Western Gull occidentalis

(last update: 22-12-2016 )

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Alvaro Jaramillo
John Cant
Mars Muusse

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adult: February

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 1 - summary and distribution (this page below)
PART 2 - similar species
PART 3 - rarer species
PART 4 - Glaucous-winged Gull X Western Gull

31 WESTERN GULL Larus occidentalis

LENGTH 21-26 IN. (53.5-66 cm)

Identification summary

This large, dark-backed, pink-legged gull is common along the U.S. Pacific Coast, where it is characteristic of the California Current coastal upwelling ecosystem. Heavily built and broad-winged (typically showing a skirt at rest). Bill typically stout and bulbous tipped, but relatively slender on some females; compared to Glaucous-winged Gull, bill of Western often looks shorter and more bulbous-tipped. At rest, tail tip about equal with or slightly shorter than tip of P7. Adult has slaty gray upperparts (Kodak 8-11, see Taxonomy) and reduced dusky head and neck streaking in basic plumage. Juvenal dark brownish overall with solidly blackish tail. PA1 starts Sept.-Nov. Subsequent ages variable in appearance.

All ages have pink legs; adult eyes pale to dusky; orbital ring chrome yellow. No other large dark-backed gull occurs regularly in its range, so older immature and adult Western Gull normally distinctive by virtue of slaty gray upperparts and pink legs.

Most similar species are Slaty-backed Gull (N. Pacific) and Yellow-footed Gull (Gulf of California). In first cycle also see Herring Gull, and note that hybrid Western X Glaucous-winged Gulls show various combinations of parental characters - and can also resemble other species (see hybrid account below).

Taxonomy

Two subspecies recognized, distinguished mainly by darkness of upperparts although southern birds average slightly smaller (see Bell 1996):
paler-backed (Kodak 8-9.5) nominate occidentalis (Northern Western Gull) and
darker-backed (Kodak 9.5-11) wymani (Southern Western Gull).

Subspecies boundary not well known (see AOU 1957 & Roberson 2002), and tone of upperparts somewhat clinal (see Bell 1996): typical nominate occidentalis breeds from n. California south to San Mateo County (37-42° north), typical wymani from California Channel Is. south to n. Baja California Sur (27-34° north). Breeding birds in Monterey and San Luis Obispo counties (35-36.5° north) intermediate in upperpart tone (Kodak 9-10), whereas "Western Gulls” in Oregon and Washington paler above (Kodak 7-8) than typical occidentalis, perhaps reflecting introgression with Glaucous-winged Gull. (see Bell 1996)

Northern occidentalis hybridizes commonly with Glaucous-winged Gull in Washington and Oregon (see Bell 1997; Hoffman et al. 1978 and Scott 1971), and some authors consider these taxa conspecific (see Weber 1981). Yellow-footed Gull of Mexico formerly considered a subspecies of Western Gull (see AOU 1957).

Status and distribution

W. N. America; accidental in Hawaii.
Breeding. Common to fairly common resident (breeds Apr.-Aug.) on Pacific Coast and islands from s. Wash. (north to Cape Flattery) south to Baja Calif. Peninsula, Mex. (south to around 27° north). May breed sw. B.C., but distinguishing pure birds from hybrids with Glaucous-winged Gull is problematic. A few pairs have bred since 2001 in n. Sinaloa, Mex (Gonzalez B. et al. 2003).
Nonbreeding. Fairly common to rare non-breeding visitor (year-round) north to sw. B.C. and Queen Charlotte Is., south (mainly Sept.-Apr.) to the tip of Baja Calif. Peninsula, Mex., and rare but increasing (year-round) in n. Gulf of Calif. Rare visitor north to se. Alaska (mainly Sept.-Mar.), exceptionally north to Bristol Bay (June), and casual (Jan.-June) south along Pacific Coast of Mex. to Oaxaca.
Uncommon to rare in protected marine waters of Wash. away from the coast, ranging in fall and winter up Columbia R. to se. Wash.; rare inland in Ore. away from Columbia R. In Calif., generally increasing in inland valleys near the coast, especially in Los Angeles Co. Rare (mainly Sept.-Mar.) in Sacramento Valley and rare but increasing at Salton Sea. Casual in Calif. east of Sierra Nevada (scattered records year-round). Farther inland, casual at best (mainly Oct.-Dec.; also Apr.-May and July) with scattered reports east to s. Lake Michigan (Wright and Komarek 1928) and se. Tex. A bird that ranged from Ala. to Fla., 1977-1990, was thought by some to be a Western Gull (Duncan 1982), but many believed it may have been a hybrid (Stevenson and Anderson 1994).

continue >>> part 2 - similar species

occidentalis Western Gull