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Swinhoe's Storm-Petrel

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Swinhoe's Storm-Petrel first northamerican photographic record Swinhoe's Storm-Petrel HICNAELO'BRIEN,* J. BRIANPATTESON,t DESCRIPTION GEORGEL. ARHISTEAD,and GRAYSONB. PEARCEõ Size,Structure, and Molt. The bird mostclosely resembled a Leach's Storm-Petrel.It wasof roughlythe same length as a Leach'sor Band- rumpedbut oftengave the impressionof beingslightly larger than birdingnAugust trip8, 1998,off Hatteras,the authors North were Carolina,participating aboard inathe pelagic Miss either. Its wingswere relatively long with an obviousbend at the Hatteraswith CaptainSpurgeon Stowe and 29 passengers.Winds carpaljoint or "wrist."It appearedproportionally longer-winged werelight from the southeast and seas calm. Over most of theprevi- thaneither Band-rumped or Leach'sand seemed to holdits wings in ousweek, strong northeasterly winds had createda strongcurrent a relativelyoutstretched position, much more akin to Band-rumped that apparentlypushed the Gulf Streammuch farther offshore than thanLeach's. Compared to Leach's,the wingswere narrower in the usualand brought cool water well south of CapeHatteras. Indeed we "arms"but longerand broaderin the "hands"and were not so did not run into warm water until 54 l•n out, about 16 l•n farther sharplypointed at the tips (Figures1, 2, 3). The net effectwas of seaward than usual, but when we arrived at the Gulf Stream'swest- morewing area in proportionto thebody than on a Leach'sor Band- ernwall, the seabirds were numerous. By early afternoon, the waters rumpedand, when the primarieswere spread,the wingslooked southeastof Hatterashad proven highly productive, with studiesof almostpaddle-shaped, very unlike Leach's or Band-rumped.In pho- Herald(Trinidade) Petrel (Pterodroma [a.] arrninjoniana)and South PolarSkua (Catharacta maccormicki), large concentrations of shear- tographsof the Swinhoe'swith a Band-rumped(Figure 4), the Swinhoe'sappears remarkably slimmer-bodied, which suggests that waters(Calonectris diomedea and Puffinusspp.) and Black-capped Petrels(P. hasitata),and a recordcount of Band-rumpedStorm- the fieldimpression of largersize in the Swinhoe'swas at leastpar- Petrels(O. castro)for that port. tiallyartifactual, probably owing to greateroverall wing area. None At about15:15 ED$•, shortly after we startedto headback shore- of usever had a firm impressionof tail shapein thefield, but pho- ward,we flusheda smallgroup of Oceanodromastorm-petrels off tographsseem to showa tail that is slimmerthan that of a Leach's the water,induding one with entirelydark uppertailcoverts and andwith a shallowernotch (Figures 1 and2). Althoughthe slim tail rump. Giventhe recentspate of Swinhoe'sStorm-Petrel records in gavethe bird a long-tailedlook in directflight (Figure 5), whenthe the northeasternNorth Atlantic (Bretagnolleet al. 1991,Cuhitt rectricesand primarieswere spread and the full wingarea revealed, 1995) and a sightrecord from North Carolinawaters (Brinkley the bird actuallylooked quite short-tailed (Figure 1). The flight 1995), togetherwith the fact that dark-rumpedLeach's Storm- featherslooked full, with no obviouslymissing feathers, though in Petrels(O. leucorhoa)have yet to be documentedin the Atlantic somephotographs (Figures 1 and 5) the outerprimaries look more Ocean basin (see below), we suspectedthe bird might be a wornthan the inners, suggesting that the bird was midway through Swinhoe's.We pursuedthe bird to the southwestfor about 11 km primarymolt. Judgingfrom photographs(Figures 6, 7, and8), the overa 40-45minute period in anattempt to obtainphotographs and primarycoverts seem to befull aswell. Foradditional images of this betterviews. The bird remainedin flightfor theentire observation, individual,consult Patteson's website at http://www.patteson.com. but CaptainStowe occasionally brought us to within about 25 Coloration.The overallcolor was dark grayish-brown, darkest on metersof the bird, allowingreasonably good views. During this the uppertailcoverts and rump and slightly paler on theupper back time, we obtainedabout 150 photographsof the bird and noteda and nape.The tonewas similar to that of a Leach'sbut obviously combinationof plumage,structural, and flightcharacters that con- palerthan that of a Band-rumped.In photographs,the head appears firmed the identification as Swinhoe's Storm-Petrel. On several to be contrastinglygrayer (Figures 8 and 9), thoughthis was not occasionswe flushedother storm-petrels and, for briefperiods, had notedin thefield. The pale upperwing covert bar or "carpalbar" was Wilson's(Oceanires oceanicus), Leach's, and Band-rumpedstorm- veryobvious, as on a Leach's,and it broadenedmarkedly toward the petrelsflying alongside the Swinhoe's for directcomparison. leadingedge, as on that species(Figures 2, 5, 6, and 