Rare Bird Report 39-B-08

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: Lisa Weisse

 

Date of Observation: 11/27/06

 

Date Report Prepared: 11/30/06

 

Locality of Observation: Below American Falls Dam, specifically perched on the cement wall between the main channel and the outflow the runs out over the rocks on the east side of the river.  

 

Habitat: River

 

Conditions: Overcast with moderate wind and snow squalls.  Duration of observation about 45 minutes.  Distance about 100 yards.  Binocs were nikon 10 x 42 Premier LX (Cliff) and Nikon 10 x 32 Premier (Lisa), and Swarovski 80mm HD scope with 20-60 eyepeice.  

 

Did you take notes…

            not at all

 

Did you consult a field guide or other reference work?

            during the observation?  Yes

 

What guide or reference did you consult?
Olsen and Larsson, Gulls of North American, Europe, and Asia

 

Description: Large Gull in juvenile plumage.  

Structure
Overall size was larger than Ring-billed Gull in direct comparison but no other gull species were pressent for comparison.  Head evenly rounded showing no strong angulation on forecrown or hindcrown.  Eye set forward on head, not centered, or closer to bill the nape.  Body was somewhat heavy, especially in the chest, so that it appeared more bulky, or less streamlined, than Herring Gull.  Legs were forward of center on perched bird although the angle of the photos makes this impossible to see.  Rear somewhat attenuated with moderately long primary projection, P8 about even with the end of the tail or possibly extending very slightly beyond tail tip.  Primary stack thin.  Bill short and thin with slight gonydeal angle visible, shorter and thinner than typical for Herring Gull with less obvious gonydeal angle.

Plumage
Overall coloration warm medium brown with little difference between upperparts and underparts in overall shade of brown.  Neck streaked making it appear paler than body.  Lores and auriculars washed with darker brown.  Malar region pale/whitish.  Bill entirely black.  Legs fairly bright pink.  Belly smooth brown with no obvious streaking, paler at vent.  Undertail coverts evenly barred brown and whitish.  Scapulars brown centered with even white edges creating scaled appearance on mantle, lower scaps with larger white tips on feathers.  Tertials extensively brown with white tips covering about 20 percent of feathers.  Some white edging visible along lower edge of bottom two tertials.  Primaries brown (appearing slightly darker in photos than it did in the field) with obvious pale crescents on tips.  Brown of primaries was definitely not blackish as in Herring.  Greater secondary coverts evenly and crisply checkered brown and white.  Median and lesser secondary coverts similar in overall appearance to greater coverts, evenly checkered with brown and white, although the individual feathers are brown centered with white edges and extensive white tips, more white in the median coverts than the lesser coverts.  

In flight primaries had pale inner webs, except at tips which were concolor with outer webs, and brown outer webs out to at least P8.  I was unable to see the details of P9 & P10 because of distance to bird.  Secondaries were darker the the rest of the inner wing.  Inner primaries paler than outer primaries but not contrasting as much as on Herring because of the paler and less extensive brown on the primaries.  Under side of wing entirely silvery except for dark trailing edge of primaries created by brown extending across both webs at tips of primaries.  Rump did not contrast significantly with rest of upperparts.  Tail was slightly paler brown than primaries, contrasting darker than rump, and had pale tips to all rectrices as well as pale bases to outer rectrices.  

 

Behavior: Perched on cement wall and flying around near outflow at base of dam.  Once it appeared to catch something in the water and eat it.  

 

How and when did you positively identify the bird, and what clinched the identification for you?  I first saw the bird flying around and recognized that it was not a Ring-billed or California Gull.  When it perched it appeared pale and frosty with brown primaries, small black bill, and characteristic chesty but attenuated look of Thayer's Gull.  Observation with scope revealed typical field marks consistent with Thayer's Gull.

 

How did you eliminate similar species, and what were they?
Of the North American large gulls there aren't many possibilities for confusion.  Western, Herring, Lesser Black-backed, Great Black-backed, Yellow-footed, Kelp, California, Ring-billed, and Mew all have blackish primaries in juvenile and first winter plumage.  California, Ring-billed and Mew also have bicolored bills in their first winter.  Glaucous has white primaries and strongly bicolored bill.  That leaves only Iceland/Kumlien's and Slaty-backed as possibilities.  First winter Kumlien's has paler primaries, more petite bill, tertials with extensive pale centers, and indistinct tail band if present at all.  Slaty-backed has limited markings in the secondary coverts, larger deeper bill with stronger gonydeal angle, usually pale head, bulkier body, darker pink legs, and typically molt of scapulars into first winter is well under way by this date.  The real potential for confusion is with hybrids between Glaucous-winged x Western and, especially, Glaucous-winged x Herring.  Glaucous-winged x Herring typically shows somewhat angulated head, eye more centered on head, shorter primary projection, primaries with more extensive pale edges running further up edges of feathers, finer more diffuse patterning in secondary coverts, some solid dark areas in the greater coverts, and some solid dark feathers on the mantle.  Glaucous-winged x Western typically has rounded head with eye centered, bulkier body, darker legs that are centered under the body rather than being set forward, more extensive dark in primaries, and long heavy bill with strong gonydeal angle.  This individual shows all field marks consistent with Thayer's.  Nothing about this bird suggest either of these hybrid combinations.

 

Experience with this species: I see from a few to fifteen Thayer's Gulls and numerous first winter Herring Gulls annually in Idaho and Utah.  I've studied gulls pretty seriously for the past seven years.  

 

General experience birding: 14 years

 

Were photo(s), video, and/or audio obtained by you?
Yes  Photos

These photos were shot through a spotting scope and were cropped and resized, and may have been sharpened, but no other adjustments were made.