First Record of a Lesser Black-Backed Gull in Colorado

NOTES

FIRST RECORD OF A LESSER BLACK-BACKED GULL IN COLORADO

BRUCE E. WEBB, Departmentof Environmental,Population and Organismic Biology,University of Colorado,Boulder, Colorado 80309 JEANNE A. CONRY, Departmentof Biology,University of Colorado,Denver, Colorado 80202

On 11 December1976 we foundan adult LesserBlack-backed Gull (Larusfuscus) standingon the ice at LakeSangraco, a smallsand and gravelborrow area 2.5 km north of Interstate70 alongLowell Boulevard, northwest of Denver,in AdamsCo., Colorado.When first observedit was in a flock of gulls consistingof 15 adult Herring (L. argentatus),5 adult California (œ cali•rnicus)and 50 adultRing-billed (L. delawarensis)gulls. Later in theweek, a first-winterGlaucous Gull (œhyperboreus) and anadult Thayer's Gull .(L.thayeri) were also present for comparison.The LesserBlack- backedGull remainedat Lake Sangracothrough 1 January1977. This, the first recordedoccurrence of the speciesin Colorado,was also the first recordfrom the deepinterior of the United States. Description:Body color: entire ventral plumag• immaculate white. Body bulk: slighdysmaller than adultargentatus. Mantle color: deep slate-gray, distincdy palerthan primaries. Mantle much darker than that ofcalifornicus. Wing length: at restthe wing-tipextended slighdy beyond the tip of tail. Tail color:pure white. Wing color:primary tips dorsallyas blackas thoseof adjacentdelawarensis and argentams.With the wing folded,the primarieswere white-tipped,indicating relativelyrecent renewal. Two flight photographsverify our observationsthat whitesubterminal mirrors were absent from distalprimaries. Either these mirrors werelacking or theoutermost primaries might have been in molt.When seen from belowin flight,the dorsaldarkness of the primariesand secondarieswas visible throughthe extendedwing, as noticeablyas on one of the darker racesof the WesternGull (œ occidentaliswymani). Secondary color sameas mande,the color shadetransition from black primary tips to the slategray of the secondaries gradual.Head shape:forecrown angularity and supraocularridge similar to argentatus.Crown streaking:strongest on pileumwith somestreaking above and beloweye at baseof lowermandible. No streakingon forecrownor chest,or from eyeto bill. Iris color:pale straw-yellow, appearing whiter than that ofdelawarensis. Eyelidcolor: very conspicuousred, providingsharp contrast with iris colorand face.Tarsus color: legs, carefully compared with delawarensis and argentatus, were paleyellow, with no hint of pinkishalong tarsi; intensity of leg colorwas slighdy more yellow than adjacentdelawarensis, whose tarsi at this time had a tinge of pinkish color, especiallyat the "knee." Tarsus size: only slightlythicker in diameter than delawarensisand noticeablyless stout than argentatus.Left foot damaged,causing the bird to walkwith the toesin a closedposition. Bill color: brightyellow with very extensiveoval red spotat the gonydealangle. Bill shape: relativelylong and shallow;gonydeal angle not asacute as cali•rnicusor argentatus,but moreangular than delawarensis.Comparisons indicated bill aboutas long asthe longestargentatus bill nearby. All other adult dark-mantled North American gull species could be ruled out on the basisof numerouscharacteristics, but mostreadily by: (1) Legcolor pinkish to pinkish-whitein Great Black-backed(L. marinus)and Slaty-backed(L. schistisagus)gulls, as well asthe twoWestern Gull races(L. o.wymani and L. o.occiden- talis).(2) Eyelidcolor yellow in the Yellow-leggedWestern Gull (L. occidentalislivens). Western Birds 9:171-173, 1978 17 1 NOTES

