of expertise, I think I've got it narrowed down to two possibilities. Of course, it's also possible - again given my lack of expertise - that I've managed to totally miss even the family identification.

With that in mind, I'm turning to all of you for help. I've got two pictures, both taken through a 300mm telephoto lens. One of the two pictures is decent; the other is out of focus. I'm going to post the two pictures below the fold, and then I'll run through my thinking as I try to id the bird. I'd really love to get feedback from the rest of you on how well I've done.

i-de5bffe133c976236bff5339b265f0f6-roytern 1-tm.jpg

i-318d7d11e061d8bd9858549834d4139e-roytern 2-tm.jpg

The bird was observed on August 6th, along the shoreline of Big Lagoon, in the Pensacola, Florida area. It was just high enough and far enough over the water that it was difficult to estimate the size, but my impression was that it was in the mid- to large- gull size range (~18-24"). Given the forked tail and pointed bill, I feel reasonably safe in IDing this as a tern.

There are several species of tern that occur in this area, so I consulted my guidebook to see which is the best fit.

I feel reasonably confident eliminating the Sandwich Tern based on the color of the bill; Gull-billed Tern based on the shape and color of the bill; Black and Sooty Terns based on the overall color; and the Least Tern based on the size.

I'm a bit less confident when it comes to Foster's Tern and the Common Tern, but I don't think this one is either of those. Both those species are noted as having orange legs and feet, and I'm pretty sure that the one I saw had dark feet. They're also a bit smaller than I think this one was, and the head markings don't look quite right for either.

That leaves me with two choices: the Royal Tern and the Caspian Tern. I think the most likely choice is the Royal Tern, given the lightish underside of the primaries and the depth of the fork in the tail. At lest for the moment, that's what I'm calling this one.

But I could be wrong. What do you think?

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Royal Tern indeed. The identification is reinforced not only by the pattern of the underside of the primaries, but also by the head pattern and (especially) bill shape and color: Caspian Tern has a thicker, redder bill than this. Good reasoning in your post.

By Nathan Pieplow (not verified) on 23 Aug 2008 #permalink