A Cook's Petrel Specimen from California

A COOK'S PETREL SPECIMEN FROM CALIFORNIA

WM. BRECK TYLER, Instituteof Marine Sciences,University of California,Santa Cruz, California 95064 KENNETH BURTON, Dosen Bahasalnggeris, Universitas Palangka Raya, Tunjung Nyaho,Palangka Raya, Kalteng, Indonesia

On the morningof 17 November 1983, Bob Moon found a Cook'sPetrel (Pterodroma ½ookii)floundering in the driveway of his seasidehome at 2-2702 East Cliff Drive, Santa Cruz, Santa Cruz County, California(37 øN). Moon called the Native Animal Rescue Service, a local wildlife rehabilitation center, but the bird expired before the center'svolunteers could respond. They retrievedthe carcass,however, and deliveredit to us for identification. To the bestof our knowledge,it is the firstspecimen of Cook'sPetrel for the continental United States and the first land record for the speciesin the Northern Hemisphere. Our preliminaryexamination revealed that the specimenwas a small (length30 cm) tube-nosedbird in extremelyworn plumage. Its upperparts were uniformlydark gray, itswings and rump sootyblack and brown, and its underpartsall white exceptfor a narrow dark marginon the underwing.The pale blue feet and high-contrastwhite inner vanes of the otherwiseblack primaries (Figure 1) indicated that it was one of the Cookilaria petrels (Palmer 1962); the subgenusCookilaria comprises several species of small Pterodromapetrels (see Table 1). We identifiedthis specimenas ½ookiion the basis of plumage characteristics,after comparing it with a series of Pterodroma petrelsat the California Academy of Sciences.George Watson (in litt.) subsequentlycompared the specimenwith those in the National Museumof Natural Historyand confirmedour diagnosis.The specimenwas depositedin the CaliforniaAcademy of Sciences(71447). The problemof identifyingCookilaria petrels is exacerbatedby the lackof a universallyaccepted classificationfor the group. We combine those by Jouanin and Mougin (1979) and Bourne (1983), both of which accom- modate Fieming's (1941) finding (based on skull morphology) that two parallelradiations of similarspecies exist within the group. Arrangementsby other authors(e.g., Murphy 1936, Harper and Kinsky1978, Harrison1983) differ in varying degreesfrom the one presentedhere (Table 1). Historically,three racesof P. ½ookiihave been described,as follows: ½ookii from breedingislands near New Zealand; orientalis,described by Murphy (1936), from unknown breeding areas believed to be in the southeastern Pacific;and defilippiana,from Mas Atierra and other islandswest of Chile. Murphy describedorientalis as larger than ½ookiiand paler above (due to lightfeather edgings). Most authorsnow believeorientalis is invalid, having been based on freshly molted or juvenal plumage ½ookii(Palmer 1962, Bourne 1983, G. Watson in litt.). P. defilippiana,believed to be restrictedto Chileanwaters (Harrison 1983), is now consideredby Jouaninand Mougin (1979), Bourne (1983) and the AOU (1983) to be a separatespecies, the Mas Atierra Petrel. Therefore,P. ½ookiiis currentlyconsidered monotypi½. Western Birds 17:79-84, 1986 79 COOK'S PETREL SPECIMEN

Whetherobserved at seaor in the hand,a Cookilariapetrel presents an identificationchallenge. Plumage differences between the taxa are subtle, andindividual and seasonal variation within species are not adequately ad- dressedin mostfield guides. In general,our SantaCruz specimen resembles theCook's Petrel pictured in theNational Geographic Society guide (Scott 1983),but it hasthe upperwing pattern of thePycroft's Petrel (P. !ongirostris pycroftOillustrated in Harrison'sguide (1983). The crown,nape and back of the SantaCruz specimen are uniformlydark gray. The coloris muchdarker thanthat usuallydepicted for cookii(Harper and Kinsky1978, Harrison 1983), but the distinctlydarker cap typical of P. !ongirostrisis not evident. Apparently.among Pterodromapetrels, plumage becomes darker with wear;in fact,a veryworn cookii may appear as dark as a fresh!ongirostris, butthe former's cap and back will be concolor (Roberson 1980). The upper- partsof thisspecimen are so dark that the M-shaped mantle pattern described forcookii by severalauthors (Roberson 1980, Harrison1983) isnot obvious. The centralrectrices (dorsal side) are sootybrown, but the lateralones are paler,ranging from light gray to whitewith a finegray speckling (Figure 2). The lateralrectrices become sequentially paler toward the edgeof the tail. andthe inner vane of eachfeather is significantly paler than the outer vane, a patternwhich is typical for cookii(Murphy 1936). The whiteouter tail men- tionedas a diagnosticfield markfor cookiiby severalauthors (Roberson 1980, Harrison1983) is probablymost conspicuous when the rectricesare heavilyworn. In contrast,P.!. !ongirostrisusually has darker outer rectrices; P.!. pycrofti,however, is intermediate,often showing some white in the

