SEABIRD OBSERVATIONS OFF WESTERN MEXICO

STEVE N. G. HOWELL, Point ReyesBird Observatory,4990 ShorelineHighway, StinsonBeach, California94970 STEVEN J. ENGEL, 1324 SecondStreet, Bismarck,North Dakota 58501

Southern Mexico's offshore Pacific avifauna has been rather little studied. Murphy (1958) and Jehl (1974) reportedobservations from cruisesthat passedthrough Mexican waters in Novemberand December1956 and in April 19.73, respectively.Pitman (1986) mappedthe relativeabundance of 57 speciesof seabirdsin the easterntropical Pacific on the basison 4333 hoursof observationbetween 1974 and 1984; only a small numberof these4333 hours(81 noon positions,55 of which were off Baja Califor- nia), however,pertain to Mexicanwaters (i.e., within 200 nauticalmiles of Mexican territow), and 74% of the 81 noon positionswere between Octoberand March (R. L. Pitmanpers. comm.).Pitman's atlas provides an excellentlarge-scale picture but lacks data on seasonalstatus of species,and the scale employeddoes not enable one to interpret local distributions. Other recordsof seabirdsoff western Mexico are widely scatteredand mostlyderive from nearshoreland-based trips of a day or less(e.g., Binford 1970, 1989). The Middle AmericanTrench runs from the vicinityof the IslasTres Marias,Mexico, to the CocosRidge, south of CostaRica. The trenchlies some 55-110 km (mean distance 75 km) offshore between Jalisco and Guerreroand is 15-50 km (mostly20-30 km) wide. The trenchis at least 3600 m deep, mostly4300-4650 m deep from centralJalisco south, and increasesto 5000-5200 m deep off central Guerrero;submarine moun- tains in the trench off Colima (Manzanillo)and Guerrero (Zihuatanejo) reduce depths to 3600 m. On either side of the trench waters quickly shallowto 2700-3200 m, and inshorethe 1000-fathom (1800-m) contour line lies 20-55 km (mostly35-55 km) off the coast. From 28 April to 6 May 1992 we observedseabirds off westernMexico out to 100 km from shore,that is, from just offshoreof to well inshoreof the Middle American Trench, between the vicinityof Cabo Corrientes, Jalisco,and Acapulco, Guerrero. Here we describethe observed duringour visit.

ITINERARY AND METHODS

We beganobservations at dawn on 28 April at 19ø51' N, 106ø28' W, about 110 km west-southwestof Cabo Corrientesand just offshoreof the northernend of the trench. Figure 1 showsthe routesof our daylight transits off western Mexico relative to the Middle American Trench. Other than 29 April and 4 May, when we made stopsfor supplies,daily transits were 85-145 km, with variationdue to time spentobserving feeding flocks and the effectof currentson our cruisingspeed. On 3 May we circledLas RocasPotosi (17ø32 ' N, 101ø31' W), 13 km southeastof Zihuatanejo,and on 4 May we checkedvarious islets 10-18 km west of Zihuatanejo (includingLas IslasBlancas and Isla Grande). Western Birds 24:167-181, 1993 167 SEABIRD OBSERVATIONS

We madeobservations with 8 x 30 binocularsthroughout the day from the deckof the 14.5-m ketchEnchantress. Our eye levelwas about3 m abovesea level,giving a visiblehorizon of 5.5-7 km. For 30 continuous minutesof everyhour of transitwe countedall birdswithin 200 m of the vesselwithin a 90 ø sector(ahead to eitherport or starboard,depending on light, wind, and/or sailconditions). The radiusof 200 m was chosensince beyondthat distance,in windsof more than 10-15 km/h, we often were unableto identifyto speciessmaller birds such as storm-petrelsor phala- ropes.Our speedover the groundaveraged 11 km/h andwas rarely below 9 or above 13 km/h. We frequentlychanged course to observefeeding flocks within 5 km of our coursemore closely,and we notedthe compositionand behaviorof speciesin each flock (from 5 to 45 minuteswere spentat each feeding flock).We discontinuedany 30-minuteobservation period interrupted by suchcourse changes since it no longerrepresented a randomtransit. One or bothof uswere on deckduring all daylighthours and maintained a nonstandardizedwatch for seabirds(i.e., anythingwe couldsee from the vesselregardless of distanceand direction)when not making30-minute censusesor observingfeeding flocks. We took the surfacewater temperatureat the start of each 30-minute observationperiod and at the locationof eachfeeding flock. Positions were determinedby meansof a MagellanGlobal Positioning System; depths were interpolatedto the nearest50 m from DefenseMapping Agency chart numbers21017 and 21020. The warmestwaters were consistentlythose inshoreof the trench (in depthsof 900-3000 m), which averaged0.5- 1.0øCwarmer than waters from the inshoreedge of the trenchto offshore of the trench.Monthly mean sea-surface temperatures for thesewaters are 1.0-2.5øC coolerin April and 0.5-2.0øC coolerin May than the tempera- tureswe recorded(Robinson 1976). The relativelywarm watersof our cruiseprobably reflect a well-developedE1 Nifio-Southern Oscillation. Windswere generallylight (1-10 km/h), althoughfor briefperiods on 29 and 30 April they increasedto 20-25 km/h. Wind directionwas mostly from southto west off Jalisco,Colima, and Michoacan,and mostlynorth- west to west off Guerrero.

