Western Gull occidentalis
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Below you will find a description of Chapter 31 WESTERN GULL Larus occidentalis, 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 4: HYBRID GLAUCOUS-WINGED GULL X WESTERN GULL Larus glaucescens x Larus o. occidentalis
LENGTH 22-26.3 IN. (56-67 CM)
This hybrid combination - sometimes called “Puget Sound Gull” or “Olympic Gull" – is common along the Pacific Coast of N. America, breeding from British Columbia south to Oregon. Nonbreeding migrants occur regularly south to s. California and north to se. Alaska. Hybrids show various combinations of parental plumage and structure (Bell 1996, and see Glaucous-winged Gull and Western Gull accounts) and can also resemble other species, thus causing identification problems. The most common problems are distinguishing Glaucous-winged x Western hybrids from Slaty-backed and Thayer’s Gulls, and they can also be difficult to separate from Glaucous-winged Gull x American Herring Gull hybrids (see Similar Species section for details).
Because hybrids span the range of characters between parental types it is difficult to establish where a pure Glaucous-winged Gull or Western Gull ends and a hybrid begins (Bell 1996). Many birds are clearly intermediate between the parent species, but other hybrids are more subtle. Some adults look like Western Gulls, but their backs are a shade too pale, their wingtips blackish (versus black), their orbital rings mixed yellow and pink, and, in basic plumage, their heads have relatively extensive and smudgy dark markings, and their bills are pale yellowish with dark distal marks. Others look like Glaucous-winged Gulls, but their wingtips are a shade too dark, their orbital rings mixed yellow and pink, and, in winter, they have bright bills typical of Western Gull. Most breeding hybrids from the Juan de Fuca Strait north look more similar to Glaucous-winged Gulls than to Western Gulls, most from the outer coast of cen. Washington south look more similar to Western Gulls (Bell 1997).
First-cycle presumed hybrids also look variably intermediate between the parent species, and their wingtips tend to be some shade of medium brown or dark brown. Birds that look mostly like Glaucous-wingeds can have strongly checked upperparts typical of Western Gull, whereas birds that look mostly like Western Gulls can exhibit delayed PA1 molt and then attain relatively plain, medium gray scapulars typical of Glaucous-winged Gull.
Second-cycle and third-cycle plumages are equally variable.
Common breeding resident a long coasts from s. B.C. to cen. Ore. The hybrid zone extends from Puget Sound and Juan de Fuca Strait south to cen. Ore. (43-49° north) with some introgression north to Queen Charlotte Is. (54° north, Bell 1996) and occasional hybridization south to cen. Calif. (around 38° north; Bell 1997 & Howell pers. obs.). Regular nonbreeding visitor (mainly Oct.-Mar.) north to se. Alaska, south to s. Calif., rarely to n. Baja Calif., and possibly to n. Sonora (Howell pers. obs.). Status inland in w. N. America clouded by difficulties distinguishing hybrids from pure Glaucous-winged Gulls and by a general lack of interest in hybrids among birders. Small numbers winter inland at least to e. Ore.,( Marshall et al. 2003) ; Central Valley of Calif., and n. Great Basin (Floyd 2002). Up to 75 percent of some Western Gull and Glaucous-winged Gull colonies in Wash. and Ore. Comprise hybrids (Bell 1996 & 1997; Hoffman et al. 1978). In cen. Calif.(Marin County), 15-25 percent of the wintering population of Western and Glaucous-winged Gulls typically show hybrid characters, and at some sites up to 50 percent appear to be hybrids (Howell pers. obs.). Hybrids become progressively less common southward along the Calif. coast in winter.
Here we deal only with the commonest identification pitfalls caused by birds intermediate between the parent species. For hybrids that closely resemble the parent species, see differences from other species listed in the Glaucous-winged Gull and Western Gull accounts.
THAYER'S GULL (W. N. America in winter). Hybrids are larger and bulkier overall than Thayer’s Gulls with broader wings, shorter wing projection, and a larger and stouter bill. Only the smallest female hybrids are likely to cause difficulties, and even these have stouter bills than Thayer’s (noticeable in direct comparison). Wingtip patterns of hybrids can resemble Thayer`s so closely as to be indistinguishable under field conditions. But hybrids are often slightly darker above (Kodak 6-8. versus Kodak 5-6 on Thayer`s); often show some yellowish in their orbital ring; and can have winter bill patterns varying from bright (like Western) to pale with extensive dark marks (like Glaucous-winged) – unlike the pattern of adult Winter Thayer’s Gulls, which typically have a greenish bill with an orange-red gonydeal spot and a brighter yellow “saddle spot” on the maxilla above the gonys.
SLATY-BACKED GULL (N. Pacific). Hybrids average bulkier and broader winged with a more bulbous-tipped bill, paler upperparts (Kodak 6-8 versus 9.5-11.5), and their dorsal wingtips tend to be slaty blackish (or paler) rather than black. Wingtip pattern can be similar to Slaty-backed (Glaucous-winged pattern combined with Western Gull darkness) although hybrids with the darkest wingtips generally have narrow whitish tongue-tips and often lack a mirror on P9; and the white trailing edge to the secondaries averages wider on Slaty-backed. Also note Slaty-backed’s reddish orbital ring, more parallel-edged bill, and rich pink legs.