Figure1. Dorsalview of primariesof immaturefemale Cook's Petrel (P cookii: CaliforniaAcademy of Sciences71447) showing contrasting white inner vanes. 80 COOK'S PETREL SPECIMEN

outer tail (0liver 1955, Roberson 1980). In the field, P. defilippiana is not considered to be safely separable from cookii, but in the hand it can be distinguishedby the larger, more deeply groovedbill (Murphy 1936), darker eye patch, and longer tail (Harper and Kinsky 1978). All other Cookilaria petrels have even darker upperparts and more pronounced underwing patterns. Cook's Petrels breed from October to April. During the austral winter (May-September),they leavetheir breeding areas (0liver 1955) and disperse into the central and eastern Pacific. The range of dispersaland migration routes are not well known. Most records are from waters off Peru, northern Chile, and Baja California (Murphy 1936, Roberson 1980, Pitman 1986), but considerablemovement through the central Pacifichas been noted (Pit- man 1986). The northernmostrecords are from the vicinity of the western Aleutians (Roberson 1980). During the period 3 October through 1 December 1979, 21 sightingsof Cookilaria petrelswere made 50 to 200 km off the coast of California. from Point Piedras Blancas (35ø40'N) to Point Arena (39øN; Roberson 1980). These sightingsrepresent the northernmostrecords along the west coast of North America (excludingthe Aleutian records). Because P. defilippiana is nearly identical to P. cookii under field conditions,the identificationof these birds, as well as several others seen off southern California and one on the Salton Sea during summer 1984 (McCaskie 1984, R. Pitman in litt.), is cur- rently unresolved:all are under considerationby the CaliforniaBird Records Committee.

Figure 2 Dorsal view of rectricesof immaturefemale Cook's Petrel (P cookiPCAS 71447).

81 COOK'S PETREL SPECIMEN

The distributionof Cook's Petrelsat sea is almostcertainly related to patterns of sea surface temperature and associatedoceanographic factors. Bourne (1983) saysthat Cook'sPetrels prefer waterssomewhat cooler than thosefrequented by other Cookilaria,but he does not mention any specific thermalrange. Previously,it was thoughtthat Cook'sPetrels were associated with subpolar waters (4.0 to 13.9øC) in both hemispheres(Bourne, in Palmer 1962), but especiallythose of the sub-AntarcticZone (Murphy 1936). However, current evidence showsthat the speciesshould be considered subtropical.The species'nesting islands are located in subtropical waters,very near the sub-TropicalConvergence. Breeding birds could easily foragein the cold productivewaters of the sub-AntarcticZone, but in fact are rarelyrecorded there (Watson1975). Duringthe australwinter, when waters nearer the colony become colder, the birds depart the area (0liver 1955). Recent pelagic surveyshave found that ½ookiioccurs frequently in subtropical (14 to 21.9øC) and tropical (22+øC) waters (Ainley and Boekelheide 1983, Pitman 1986). In the eastern Pacific, most records are from mild oceanicwaters (approximately 15 to 25øC) seawardof cold-water upwelling zones (Murphy 1936, Roberson 1980, Pitman 1986). Pitman's data show also that Cook's Petrels are scarcein the very warm tropical

Table 1. The subgenusCookilaria, after Jouanin and Mougin (1979) and Bourne (1983).

Taxon English Name Breeding Range Center Pterodroma c Bonin Petrel Bonin and Volcano Islands (south of hypoleuca Japan), Leeward Hawaiian chain P. nigripennisa.c Black-winged Southwest Pacific Petrel P. axillaris a.c Chatham Island Southeast Chatham Island Petrel (east of New Zealand) P. cookii b Cook's Petrel Little and Great Barrier Islands (northern New Zealand) P. defilippiana • Mas Atierra Juan Fernandez Islands Petrel (west of Chile) P. ]_ongirostris Stejneger's Juan Fernandez Islands Iongirostris•'• Petrel (west of Chile) P. Iongirostris Pycroft's Northeast of New Zealand pycrofti •'• Petrel P. leucoptera Gould's Petrel CabbageTree Island leucoptera (eastern Australia) P. leucoptera Collared Southwest Pacific brevipes •'• Petrel P. leucoptera New Caledonian New Caledonia caledonica • Petrel (unclescribed)• Solomon Islands

øTogetherform a superspecies(Jouanin and Mougin 1979). bTogetherform a superspecies(Jouanin and Mougin 1979). ½Togetherform a superspecies(Bourne 1983). aTogetherform a species,the White-winged Petrel, P. leucoptera (Bourne 1983).

