Interesting Waterbirds Part 2

The stakes are considerably higher in assessing the identification of Ang's mystery tern. After all, possible contenders include River Tern, which would be a first for the country, and Chinese Crested Tern, one of the world's rarest birds, for which there are no certain records away from the breeding area for close to a century!

Helpfully, Ang captured two other terns, which we can identify, in the same pictures - a Gull-billed (right) and a Little (left). Even allowing for the fact that the mystery tern is not on the same plane as the other two, we can confidently say that it is about the size of the Gull-billed Tern, which means that it is too big to be a River Tern.

Going back to first principles - eliminating the default common species - the only large terns which commonly occur on the West coast of Peninsular Malaysia are Gull-billed, Great Crested and Caspian Terns.

None of these should show a bill pattern like the bird in the photos, so does the bird fit with Chinese Crested Tern?

Close inspection of the photo, even though it is blurred, reveals that the yellow colouration appears to be limited to the lower mandible, rather than both, which should be the case with Chinese Crested. Futhermore, the head shows no hint of a shaggy crest, but is smoothly rounded in shape, with the black markings limited to an isolated block on the ear coverts. All of these are wrong for Chinese Crested Tern.

So, back to our default species again! In fact, the bird looks in all aspects except the bill like a 1st winter Gull-billed Tern. A brief scan for Gull-billed Tern images on the web turns up this picture (scroll down) and this one. Mystery solved! It seems 1st winter Gull-billed Terns can show a yellow base to the lower mandible, something I didn't realize before!