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Another Look at the Western and Yellow-Footed Gulls

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Another Look at the Western and Yellow-Footed Gulls ANOTHER LOOK AT THE WESTERN AND YELLOW-FOOTED GULLS GUY McCASKIE, San Diego NaturalHistory Museum, P. O. Box 1390, San Diego, California 92112 With recognitionof the Yellow-looted Gull (Larus livens) as a distinct species,separate from the Western Gull (Larusoccidentalis) (American Or- nithologistsUnion 1982), there is renewed interestin the dark-mantledgulls of westernNorth America.In orderto correctlyidentify these birds it is impor- tant to understandhow they progressfrom juvenal to adult plumage, and when molts occur. JonathanDwight's definitive work, The Gullsof the World (1925), includes much valuableinformation on plumagesand molts, but was designedfor museumworkers with specimensin hand, and is now difficultto obtain,being longout of print. Recently,Peter Grant (1982) provideda masterpieceon all the gullsknown to occur in the westernPalearctic, Gulls: A Guide to Iden- tification, with emphasison field identification.It is from this book that the chartshowing the sequenceof plumagesand molts,as well asthe topograph- ical terminology,is taken. Mostgulls molt twice a year, with a completemolt in late summerand early fall, and a partialmolt involving only head and body feathers in latewinter and earlyspring. The time takento advancefrom juvenalto adultplumage varies amongspecies but, in general,the largerthe birdthe longerit takesto attain adult plumage. Small gulls reach adult plumage in their second winter, medium-sizedgulls in their third winter, and largegulls in their fourth winter. Needlessto say, there are exceptions(e.g., some large gulls reach adult plumagein third winter). In addition,some individuals, particularly among the larger species,show signsof immaturityduring the first year as adults (e.g., adult-plumagedbirds with dark markingsretained in the tail), while othersadvance towards adult plumagein a more rapid or retardedrate than expected(e.g., a largespecies of gull in third-winterplumage could possibly be in its secondwinter if progressingat a more rapid rate than expected,or in itsfourth winter if retarded),and sickor injuredbirds may failto molt at the ap- propriatetimes. Figure 1 indicatesthe typicalsequence of plumageand the approximateseasons of moltsfor the three sizesof gullsas they advancefrom juvenileto adult. It also includesthe age terminology(e.g., first-winter)in commonuse, which is usedthroughout this paper. A basicknowledge of the variousplumages of WesternGulls is necessary to identifyYellow-looted Gulls with certainty,and is essentialto thoseinterested in the differencesthat helped establishthe Yellow-looted Gull as a distinct speciesin the mindsof those studyingthe bird. Consequently,a complete review of WesternGull plumagesis in order. WESTERN GULL Larus occidentalis This speciesis the commondark-mantled gull of the Pacificcoast of North America, nesting from the vicinity of Destruction Island in northern Washingtonsouth to Guadalupe Island off the west coast of central Baja Western Birds 14: 85-107, 1983 85 WESTERN AND YELLOW-FOOTED GULLS WESTERN AND YELLOW-FOOTED GULLS California.There are two races,the nominateform occidentalis,nesting from the Farallon Islandsoff central California northward, and the darker-mantled form wymani, nestingfrom the vicinityof MontereyBay southward(Figure 2). At the northernlimit of the rangeWestern Gulls interbreed with the closely related Glaucous-wingedGull (Larus glaucescens),resulting in many inter- grades(Scott 1971, Hoffman et al. 1978). In winter somebirds move north- ward into coastalsouthwestern British Columbia, with a stragglerrecorded as far north as near Chignik on the Alaska Peninsula (Kesseland Gibson 1976:46), while others move as far south as the southerntip of the Baja Californiapeninsula, with stragglershaving reached Guaymas, Sonora, on the coastof mainland Mexico (Devillerset al. 1971). However, the speciesas a whole is mostlysedentary, remaining in the immediatevicinity of the coast. Feedingareas (e.g., garbagedumps) and bathingspots attract flocks a few milesinland along the coastalplain, and spawningsalmon have been reported to attract individualsup major rivers (LaFave 1965, Weber 1981), but the speciesis otherwiseaccidental inland. L. o. occidentalis L. o. wymani L. livens Figure2. Breedingrange of WesternGull (Laruso. occidentalisand L. o. wyrnani)and Yellow-looted Gull (L. liuens). 