Thayer's Gull (thayeri)

(last update: January 22, 2013)

Thayer's Gull 1cy November (1st cycle)

Below you will find a description of Chapter 36 THAYER'S GULL Larus thayeri, as published in one of the best Gull publication: "Gulls of the Americas" by Steve Howell & Jon Dunn.

"we" in the text below refers to the original authors. If any errors occur in this text, please let me know and mail to marsmuusseatgmaildotcom.

PART 1: IDENTIFICATION SUMMARY & TAXONOMY
PART 2: FIELD IDENTIFICATION SIMILAR SPECIES - ADULT
PART 3: FIELD IDENTIFICATION SIMILAR SPECIES - 1ST CYCLE
PART 4: FIELD IDENTIFICATION SIMILAR SPECIES - 2ND CYCLE
PART 5: RARER SPECIES
PART 6: DESCRIPTION AND MOLT - ADULT & 1ST CYCLE
PART 7: DESCRIPTION AND MOLT - 2ND & 3RD CYCLE

BELOW: PART 3

Thayer's Gull 1st cycle (2CY), mid-January 2014, central California, US (Amar Ayyash).	Thayer's Gull 1st cycle (2CY), mid-January 2014, central California, US (Amar Ayyash).
Thayer's Gull 1st cycle (2CY), mid-January 2014, central California, US (Amar Ayyash).	Thayer's Gull 1st cycle (2CY), mid-January 2014, central California, US (Amar Ayyash).
Thayer's Gull first cycle, January 28 2012, Pacific County, WA (Charlie Wright). Few scapulars replaced for 2nd generation feathers.	Thayer's Gull 1st cycle, February 16 2010, Winthrop Harbor, IL (Erik Bruhnke). Note pale windows on inner primaries which continue to the inner webs of the outer primaries. Also note the lack of dark pigmentation on the inner web tips of P1-P4, with just a bit of faint darker on P4.

FIELD IDENTIFICATION

SIMILAR SPECIES

Thayer’s Gull is medium-large, fairly long winged, and relatively slender billed, but its measurements overlap with most other large gulls, so identification should be based on a suite of characters. In particular, note overall size and structure (in comparison to gulls of known identity), especially bill size and shape, and wingtip pattern.

First Cycle.

First-cycle Thayer’s Gull variable, with the main confusion species in regular winter range being American Herring and Glaucous-winged Gull (especially compared to bleached Thayer’s). Note size and structure (especially bill), and outer-primary patterns.

