Yesterday (29/6/10) early morning Yosef Kiat, Yotam Lenhardt, Ohad hatsofe and Asaf Mayrose ringed this interesting tern at Ma'agan Michael, among the Common Terns breeding there. Yosef passed the images and biometrics to me (thanks Yosef!) and I will be happy to get some expert opinion on it, as I am quite puzzled by this bird. Plumage-wise, it shows all the important characters for White-cheeked Tern - dark grey belly and breast contrasting to white cheeks, dark grey upperparts and most important - grey rump, uppertail coverts and tail.
However, size and structure are not good for White-cheeked Tern, and are in fact closer to Common Tern. In this table you can see the biometrics of the mystery tern compared to the Common Terns ringed there yesterday:
I would expect White-cheeked Tern to be significantly smaller with darker bill, and darker and shorter lags. This bird is quite close to the average Common Tern in size and structure. I am aware of the huge variation demonstrated by Common Terns both in plumage and structure, but I have never seen nor managed to find info about such dark birds, nor did I find anything about hybrids of these two species.
It is worth noting that White-cheeked Tern is a scarce summer non-breeding visitor to Eilat, but extremely rare on the Mediterranean coast. During the last 4-5 summers one 'White-cheeked Tern' has oversummered at Ma'agan Michael among the breeding Common Terns (I've seen that bird several times). This bird ringed makes me wonder whether we missed something all of these years.
So, if anyone has interesing info or ideas about this bird, please drop me a line - as a comment here or by email. Thanks!