87 WESTERN AND YELLOW-FOOTED GULLS The only occurrencesfrom the true interiorin the westernUnited Statesare one (L.o. occidentalis)collected along the ColoradoRiver above Parker Dam (Monsonand Phillips1981) and four seenon the Salton Sea in southeastern California(a third-winterbird along the southshore between 17 Januaryand 13 February1965, anotherthere on 29 March1969, an adultjudged to be u•yrnaniat Salton City on 26 September1982, and a third-winterbird, also judgedto be u•yrnani,there on 11 and 28 November1982--McCaskie pers. obs.).Reports of one seennear Denver,Colorado (Am. Birds32:1038 , 1978) and anotherseen on Lake Mead, Nevada (Am. Birds35:210, 1981) lacksubstantiating details, and all otherpublished records from the SaltonSea are now believed to have been L. livens. The WesternGull is a largegull, takingabout 31/2 years to acquireadult plumage.It isabout the size of a HerringGull (Laru$argentatu$), possibly a bit larger,but stockier,with a stouterbill and shorterwings, having much the bodysize and shapeof a Glaucous-wingedGull. The southernform u•yrnaniis darkermanfled than the northernbirds at all ages,and acquiresa paleeye as an adult. The nominate form occidentalismore closely resemblesthe Glaucous-wingedGull, normallyhaving a darkeye as an adult,and having some darknesson the head and neck in winter plumage.However, the distinctionbetween the southernbirds and the northernbirds is not obvious, therebeing more of a clinaltrend, with the darkest-mantledbirds in the south andthe lightest-mantledbirds in the north,the latterbeing quite similar in ap- pearanceto the moresouthern Glaucous-winged Gulls. The followingdiscus- sionand detaileddescriptions apply to the southernbirds. In juvenalplumage the WesternGull appears much like a juvenileHerring Gull, but is darker,the overallcoloration being sooty-gray rather than gray- brown,and the paleedges to the feathersof the upperparts being narrower, givingthe bird a lessmottled appearance than juvenileHerring Gulls. The rumpof the WesternGull appearswhiter than that of the HerringGull, there beinga greatercontrast between the backand rumpthan on a HerringGull. The tailis also darker, helping to emphasizethe pale rump. The flight feathers on bothspecies are blackish,but WesternGulls lack pale areason the inner websof theinner primaries that form pale windows in thewings of youngHer- ring Gulls. First-winterWestern Gulls are similarto the juveniles,but have more con- spicuouswhite in therump, and lack the patterning on the upperparts formed by the pale edgesto the mantleand scapularfeathers on the juvenile,the overallappearance being streaked and smudged.First-winter Western Gulls do not exhibitthe pale-headedappearance evident on mostfirst-winter Her- ring Gulls. First-summerWestern Gulls are similarto first-winter,but tend to bepaler about the headand underparts,with the now 1-year-oldwing and tail feathersshowing evidence of wearand fading,varying from littleto extensive (someindividuals of all gullspecies have primaries and rectricesthat appearto be mere feather-shaftsin summer). The minor differencesthat separate juvenileWestern and Herringgulls hold true throughthe firstsummer. In second-winterthe head and bodyare mostlywhite, and the mantleand scapularsshow much of thedark gray worn by the adult.The birdacquires the eye colorof the adultlate in the winter(pale yellow or grayto whitishfor most u•yrnani,but dark brown to blackishfor mostoccidentalis). The wingsappear 88 WESTERN AND YELLOW-FOOTED GULLS much as they did in first-winter,with only a minimal amount of dark gray leatheringon the wingcoverts, but the primariesremain entirely blackish. The tail showsa little more white at the basethan in first-winter,but is nonetheless mostlydark, contrastingsharply with the whitishrump. From this stageon- wardthe colorationof the mantleand scapularsmakes differentiating Western and Herringgulls much easierthan in first-winterand first-summer.Second- summerbirds are whiter aboutthe head and underpartswith relativelycon- spicuouswhite rumps. The darkgray on the upperparts becomes a littlemore extensive,but the wingsand tail are the same as in second-winter,though showingthe effectsof wear. Third-winterbirds look more similarto adults, havingthe dark gray of the upper parts extendingacross the wing coverts.However, the primariesare uniformblackish without the whitemirrors. There is normally some brown col- orationevident on the wing coverts,and someblack is alwayspresent in the tail. The bill may startto acquiresome of the yellow color of the adult, but is morenormally pinkish with blacktowards the tip (billcolor is variableon most immature gulls, and is not recommended for use in aging individuals, especiallyafter the firstwinter). Third-summer birds look muchthe same,but never show any duskycoloration about the head and neck, and many in- dividualshave bare partsmatching those of the adult. Adult Western Gulls in the winter are entirely white about the head and throughoutthe underparts.The mantleand uppersides of the wingsare dark graymerging into the blackprimaries. White tipsto the primariesand second- ariesform a prominentwhite trailingedge to the wing, and the outermost primary containsa
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