[cobirds] Aurora Reservoir (Arapahoe County): A bounty of gulls etc. today

Folks- ever stare through your scope for so long that your weak eye goes wonky?  Well, that happened to me today which is a good indicator of how target-rich Aurora Reservoir was.  Was indeed a treat to meet Joe Roller, Glenn Walbek, and Paul Differding awaiting us as we rolled in at the cozy hour of 10-ish this morning, with the Black Scoter centered in Glenn's scope.  Gulls were on the wane though, so for awhile my dad Jim & I focussed our attention on the thousands of geese on the lake (being stirred every half-hour or so by Bald Eagles.)  Of the 5000+ geese I'd say about a quarter were Cacklers, at least three Ross's, and 10+ Snow.  A pair of adult Greater White-fronted Geese anchored a point off the swim beach for a time.  Of interest to warped minds like mine was a hybrid Greater White-fronted x Canada Goose.

At their nadir I could probably have counted all the gulls in my view on my fingers & toes, but after 11 am, legions of gulls began streaming in to the lake, many coming in to the swim beach cove to bathe and roost on the ice.  Many remained far out on the mainly open lake, though.  Though we searched near & far, high & low, wet & dry, we didn't see any Slaty-backed candidates for several hours among the thousand  or so gulls in view.  We were rewarded with 8 other gull species, however (Ring-billed, Herring, California (1), Thayer's (at least 1 juv, probably missed more), Lesser Black-backed (5 adults and at least 1 juv), Glaucous (2 ad & 2 juv) Great Black-backed (1 juv), and Glaucous-winged (1 Juv.)  In mid-afternoon, shortly after John Drummond and Ron & Jeannie Mitchell arrived, we spotted a bird far out on the lake that we studied extensively, concluding from what we could see that it fit the description of an adult Slaty-backed Gull.  My eBird notes on the bird:

Adult, first found 31 Dec. by David Moulton. Size of a large Herring Gull (determined by direct comparison), not as big as nearby Glaucous Gulls. A bit darker mantle than Lesser Black-backed Gulls (also seen in direct comparison) but not as dark as adult Great Black-backed would be expected. Extremely prominent wide white skirt at end of primaries that wrapped under the bottom of the wing due to white secondary tips. Also had bold tertial crescent just below mantle. At distance I couldn't make out streaking on head or nape but did get impression of dark eye line. Underwing overall lighter than GBBG (and LBBG), with black only at furthest tip, pale gray primary bases, and bold white trailing edge. Extremely distant digiscoped video obtained, don't know if it will be useful. Reasonable pics by David Moulton from 31 Dec. and Glenn Walbek communicates that he got diagnostic flight shots today.

Glenn Walbek returned to the lake and viewed the bird from another vantage.  As it departed the lake around 3 pm he got flight shots of the bird.  I hope he, John, or anyone else viewing the bird will share their comments on this gull.

Congrats on the great find, David, and thanks for getting the news out, Joe!

And good luck to any searchers tomorrow.

Enjoy- Bill Schmoker, Longmont

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|           Bill Schmoker              |
|  bill.schmoker@gmail.com  |
|       http://schmoker.org         |
| http://brdpics.blogspot.com |
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