[cobirds] Larimer County Gulls (long)

With multiple reports on eBird of an adult Slaty-backed Gull (SBGU) and three distinct Great Black-backed Gulls (GBBG) (adult, third cycle and first cycle) reported this past week at the Larimer County Landfill (one of my local patches), I spent most of the day today looking for these rare gulls. Here are some notes that may aid others in looking for these gulls in coming days.

Gulls typically do not arrive at the landfill until 9 am (from overnight roost locations). So, between 7:15 and 9:15, we (Dave Wade, Walt Wehtje and I) checked some local lakes. No gulls at Robert Benson Lake or Fossil Creek Reservoir. Hundreds of gulls were at Donath Lake, including immature Glaucous Gull. Hundreds of gulls were on the ice shelf at north end of Boyd Lake. As we approached that group, they were flushed by an Eagle. It was almost 9 am, so we headed to the landfill. Passing Robert Benson Lake, we found a large flock of gulls on the ice. These new arrivals may have been the birds that flushed from Boyd. We watched them for about 10 minutes until they flushed and headed NW to the landfill.

Before they flushed we observed an adult black-backed gull that may have been the SBGU (low quality photo below). It seemed smallish but sported a wider white tertial crescent and a much smaller red gonydeal spot than other nearby adult Lesser Black-backed Gulls, and had the typical gray smudge around the bright pale iris. We couldn't see legs. I could not discern the color of the feet in the photo below. Video of the bird in flight was inadequate to assess pattern on primaries. In the photo, the shade of slate gray of the mantle is different than the LBBG (rear left). Input on this bird's identity is welcome.

Expecting to refind this bird at the landfill, I was disappointed when we couldn't find it there among at least 10 LBBGs of various ages. Nor did we find any birds large enough to be a GBBG. We did find several Thayer's Gull and an adult Herring x Glaucous-winged hybrid ("Cook Inlet Gull"). There was also a probable first-cycle Cook Inlet Gull as well. I will add some photos of these birds on the CFO Facebook page later.

We left the landfill after a couple hours in order to photograph the gyrfalcon. The gulls left too. Several hundred had flown NW to Horsetooth Reservoir, but EJ Raynor and I couldn't find any SBGU or GBBG there either. Many gulls eventually returned to the landfill in the early afternoon.

Larimer Landfill is open to birders who check in at the entrance booth and follow staff instructions. They are closed Sundays. On Monday, they will close early (2:30 pm). They will be closed Dec 25 and Jan 1.

If you search for these gulls, please try to get documentary photos and share your find with CFO, COBIRDS, Colorado RBA, etc.

In the photo below, the candidate SBGU is on the right, behind a California Gull. To its left is Ring-billed Gull. To its right is Herring Gull. A sleeping LBBG is at rear left.

Nick Komar
Fort Collins CO

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