Rare Bird Report #27-B-09

Idaho Bird Records Committee Rarities Report Form

Species: Thayer’s Gull

Reporter: Cliff Weisse, 4125 Beaver Springs Rd., Island Park, ID 83429, 208-558-7789, cliffandlisa@octobersetters.com

Other Observers: Jay Carlisle, jaycarlisle@boisestate.edu , Darren Clark, clarkd@byui.edu / Steve Butterworth, tntbutters@cableone.net / Heidi Ware, heidithebirdnerd@yahoo.com / Louie and Ileana Quintana, lnquintana@yahoo.com

Date of Observation: 14 February 2009

Date Report Prepared: 16 February 2009

Locality of Observation: Riley Pond at Hagerman State Fish Hatchery, Hagerman, Idaho

Habitat: Pond within a wetland complex. Gulls stage at this location before going to roost on the Snake River.

Conditions: Sighting took place at about 5:15 PM with sun low and from the side. Distance was about 75 yards. Duration of sighting maybe 20 minutes, long enough so we were satisfied and didn’t miss anything we wanted to see. Optics used included scopes @ up to 60x and binocs of 8-10x. Wind was light enough to be irrelevant.

Did you take notes… Yes, the day after the observation.

Did you consult a field guide or other reference work? No

Description: Description adapted from field notes written on 2/15/09

First winter “large gull”; overall brown and whitish. Brown was similar shade to Herring Gulls nearby and whitish was more of a very pale gray/not stark bright white. Overall this bird appeared paler/faded compared to Thayer’s Gulls seen in fall.

-Size between Herring Gull and California Gull in direct comparison, closer to CAGU. This was a small looking gull, obviously smaller than Herring in direct comparison.

-Body with attenuated rear end and deep chest making it look front heavy
-Head rounder and less angulated or squared off than Herring Gull (from past experience and in direct comparison to 2 Herring gulls present)

-Bill short, narrow with barely noticeable gonys. Length noticeably shorter and thinner than HEGU and CAGU (both species present for comparison) Black all the way to the base.

-Eye dark brown

-Legs pink

-Upperwing coverts extensively checkered with brown and white. Clean change from brown to whitish, not muddy or fading from one color to the other. Greater coverts checkered with whitish all the way to the base with no solid brown areas.

-Tail brown, not blackish and paler than first winter HEGU present, with pale bases to the rectrices forming wide brown tail band that covered most of the tail.

-Scapulars pale gray with fine subterminal dusky bands, some feather appeared to have two bands

-Primaries dark brown but not blackish, noticeable paler than first winter HEGU in direct comparison. Tips of primaries had narrow pale “crescents” on edges. Pale edges limited to tapered portion of each feather and not extending along the parallel sides of the feathers.

-Primary extension medium long with three primaries extending beyond end of tail. The tail fell between the third and fourth visible primary tip, being a bit closer to the fourth than the third.

-Tertials paler brown than primaries matching overall tone of body closely so the primaries contrasted obviously darker than the rest of the bird. Tips of tertials had whitish edges and notches.

-Upper wings in flight showed large pale window on inner primaries/outer secondaries that extended well out into primaries. Outer primaries had pale inner webs and dark outer webs creating striped look when fully spread.

-Under wing (seen well during bathing and during an extended wing stretch) was slivery with obvious dark trailing edge to primaries. Width of dark trailing edge was less than one primary’s width.

Behavior: At first the bird was just standing on a submerged rock, then it swam away from the rock and bathed/preened. Eventually it lifted off and flew towards the roosting area.

How and when did you positively identify the bird, and what clinched the identification for you? I first noticed a first winter large gull standing on a submerged rock. It had a small, entirely black bill which made me consider Thayer’s. Clinching field marks included completely checkered wing coverts, long primary extension, deep chested shape with long attenuated rear end, and the underwing and upperwing pattern in fight.

How did you eliminate similar species, and what were they?
Possible species for confusion include all large gulls with dark primaries in first cycle plumage and some hybrids.

-Herring Gull is eliminated by a combination of primaries being paler brown with narrow pale tips, small narrow entirely black bill lacking obvious gonys, rounded head shape, longer primary extension, lack of solid dark area in greater coverts, small size, pale inner webs on outer primaries, and front heavy body with deep chest and long rear end.

- Slaty-backed Gull has muddier checkering on wing coverts that is much less extensively whitish, shorter primary extension, larger (especially longer) bill with obvious gonys, deep bellied structure, and is much larger.

-Lesser Black-backed Gull does not have pale inner primaries/outer secondaries in flight, has black primaries, and has black tail with extensively white base.

-Glaucous-winged Gull is much paler overall with paler primaries that have extensive pale edges running along sides of feathers, large bill, and deep-bellied shape.
Western Gull is larger, has less extensive checkering in coverts including solid dark bases to greater coverts, deeper bellied shape, shorter primary extension, black primaries lacking pale window on wing in flight, large bulbous bill, and black tail lacking pale base.

Californian Gull has sharply bicolored bill, lacks pale window on primaries, has much longer primary extension, and has blackish primaries with dark inner webs on outer primaries.

-Glaucous-winged x Herring Gull has larger bill, shorter primary extension, heavier, deeper-bellied body shape, and some solid dark areas in greater coverts

-Glaucous-winged x Western Gull has shorter primary extension, very large bill with strong goyndeal expansion, less extensive pale window on inner primaries/outer secondaries in flight, deep-bellied body shape, and is larger.

Experience with this species:
Weisse- I’ve seen multiple Thayer’s, mostly first winter but some adults, every year for ten or more years. I spend some time every year studying gulls in the field and a lot of time studying gulls via photos posted to the internet.

General experience birding: 14 years for Weisse

Were photo(s), video, and/or audio obtained by you? No