8). Fromabove, The bird wasinitially found at 34ø37'05"N, 75ø18'30- W, about the outer severalprimaries appeared to have somewhite toward 68.6 km south-southeastof Hatteras Inlet, in water 2378 meters theirbases, though it wasvery inconspicuous and only visible when deep,and was pursued to 34ø32'00"N, 75o25'45"W, about77.8 km thebird wasat itsvery dosest (about 25 m). Wehad this impression south-southeastof Hatteras Inlet, in water2378 m deep.Sea surface especiallyas the bird banked,perhaps because it spreadthe pri- temperaturein thesewaters ranged from 28.8 to 29.2ø C. For most mariesfarther apart but no doubtalso because the wing motion was of theobservation, distances from the bird rangedfrom about 50 to slowestthen. Had we not lookedspecifically for whitein the pri- 75 m. Lightingconditions ranged from goodoff the port bow to maries,it probablywould have gone unnoticed. Photographs con- harshand backlit off thestarboard bow, but CaptainStowe was able firm thatthere is a veryrestricted white patch at thebase of theouter to keepthe bird in goodlight most of the time. primaries,visible in both spreadand folded wing positions (Figures * E0.Box 436, Cape Nay Point, New Jersey 08212; t RO.Box 772, Hatteras, North Carolina 27943 $ 25124•t FerryNeck Road, Royal Oak, Naryiand 2! 662; õ 338Faire Chase, Chesapeake, Virginia 23322 6 NORTH AHERICAN BIRDS Michael O'Brien Figure 1 Michael O'Brien Figure2 J. BrianPatteson Figure3 I. BrianPatteson Figure4 I. BrianPatteson Figure5 MichaelO'Brien Figure6 GeorgeL. Armistead Figure7 GeorgeL. Armistead Figure8 FiguresI to 9. In thesephotographs, most of the generalproportions and plumage charactersof this Swinhoe'sStorm-Petrel can be discerned.Figures 1, 2, and 3 showthe overallshape of the bodyand wings fairly well: the baseof the wingappears a bit narrowerthan in Leach's,whereas the primaryarea looksa bit broaderin comparison. In Figure4, whereSwinhoe's is picturedwith a Band-rumped,the formerlooks somewhat slimmerof body,perhaps owing to the impressionof greaterwing area. Impressions of tail lengthvaried from long (Figure5) to rathershort when the wingswere extended (Figure1). Figures6, 7, and 8 showa hird with intact,full primarycoverts, though Figures1 and 5 suggestthat the outerprimaries are moreworn than the inner (possibly newer)primaries. The dorsalsurface of the birdwas largelyflat brown,set off by a pale carpalbar (Figures2, 5, 6, and8). Othenvise,the onlypale pigmentationon the bird consistedof a patchat the visiblebase of the primaries(Figures 6, 8, and9). Michael O'Brien Figure9 VOLUME53 (1999)t ISSUEI 7 6, 8, and 9). Whetherthis white patchis confinedto the primary Howeverthey arrive in the Atlantic,the intriguingquestion shaftsor includeswebbing isnot possible m determine. remainsas to whetherthese Atlantic Swinhoe's are breeding. It has FlightStyle. The flight styleseemed distinct from both Leach's beensuggested that a smallbreeding population of Swinhoe'smay and Band-rumpedstorm-petrels. The wing strokeswere relatively existsomewhere in the northeasternAtlantic (Cubitt et al. 1992),but slowand "loping" (not snappy)and the flightwas quite steady and asyet none has been discovered. Interestingly, of thebirds that have direct,not unlikethat of a tern.It lackedthe nighthawk-like erratic been captured,several have possessed vascularized brood patches, bounding,quick directionchanges, and side-to-siderocking of whichwould appear to lendmore weight to suspicionof localbreed- Leach'sor at leastthese patterns were very much reduced from those ing (Parkin& Cubitt1995). Also, an apparentmorphometric diver- of Leach's.Though the wing strokes were deep, they were shallower genceexists between Atlantic and PacificSwinhoe's Storm-Petrels, with the Atlanticbirds showinglarger wing chordmeasurements thanthose of Leach's,not risingas high on the upstrokeand seldom (Cubittet al. 1992),though, as Cubitt (1995) notes, this conceivably exhibitingthe deep"V" sotypical of that species.The glideswere on couldbe explainedby differencesin the recordersand/or their mea- bowedwings as in Leach'sand Band-rumped,but the glideswere suringtechniques. DNA analysishas found Atlantic Swinhoe's to be longerthan those of Leach's(usually one-two seconds). Compared to geneticallyinseparable from Pacific birds (Dawson 1992), which sug- Band-rumped,the wing strokeswere deeperand slowerand the geststhat they are relatively recent visitants from the core population, glidesprobably averaged shorter. How much the bird's flight patterns perhapsprospecting new nestingareas, and that AtlanticSwinhoe's wereinfluenced by pursuit is not known, but onewould expect a bird probablyhave not been long isolated from Pacific populations. to movefaster and more directly while being chased. We do,howev- er, oftenchase Leach's and Band-rumpedstorm-petrels on pelagic IDENTIFICATION
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