AMERICAN HERRING GULL averages larger with stouter bill that generally has more-marked gonydeal expansion, often shows shallower sloping forehead. Plumage slightly to distinctly darker overall than Thayer’s, blackish to brownish black wingtips usually have indistinct or no whitish fringes, upperparts often have finer and less "frosty" pale patterning. Inner webs of outer primaries more extensively dark, so pale inner primary panel on upperwing contrasts more with more solidly dark outer primaries; underside of outer primaries darker (but can be reflectively pale). Some darkly pigmented Thayer’s can be very similar to Herring Gull (or hybrids), and problem birds may not be identifiable.
American Herring Gull (smithsonianus) 1st cycle ACX January 07 2012, Merritt Island, FL (Danny Bales). Banded as pullus in 2011 at Portland, ME. Rearscaps still juvenile.	American Herring Gull (smithsonianus) 1st cycle 1106-13848 December 07 2013, New Buffalo, MI (Amar Ayyash). Banded on June 24 2013 in Door County WI. Bi-coloured bill. All scaps replaced.
GLAUCOUS-WINGED GULL averages larger and bulkier with stouter bill, but some male Thayer’s approach small female Glaucous-winged. Wingtips of Glaucous-winged are pale to medium brown, similar in tone to upperparts, which typically have finer and less "frosty" patterning than Thayer’s. In flight, Glaucous-winged looks plainer and less contrasting, without a dark secondary bar and contrasting two-tone outer primaries.
Glaucous-winged Gull (glaucescens) 1st cycle, December 25 2006, Orange Co (John Avise). Replaced upper scaps and several feathers missing in upper row of lower scaps.	Glaucous-winged Gull (glaucescens) 1st cycle, January 20 2007, Orange Co (John Avise). Compare shade of primary tips relative to brown tone in wing-coverts.
KUMLIEN’S GULL paler and milkier overall with medium brown to whitish wingtips (lacking strong inner web/outer web contrast on P6-P10), lacks distinct dark secondary bar and tail band, and tertials paler and more finely patterned. Some presumed hybrids with Thayer’s look intermediate and are not safely assigned to either taxon.
Kumlien's Gull (kumlieni) 1st cycle F7 January 23 2013, Newfoundland (Peter Adriaens). Note dark outer-webs of outer primaries, especially P9-P7.	Kumlien's Gull (kumlieni) 1st cycle H8 January 24 2013, Newfoundland (Peter Adriaens). Note dark outer-webs of outer primaries, especially P9-P7.
VEGA GULL averages larger with a stouter bill that generally has a more marked gonydeal expansion; often shows a shallower sloping forehead. Blackish to brownish black wingtips usually have no or indistinct whitish fringes. Inner webs of outer primaries more extensively dark so that pale inner primary panel on upperwing contrasts more with more solidly dark outer primaries; underside of outer primaries darker (but can be reflectively pale); uppertail coverts and tail base contrastingly white, barred blackish with contrasting black distal tail band.
Vega Gull (vegae) 1st cycle, January 25 2011, Choshi, Japan (Terry Townshend). Dark secondary bar. Pale panel on inner primaries reaching outer secondaries S1-S3 and still innerweb of P6 largely pale.	Vega Gull (vegae) 1st cycle, January 27 2011, Choshi, Japan (Terry Townshend). Very similar to European Herring Gull. 1st gen flight feathers. Tail pattern typical. Beginning to gain some grey scapulars.
CALIFORNIA GULL (w. N. America) similar in overall size but has longer and narrower wings with long wing projection. Almost all winter birds told readily by sharply bicolored pink-and-black bill (can be mostly dark into early winter), which is also relatively longer with only a slight gonydeal expansion. California has earlier and more extensive PA1 molt (often including wing coverts) and has blacker wingtips; in flight, note California’s all-dark primaries (lacking a distinct pale inner panel) and solidly blackish tail.
California Gull (californicus) 1st cycle, October 27 2012, New Buffalo, MI (Amar Ayyash). By October already bi-coloured bill and advanced post-juvenile moult, including 2x GC.	California Gull (californicus) 1st cycle, July 30 2010, Qualicum Beach, BC (Guy Monty). Juvenile, with dark inner primaries.
WESTERN GULL (w. N. America) larger and bulkier with broad wings and stout, bulbous-tipped bill. Plumage generally darker and browner with black wingtips; upperwing lacks contrasting pale panel on inner primaries, and outer primaries solidly dark; tail solidly blackish.
Western Gull (occidentalis) 1st cycle, November 18 2006, Bolsa Chica, CA (David Marotta). Note the dark mask of a Western Gull and the rather dark plumage including tertials.	Western Gull (occidentalis) 1st cycle, November 08 2010, Oregon (Hartmut Walter). Dark outer primaries.
HYBRID GLAUCOUS-WINGED GULL X AMERICAN HERRING GULL larger overall with longer and stouter bill, but plumage can overlap with Thayer’s. Some presumed hybrids resemble Thayer’s closely and may be difficult to identify without known species for size and structure comparison, but most hybrids are bulkier with larger bills.
Cook-Inlet Gull (glaucescens x smithsonianus) 1st cycle, January 18 2010, Morro Bay, CA (Amar Ayyash). The white edges to the primaries may suggest Thayer's but the size of this individual is much more telling. The upperparts are a bit more frosty than Thayer's and the bill is that of Herring - certainly not Glaucous-winged.	Cook-Inlet Gull (glaucescens x smithsonianus) 1st cycle, January 11 2007, Rio del Mar, CA (Jeff Poklen).
HYBRID GLAUCOUS-WINGED GULL X WESTERN GULL. Besides structural differences (with broad wings and bill size/shape the most obvious), the following characters help separate hybrids from Thayer’s – but a hybrid may resemble Thayer’s Gull in each feature. Hybrids more often have the tertials, tail, and wingtips similar in tone; hybrids have more-diffuse contrast between outer and inner webs on the outer primaries; hybrids often have more solidly dark tails; hybrids often have more advanced molt timing, with extensive scapular molt under way by midwinter (and often with coarser A1 scapular patterns); and hybrids often have duller, dusky pinkish legs.
Olympic Gull (occidentalis x glaucescens) 1st cycle, October 15 2006, Vancouver Island, BC (Guy Monty). Intense mottling with brown (not black) primaries.	Olympic Gull (occidentalis x glaucescens) 1st cycle, October 28 2011, Clallam County, WA (Steve Mlodinow). Note the dark mask of a Western Gull and the rather dark plumage overall with more GW Gull like primaries. Overall much bulkier than Herring Gull.
HYBRID GLAUCOUS GULL X AMERICAN HERRING GULL larger with longer and stouter bill that often has brighter pinkish basally. Wingtip and tail pattern might resemble Thayer’s, but range of variation poorly known; should be separable by size and structure, especially bill.
Nelson's Gull (smithsonianus x hyperboreus) 1st cycle, November 08 2009, Palmer Landfill (Kirk Zufelt). Like Glaucous Gull, except dark pigmentation in the outer primaries (Herring influence).	Nelson's Gull (smithsonianus x hyperboreus) 1st cycle, January 14 2007, Sault Ste. Marie Landfill (Kirk Zufelt). Like Glaucous Gull, except dark pigmentation in the outer primaries.
SLATY-BACKED GULL (n. Pacific) should be separable by structure, being larger and bulkier with broad wings, shorter wing projection, longer legs, and stouter bill. Its plumage, however, is similar to Thayer’s, although the upperparts tend to be more uniform and less contrastingly patterned (suggesting a dark Glaucous-winged Gull), tail averages darker, and dark/light contrast on outer primaries averages less distinct at shaft.
Slaty-backed Gull (schistisagus) 1st cycle, February 13 2013, Rausu, Japan (Mike Buckland). Bulky bird. Few upper scapulars replaced. Simple tertial pattern.	Slaty-backed Gull (schistisagus) 1st cycle, February 13 2013, Rausu, Japan (Mike Buckland). Bulky bird. Few upper scapulars replaced. Simple tertial pattern. Note skirt.
LESSER BLACK-BACKED GULL (widespread, but rare in West) slimmer and longer winged with slender black bill, typically narrower pale edgings to upperparts (lacking boldly patterned or checkered look common to Thayer’s. Upperwing lacks contrasting pale panel on inner primaries, and outer primaries solidly dark; underside of outer primaries darker (but can be reflectively pale); uppertail coverts and tail base contrastingly white, barred blackish, with contrasting black distal tail hand.
Lesser Black-backed Gull (graellsii) 1st cycle HC63 November 21 2010, Madrid, Spain (Delfín González). Banded in N Germany. Most scapulars still juvenile, wing-coverts and tertials still juvenile.	Lesser Black-backed Gull (graellsii) 1st cycle, April 10 2011, Agadir, Morocco (Frode Falkenberg). P1-P10 still present, hence complete moult not started. Post-juvenile moult on winter grounds included all scaps but also several injner wing-coverts and upper tertials. Note dark inner primaries.

CONTINUE PART 4