RESULTS AND DISCUSSION

Table1 presentsthe resultsof our standardizeddaily censuses as birds per hour (totalindividuals recorded per day duringcensus periods divided by total daily hoursof census).The followingspecies accounts describe the abundanceand distributionof the 30 speciesof seabirdsrecorded more than 3 km from shoreand other speciesof note. Commoncoastal species suchas the Brown Pelican(Pelecanus occidentalis) and LaughingGull (Larus atticilia) are thus omitted from the accounts. Whileterms such as "rare,""common," etc., willalways be subjective,we considerthem useful;data for each speciesindicate the basisfor our assessmentsof abundance.The numbersgiven in parenthesesafter each species'name indicatethe total numberobserved (including standardized and nonstandardizedobservations and feedingflocks, but not individualsat 168 SEABIRD OBSERVATIONS

inshorerocks and islets), maximum number recorded per day, and number of daysrecorded. An asteriskafter the daily maximumindicates that the hightotal wasrecorded on two days.Water temperatureranges are givenat the end of each speciesaccount. SpeciesAccounts LaysanAlbatross, Diomedea immutabilis (1; 1/1). Rare. We saw one on 28 Apr associatingwith a feedingflock over the northernend of the trench;29.4øC. This specieshas colonizedislands off westernMexico in recentyears (Howell and Webb 1992); Isla San Benedictois the breedingsite nearestour observation. Pink-lootedShearwater, creatopus (481; 160/8). Common to fairly common; lesscommon in the southernhalf of our studyarea (Table 1). We recorded Pink-looted to within 20 km of shore but most were more than 25 km offshoreover watersmore than 1000 m deep. This speciesoften was a common memberof feedingflocks over midshore and offshorewaters. 25.6-29.4øC. Wedge-tailedShearwater, P. pacificus(124; 75/5). Fairlycommon to uncommon in southernwaters warmer than 27.5øC, with only one seen north of Michoacan. Wedge-tailedShearwaters occurred to within 3.5 km of shorebut mostwere 20-75

22•

Figure1. Routesof seabirdobservations off westernMexico (solid lines with arrows), 28 April-6 May 1992. Dashedline indicatesbottom of the MiddleAmerican Trench. A, Islas Tres Marias; B, Isla Isabel; C, Islas Tres Marietas; Ac, Acapulco; Mz, Manzanillo;Zh, Zihuatanejo.

169 Table 1 SeabirdsObserved (to Nearest0.1) per Hour During30-Minute Countsof 90 ø Sectorsoff WesternMexico, 28 April-6 May 1992