The likelihoodof thesimilar Dominican or KelpGull (L. dominicanus),a coastal South Americangull, or oneof the dark-backedyellow-legged Middle Eastern races of the Herring Gull finding its wav to the deep interior United Statesseeins extremely remote.Also, dominicanusshould be in worn plumagein December. Two observersexperienced with L. fuscus,P. Gent and W. Brockner, concurred with our identification, as did most other observers who studied the bird. Judgingfroin the overwhehningnumber of typicalL. fuscus features, particularly tarsuscolor and thickness,eyering and iris color,bill shape,body shape, size and mantlecoloration, the possibilities that it wasa dark-backedargentatus orwas of hybrid origin("an intergrade") seem remote. The statement that it "lackedseveral diagnostic features"(Kingery 1977) thus seems unfounded. Based on examination ofstudy skins, photographsand descriptions ofL fuscusin theliterature (Dwight 1925, Witherby et al. 1941, Deinent'evet al. 1951,Voous 1963), consideration of probablehybrid characteristicsof dark-backed gulls (Jehl 1960, Andde 1972),and discussions with individualsfamiliar with the species,we feel that the ColoradoLesser Black-backed Gull bestfits the Britishrace Larusfuscus graellsii. The seasonalpattern of manyNorth American winter and springLesser Black- backedGull records corresponds well with the migration pattern of theOld World populations.In the Old World,Wallace (1973) obserxed coastal wintering of L.f fuscusand L.f graellsii,mainly adults, at Lagos,Nigeria (7øN,5øE), far southof the breedingrange. He founda generalincrease in nulnbersfrom November through FebruaD , with L.f graellsiipeaking in January and dropping off sharply Ipresumably asbirds returned north) in Februaryand March. Details of the species' inland passage throughsouthern Europe are sumlnarized by Voous (1963), where in hisdiscussion of thelongdistance transcontinental route of L.ffuscus he mentions its occurrence on central Al¾ican lakes.

Figure1. AdultLesser Black-backed Gull (Larus fuscus, probably L. f graellsii).11 December1976 through 1January 1977, Lake Sangraco, Adam• Co., Colorado. Photoby Mike Pogue, courtes), Denver Museum ofNatural Histor3.. 172 NOTES

Table 1. Summaryof 67 LesserBlack-backed Gull recordsin North Americanorth and south of 39ø latitude, 1968 through 1977.

November- January- March- December February April North 29 (82.9%) 3 (8.6%) 3 (8.6%) South 9 (28.1%) 14 (43.8%) 9 (28.1%)

Basedon a surveyof 10years of easternUnited States L. fuscus records in Audubon FieldNotes and AmericanBirds ( 1968-1977),there is an indicationof movementsouth duringJanuary through February (Table 1). The test for equality of percentages(Sokal and Rohlf 1969:608)indicates significant north-south differences (p<0.05) between eachof thethree 2-month periods of Novemberthrough December, January through February,and March through April. In thenorthern region (north of 39ø latitude), 83 percentof the 35 recordsoccurred during November and December,but in the southernregion the highestnumbers were recorded during January and February, when43 percentof the 32 recordsoccurred. In northernlocalities, 35 percentof all recordsinvolved birds remaining longer than two weeks (i.e. probablyoverwintering individuals);whereas, in southernlocalities only 13 percentof the recordsinvolved birds remaininglonger than two weeks.The shortduration of •mostrecords at southernUnited States coastal localities suggests that L.fuscus is transientthere. This leadsus to the speculationthat someindividuals of the North AmericanL. fuscus populationmay migrate through the UnitedStates, perhaps to winterfarther south alongthe coasts of Mexico,Central and South America, and in theCaribbean Islands. Basedon thecorrespondence between distributional patterns of theUnited States and the Old World, we feel that occasionalLesser Black-backed Gulls can be expectedin the interior United Statesin passagefrom their northeasternsummer localities to distantcoastal wintering localities. We gratefullythank R/•bert Andrews, Laurence C. Binford,Winston Brockner, Davis Finch, Tom Gatz, Peter Gent, Will Russelland Ronald LeValleyfor useful discussions and comments.

LITERATURE CITED

Andrle,R. F. 1972.Another probable hybrid of Larusmarinus and L. argentatus.Auk 89:669-671. Dement'ev,G. P.,N. A. Gladkovand E. P. Spangenberg,eds. 1951. Birds of theSoviet Union,Vol. 3. Englishtranslation by A. Birron,Z. $. Coleand E. D. Gordon,Israel Programfor ScientificTranslations, Jerusalem, 1969. Dwight,J. 1925.The gulls(Laridae) of theworld; their plumages, moults, variations, relationshipsand distribution.Bull. Am. Mus. Nat. Hist. 52: 63-409. Jehl,J. R.Jr. 1960.A probablehybrid ofLarus argentatus and L. marinus.Auk 77:343- 345. Kingery,H. E. 1977.The winterseason. Mountain west [region]. Am. Birds31:356. Sokal,R. R. and F.J. Rohlf.1969. Biometry. W. H. Freemanand Co., San Francisco. Voous, K. H. 1963. Geographicvariation of Larusfuscusin northwesternEurope. Ardea 51:16-24. Wallace,D. I. M. 1973.Sea-birds at Lagosand in theGulf of Guinea.Ibis 115:559-571. Witherby,H. F., F. C. R. Jourdain,N. F. Ticehurstand B. W. Tucker. 1941.The handbookof Britishbirds. Vol 5. H. F. and G. Witherby, Ltd., London. Accepted25 October1978 173