[cobirds] More on Larimer gulls (long)

I spent most of the day looking for the recently reported rare gulls in Larimer County. Seems there has been some turnover, with some new arrivals and possibly some departures. Morning gull numbers were lower at Boyd Lake's North end and Donath Lake. At 9 A.M. there were several hundred gulls on the ice at Robert Benson Lake, but just one with dark back (immature Lesser Black-backed Gull). At 9:30 A.M. there was a large number of gulls at the landfill. The regulars were all there (hundreds of Ring-billed, dozens of Herring, less than a dozen Lesser Black-backed, several Thayer's, one Glaucous). A Nelson's hybrid (Glaucous x Herring) was interesting but not new. There was no sign of the Slaty-backed Gull, Great Black-backed Gulls, or Cook Inlet hybrid gulls reported over the weekend.
After an hour, I left the assembly of birders to see if I could find the missing rarities at nearby lakes. Horsetooth Reservoir had a few hundred gulls, all regulars. Warren Lake had almost 400 gulls including 6 Lesser Black-backed and 1 Nelson's. Fossil Creek Park had a few Herrings. Robert Benson, Donath, Horseshoe, Boyd, Houtts, Equalizer, Lake Loveland and Rist Benson were all empty of gulls. So, at 2 pm I returned to the landfill. There were still hundreds of gulls present, many sitting on the east facing hill, north of the trash heap.
I suspected these east facing gulls were out of sight for the birders on the hill, so I asked permission to scope them from the entrance road below. I quickly focused on a large, massive-billed black-backed gull, hoping it was one of the GBBGs (which I've yet to any this winter). To my amazement, it appeared to be a subadult Western Gull (full description in eBird). Just as I set up my camera to document this mega-rarity (County Life Bird for me), a rocket flushed the birds. I lost it in the chaos and the flock landed out of sight. At 2:30, the landfill closed.
Hoping that some of these gulls moved to Horsetooth Reservoir, Peter Burke and I drove to the south end. Few gulls were present but we were rewarded by another Mega-rarity: an immature Black-legged Kittiwake at the Spring Canyon Dam.
Then, checking out gulls further north, opposite Dixon Dam, a group of assembled birders studied a large Herring-sized gull with a darker gray mantle that looked like an adult Siberian Herring Gull (aka Vega Gull-full description in eBird) until the sun set behind the foothills.
So a slow start to the day, and an exciting end. I'm hoping this detailed account will help others view gulls in Larimer County and that folks document and share their observations with the birding community.
Good birding and Merry Xmas!



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