28 29 30 1 2 3 4 5 6 Apr Apr Apr May May May May May May

Pink-looted 3.0 2.0 0.2 0.8 0.6 1.1 -- 1.7 2.2 Wedge-tailed Shearwater *a __ -- 0.3 0.9 1.4 -- 0.2 -- SootyShearwater + -- 0.2 + + 0.6 -- 0.5 0.2 Christmas Shearwater ------0.5 + 0.6 Townsend's Shearwater 0.3 0.5 + -- 0.6 2.9 -- 0.5 8.7 Audubon's Shearwater ------+ 2.0 1.0 -- 0.2 Leach's Storm- 2.9 0.5 + -- -- 0.3 -- 0.8 9.1 Galapagos Storm-Petrel 4.8 0.5 0.4 1.3 3.4 1.4 2.0 0.2 5.5 Black Storm-Petrel 6.1 5.0 1.6 1.3 2.8 4.0 6.0 2.0 3.5 Least Storm-Petrel -- 1.0 -- -- 1.5 0.3 11.0 * 0.5 Red-billed Tropicbird -- + -- + 0.2 MaskedBooby -- -- 0.2 0.5 0.3 -- * BrownBooby 5.6 2.0 6.2 8.0 3.1 19.7 5.0 0.7 12.5 Red-lootedBooby -- -- 0.7 2.0 + + -- 0.2 + Magnificent Frigatebird + + 0.2 * + -- ß * Red-necked Phalarope 3.8 1.0 0.4 1.0 5.2 6.6 -- 1.2 0.7 Red Phalarope 1.3 2.5 0.2 0.7 6.2 2.0 2.0 1.5 5.6 Phalaropesp. 1.3 1.0 0.2 4.0 1.9 0.9 3.0 3.7 6.2 PomarineJaeger 0.2 1.5 0.4 + 0.3 0.3 + 0.3 0.5 , * ParasiticJaeger 0.6 -- + + + -- Long-tailedJaeger -- * + * * 0.9 + + * Jaegersp. 0.2 -- Sabine's Gull 0.2 1.0 -- 0.5 0.9 1.1 + + + Common Tern + + ß , + -- 0.4 Arctic Tern * 1.0 + 1.0 + -- -- 0.2 -- Common/ Arctic Tern 0.3 + Least Tern -- * + 0.4 Bridled Tern -- + -- + -- 13.1 24.0 -- + Black Tern 0.8 6.0 0.4 1.5 10.8 4.0 1.0 0.3 6.5 * BrownNoddy ------3.1 + Hours of observation 6.25 2.0 5.5 4.0 3.25 3.5 1.0 6.0 5.5 Sea surface temperature 25.6- 27.5- 27.0- 27.7- 28.• 28.• 28.4 27.7- 27.2- (rangein øC) 28.5 27.6 28.4 29.4 29.4 29.3 30.2 28.8 a+, seenoutside census periods; * seenoutside census periods and only with feedingflocks. 170 SEABIRD OBSERVATIONS

km offshoreover waters 900-5000 m deep. We found most birdswith feeding flocks,and exceptfor two or threedark-morph birds on 2 May we sawonly the light morph,agreeing with the resultsof Pitman(1986). 27.7-29.5øC. SootyShearwater, P. griseus(29; 5*/7). Uncommon,seen mostly as singlebirds flying northwest,but a few birdsassociated loosely with feedingflocks. Sooty Shearwatersoccurred 18-95 km (mostlybeyond 35 km)offshore over waters greater than 1000 m deep. 27-29.5øC. Christmas Shearwater, P. nativitatis (47; 27/4). Fairly common over waters warmer than 28.0øC and 2700-5000 m deep in a belt 35-55 km offshoreof Guerrero;uncommon off Michoacan,where we sawthe northernmostbird at 17o53' N, 103o41' W. We foundmost ChristmasShearwaters with feedingflocks. 27.7- 29.4øC. The A.O.U. (1983) consideredthis species "accidental at seabetween Clipperton Islandand the mainlandof Mexico,"presumably on thebasis of an undatedspecimen collected at 12o05 ' N, 107 ø W (Loomis 1918), about 315 km northeast of Clipperton.Pitman (1986), however,reported seeing this speciesat ratesof 0.75- 2.0 per hour off westernMexico (from Michoacanto Oaxaca)between June and September;he collecteda specimen(Los Angeles County Museum) on 2 June 1982 at 15o20' N, 99o22' W, about 180 km south-southeastof Acapulco.In addition,D. G. Ainleyand L. B. Spear (pers.comm.) observed four ChristmasShearwaters 102- 192 km offshorefrom Guatemala to Oaxaca,Mexico, on 9 and 10 April 1992. This speciesappears to be a fairlycommon visitor, at leastduring some years from April to September,to watersoff southernMexico. Townsend'sShearwater, P. auricularis(404; 133/7). Fairlycommon, occurring 25-95 km offshoreover waters 1000-4500 m deep. We detectedmost birdsin feedingflocks or as raftinggroups of up to 45 birds.The morningof 28 April we notedones and twossteadily flying east, i.e., on a directroute from the species' nestinggrounds on IslaSocorro to coastalwaters off Jalisco.The plumageof mostof the birdswe saw in the feedingflocks was worn and faded,and severalbirds were moltingflight feathers;we noted only 15-20 birds(including those on 28 April mentionedabove) that appearedto be in fairlyfresh plumage. 25.6-29.4øC. Jehl (1982) concludedthat youngTownsend's Shearwaters fledge between late May and late July. Thus, while some of the birdswe saw may have been failed breeders,and othersbreeding adults ranging to feedinggrounds off westernMexico, the majorityprobably were immaturesand nonbreeders.Jehl (1982), in plottingthis species'seasonal distribution, cited only one recordfrom watersoff westernMexico fromApril to June,and saw none off westernMexico in earlyApril 1973 or April 1976. Immatureand nonbreedingTownsend's Shearwaters may arriveregularly off southwesternMexico in late April. Townsend'Shearwaters associated with Audubon'sShearwaters at feedingflocks, enablingdirect comparison of thesetwo species.While most guides emphasize the whitepatches on the flanksof Townsend's,most birds we sawwere sitting on the waterand made only short feeding flights, so the flankpatches often were not visible. In addition,many Townsend'swere very worn, faded,and brownishabove, and the white flank patchesoften were inconspicuous.The featureswe found usefulin separatingthese two speciesapply only to nominateTownsend's and the sympatric subspeciesof Audubon's Shearwater (P. I. subalaris,nesting in the Galapagos). Townsend'sis largerand longerwinged than Audubon's,and this was obviousin directcomparison. Although the NationalGeographic Society (1987) and Naveen (1983) mentionedthe longtail of Audubon's,we did not noticethis, and Murphy (1936) pointed out that "the tail in the Galapagosrace is both relativelyand absolutelyshorter" than in Atlanticpopulations. The quickerwingbeats of Audubon's usuallyidentified it in prolongedflight, but in feedingflocks flight manner was not