AMERICAN HERRING GULL (widespread). Some pale backed, blackish winged hybrids can suggest American Herring Gull but their upperparts are slightly darker (Kodak 6 versus 4-5), wingtips tend to be more extensively blackish, eyes usually dusky (but can be staringly pale), and bill often relatively bulbous-tipped. Also note the thickset and broad-winged structure of hybrids, and check orbital ring for pinkish.
VEGA GULL (w. Alaska). Hybrid features such as upperpart tone, dusky eyes, and extensive white tongue-tips and mirrors in wingtip can all suggest Vega Gull - but note thickset and broad-winged structure of hybrids. Also, hybrid wingtips tend to be blackish, not contrasting sharply with gray primary bases; bill often relatively bulbous tipped; and orbital ring pinkish to yellow (reddish on Vega).
HYBRID GLAUCOUS-WINGED GULL X AMERICAN HERRING GULL Can appear similar to hybrid Glaucous-winged x Western, but features indicative of American Herring Gull parentage may be discerned: structure often slighter overall with relatively longer and especially narrower wings and more slender, parallel-edged bill; upperparts average paler (Kodak 5-6, versus 6-8) with more-contrasting and often more-restricted blackish wingtips; and basic head streaking often finer.
HYBRID SLATY-BACKED GULL X VEGA GULL (N. Pacific) not well known. Presumed hybrids (BM #1908.1.8.457 -NE China-) have slaty gray upperparts (Kodak 9) with wingtip pattern similar to Vega Gull: fairly well-demarcated black wingtips both above and below, narrow white tongue-tips on P5-P7 (but not on P8), and no white mirror on P9.
HYBRID GLAUCOUS-WINGED GULL X SLATY-BACKED GULL (N. Pacific) not well known. Could be expected to resemble hybrid Glaucous-winged Gull x Western Gull; critical study needed.
THAYER’S GULL Besides structural differences (with bill size and shape the most helpful), 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 hybrids often have duller, dusky pinkish legs.
SLATY-BACKED GULL Hybrids average bulkier and broader winged with a more bulbous-tipped bill. Slaty-backed upperwing has contrastingly pale inner primaries and more-distinct inner web/outer web contrast on the outer primaries; also note more-extensive white tail base barring and relatively pale greater coverts on Slaty-backed.
AMERICAN HERRING GULL Structure less thickset overall with relatively longer and especially narrower wings and more slender, parallel-edged bill; primaries and tail black or blackish, darker than hybrids, and with stronger contrast on upperwing between pale inner and blackish outer primaries; bill often has more pinkish at base.
VEGA GULL differs in same respects as American Herring Gull, but tail averages more extensively whitish basally with a broad blackish distal band, and underparts paler overall with whitish ground color and dusky brown streaking and mottling.
HYBRID SLATY-BACKED GULL X VEGA GULL Not well known. Suspected hybrids look intermediate between parent species in structure and plumage characters such as tail pattern and primary pattern, so wingtips blacker than hybrid Glaucous-winged x Western gulls and tails with more-distinct white barring at base; critical study needed.
HYBRID GLAUCDUS-WINGED GULL X SLATY-BACKED GULL not well known; characters relative to hybrid Glaucous-winged Gull x Western Gull need study.
Plumage highly variable; best identified by structural characters (see adult and first cycle).
Differences from other species and hybrids much as adult cycle, which see.
HABITAT AND BEHAVIOR
Much as parent species. In cen. California in winter, hybrids resembling Glaucous-winged Gulls feed mostly with Glaucous-winged Gulls (for example, in rocky intertidal areas), whereas Western-like hybrids feed mostly with Western Culls (for example, on sandy mudflats in tidal lagoons and harbors; Howell pers. obs).
DESCRIPTION AND MOLT
Descriptions focus on differences relative to parent species and are not comprehensive. Thus, leg color (pink) and adult tail color (white) are like both parent species and are not noted below.
Complete PB molt (mainly June-Dec.) produces adult basic plumage: variably intermediate between Glaucous-winged Gull and Western Gull. Mantle and primary melanism are good indicators of hybrid origin (Bell 1996). Some birds are almost as pale above as Glaucous-winged Gull but have blackish wingtips, but birds with dark upperparts and pale gray wingtips do not occur (wingtips mainly Kodak 9-16). Back and upperwings gray to pale slaty gray (mostly Kodak 6-8), often with more-extensive white mirrors and tongue-tips in outer primaries than Western Gull; darkest and most Western-like hybrids tend to have reduced white tongue-tips and mirrors and paler, less-contrasting underside to wingtips. Eyes pale lemon (can be more staringly pale than typical of either parent) to dusky. Orbital ring pinkish to chrome yellow; can show patches of both colors or can be an intermediate color such as a fleshy ocher. Bill pale greenish yellow to orange-yellow with orange-red to red gonydeal spot, and often a dark distal mark. Partial PA molt (Oct-Feb.) produces adult alternate plumage: head and neck clean white. Orbital ring, bill, and legs brighter, bill lacks dark distal marks.
Second and Third Cycles.
Molt timing and age-related plumage variation parallel parent species (for example, complete PB molts mainly May-Nov., partial PA molts mainly Sept.-Mar.). Plumage and bare parts variably intermediate between parent species; see Adult Cycle (above).
|occidentalis Western Gull|
|Western Gull occidentalis adult, May 24 2012, Commercial Wharf, Monterey, Monterey Cty., California, USA. Picture: John Cant.|