82 COOK'S PETREL SPECIMEN regionsof the easternPacific, suggesting that recordsfrom tropicalwaters are probablybirds in transit. AlthoughCook's Petrels have beenrecorded in cold (415øC) water (Wahl 1978), the watersadjacent to the coastof Californiaprobably are usuallytoo cold (10-17 øC) for the species.The 1979 sightingsand the presentspecimen were recorded during the warmestseason (early autumn) and in unusually warm years.Weak and very strongEl Nifio conditionsprevailed off PointSur (36øN) during 1979 and 1983, respectively(Breaker 1983). Hydrograph- ically, waters in the reported sightinglocations (Davidson Seamount and northwest of Point Arena) are characterizedby persistentwarm-core (16-19øC) eddiesthat closelyapproach the Californiacoast (Simpson et al. 1983, K. Briggs pers. comm.). As evidenced by its worn plumage (G. Watson in litt.) and underdevelopedovary (L. Thompson pers. comm.), the Santa Cruz specimenis an immaturefemale. Given the October-Novemberdates of oc- currencerelative to the nestingseason, many of the Californiarecords likely have been of immature or nonbreedingindividuals. Most late records of austral-breedingSooty Shearwaters(Puffinus griseus) in Californiawaters also pertain to immatures (E. Chu pers. comm.). Our examinationof the specimen'sdigestive tract revealed only irregular stripsof thin translucentplastic (up to 1 x 3 cm) in the foregut, and several squidbeak fragmentsand four smallplastic particles (various colors, all < 1 cm across) in the gizzard.

ACKNOWLEDGMENTS We thank Luis Baptistaand Mary Marcussonfor providingaccess to the collectionsof the CaliforniaAcademy of Sciences.George Watson verified the identificationof the specimen.Robert Pitman provided expert guidance and accessto his unpublisheddata. Ken Briggs, David Lewis and Martha Brown reviewed earlier draftsof this manuscript,and Cyndi Hitchcockdid the typing. We also thank Bob Moon and the Native Animal RescueService for bringingus this unique specimen.

LITERATURE CITED

Ainley, D.G. & R.J. Boekelheide. 1983. An ecologicalcomparison of oceanicseabird com- munities of the South Pacific Ocean. Studies Avian Biol. 8:2-23. AmericanOrmthologists' Union. 1983. Check-listof North Americanbirds. Sixth ed. Am. Or- nithol. Union. [Washington,D.C.]. Bourne, W.R.P. 1983. The appearanceand classificationof the Cookilariapetrels. Sea Swallow 32:65-71. Breaker,L.C. 1983. The space-timescales of variabilityin oceanicthermal structure off the central Californiacoast. Ph.D. thesis,Naval PostgraduateSchool, Monterey,CA. Fleming, C.A. 1941. Notes on Neozelanicforms of the subgenusCookilaria. Emu 41:69-80. Harper. P.C. & F.C. Kinsky.1978. Southernalbatrosses and petrels.Victoria Univ. Press,Wel- lington, New Zealand. Harrison,P. 1983. Seabirds:an identificationguide. Houghton Mifflin Company, Boston,MA. Jouanin,C. & J.-L. Mougin. 1979. Procellariiformes.Pp. 76-78 in E. Mayr & G.W. Cottrell,eds., Check-listof the birdsof the world, Vol. 1, 2nd ed. Mus. Comp. Zool., Cambridge,MA.

83 COOK'S PETREL SPECIMEN

McCaskie,G. 1984. SouthwestPacific coast region. Am. Birds38:1061. Murphy, R.C. 1936. Oceanicbirds of South America, Vol. I. MacmillanCo., New York. Oliver,W.R.B. 1955. New ZealandBirds. Second ed. A.H. & A.W. Reed, Wellington,New Zealand. Palmer, R.S., ed. 1962. Handbook of North American birds, Vol. I. Yale Univ. Press,New Haven, CT. Pitman,R.L. 1986. Atlasof seabirddistribution and relativeabundance in the EasternTropical Pacific.Natl. Mar. Fish.Serv. Admin. Rep. LJ-86-02. La Jolla, CA. Roberson,D. 1980. Rare birds of the west coast of North America. Woodcock Publ., Pacific Grove, CA. Scott,S.L., ed. 1983. Fieldguide to the birdsof NorthAmerica. Natl. Geogr.Soc., Washington, D.C. Simpson,J.J., T.D. Dickey,& C.J. Koblinsky.1982. CaliforniaCurrent eddies -- A. Eddiesand seasonalvariability. Abstr. Eos. 63:82. Wahl,T.R. 1978. Seabirdsin the northwesternPacific Ocean and south central Bering Sea in June 1975. West. Birds 9:45-66. Watson, G.E. 1975. Birds of the Antarcticand Sub-Antarctic.Amer. GeophysicalUnion. Washington,D.C.

Accepted 17 February 1986

Cook's Petrel Sketch by Keith Hansen