Slaty-backed Gull (age? more than one?)

Slaty-backed Gull with Glaucous-winged, Herring, and Thayer’s Gulls all nearby.

Today I saw a very dark mantled gull that I am reasonably confident is a Slaty-backed Gull. The heavily streaked head with quite pale eyes and pink legs seem to leave no other alternative, as far as I can tell. Gulls of the Americas suggests that Western Gulls can have pale eyes, but they don’t have streaking, and none of the pictures of Western Gulls in the book showed eyes that were anything close to as pale as those on this bird. I’m trying to gain a little more understanding of gull identification, so I decided to spend a little more time with it and have some observations/questions.

Slaty-backed Gull Slaty-backed Gull
Slaty-backed Gull (click on thumbnails for larger)

This bird seems to be pretty much an adult, though there are some elements that are not so adult-like. Perhaps this is a fourth-cycle type gull then? The beak seems quite pale at the base, the red gonys spot seems to be somewhat faint, there is some darkness still in the tail feathers, and the some of the tertials and primary coverts seem to be a bit brownish relative the the rest of the dark gray feathers. It also seems that the outer primaries are not fully grown yet.

The other question I have relates to a bird that was photographed at the park almost two weeks ago by Geof Smith. I think it is also a Slaty-backed Gull, and I’m curious if it is possible to tell whether they are different birds, or whether they are most likely the same bird.

Slaty-backed Gull Slaty-backed Gull
(Photos by Geof Smith)

The gull Geof photographed was further away, (so the crop/enlargement is somewhat blurry) but it appears that it might have somewhat less streaking than the gull I saw. It also seems to have shorter primaries. I am not sure how fast the feathers grow, so I do not know whether the apparent difference in length is reasonable for two weeks difference in growth. It seems more likely to me that these are the same birds, rather than two different birds of this species. That said, at the very least, this is the second Slaty-backed Gull I have seen in Sitka this year (April 2007 Slaty-backed Gull), and the third since the beginning of 2006 (January 2006 Slaty-backed Gull).

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