Aurora Reservoir Gulls

[Glenn and Laurie]

Folks,

 

I saw the Moulton's pics of a 'mystery gull' at Aurora Reservoir late last night and figured I should go take a look.  I agree completely with Paul Hurtado, the photos strongly suggest if not confirm a Slaty-backed Gull.  More on that in a minute.  First more on Aurora Reservoir.... Aurora Reservoir is a) 95% open and b) within 1 mile of the county landfill.  A larophile's dream come true.  The only setbacks are c) it is difficult to see birds on all sides of the lake and g) it costs $10 every time you go. 

 

When Joe and I arrived this morning, there were only about 100 gulls on the lake.  We saw at least 3 Glaucous Gulls, a couple of Lesser Black-backed Gulls and a Thayer's Gull.  The white-cheeked geese numbers however were in the thousands.  Several Snow Geese, 2 Greater White-fronted Geese and 2 Ross's Geese were fairly easy to find.  Most of the white-cheekers were Cackling Geese and there was a bunch.  We stayed until about 10:30am and the gull numbers never varied much.  About that time Bill and Jim Schmoker arrived.  They stayed for hours and alerted me when the gull numbers increased.  They had added some Lesser Black-backed Gulls and a Glaucous-winged Gull to the increasing numbers of gulls.  At about 1:30pm Bill called with a 'candidate' for the Moulton, Slaty-backed, although distant.  I decided to go back and check.  Bill got me on the suspect bird which we studied from a distance.  Here is what we noted:

 

A dark-backed gull, the same size as nearby Herring Gulls, but slightly darker than nearby Lesser Black-backed Gulls (there were many to compare).  The bird had noticeably broad white tips to it's flight feathers, giving it a broad, white trailing edge to it's wings.  The gull was certainly darker then the Lesser Black-backs but not as dark as a Great Black-backed nor as large.  When spread-winged, noticeable white tips and sub-terminal white spots were seen.  Photos of the bird in flight confirm the 'string of pearls' indicative of Slaty-backed Gull.  Though my photos are distant, I can see the sub-terminal white spots and the tri-colored under-wing of Slaty-backed Gull.  The size and mantle color also suggest Slaty-backed Gull, which I believe is the correct ID of this bird and the bird photographed by the Moultons.  As a cautionary note, there are many Lesser Black-backed Gulls, probably a couple of Great Black-backed Gulls and several white-primaried-gulls visiting Aurora Reservoir.  Please use caution and discretion when observing these birds as they are tricky and subtle differences are most important.

 

Glenn Walbek

Castle Rock, CO

http://www.pbase.com/gwalbek/2013_birds