171 SEABIRD OBSERVATIONS veryuseful since both species mostly made short rapid flights. We alsowere unable to discernon birdsin feedingflocks reported differences in underwingpattern (e.g., Harrison1983, 1987), althoughthis pattern may be seenbriefly when birdsflush from the water (Figure2). Whenthe birdswere sittingon the waterthe moststriking difference was the white sidesof Townsend's,suggesting a TuftedDuck (Aythyafuligula), versusthe mostly dark sidesof Audubon's(Figure 3). This differencemay be due in part to a tendency of Audubon'sto holdits wingsmore drooped,since 41 of 100 specimensof P. I. subalarisat the CaliforniaAcademy of Scienceshave clean white flanks;the other 59 havedistinct dark streakingon their flanks. Face and neck pattern was usefulin flightand at rest:Audubon's has a clean-cut dark cap that extendsto just belowthe eyes and may show indistinctwhite eye- crescents:Townsend's has a more extensive and not clean-cut blackishcap that extendswell belowthe eyesand overthe auriculars(Figure 3). Townsend'shas a dark half collar on the sidesof the lower neck; Audubon'shas a cleaner-cutand smaller patch(Figure 3). Atlanticpopulations of Audubon's,however, have a darkhalf collar similarto Townsend's(Murphy 1936, Howell pers. ohs.). The feet of Audubon'sappeared dusky flesh, those of Townsend'sflesh pink, not black (contra Harrison 1983, Ridgely and Gwynne 1989). This is evident in photographsof Townsend'staken by Howell (unpubl.)and Jehl (1982). Six speci- mensof Townsead'sat the CaliforniaAcademy of Scienceshave obviouslypale legs and feet with blackishonly on the outer sideof the outer two toes. as did two live Newell'sShearwaters [P. (a.) newelli]examined near Hawaii (Howelland L. B. Spear field notes).Thus the feet of Townsend's(and Newell's)appear mostlyflesh at sea, perhapsbrighter and pinkerthan the feet of Audubon's.

Figure2. Townsend'sShearwater off Jalisco,28 April 1992. Note extensivedusky smudgingon sidesof faceand neck, relatively faded and brownish upperwings. white flank patch, and extensivelyclean white underwing. Photo by Steve N. G. Howell

172 SEABIRD OBSERVATIONS

Becauseof plumagewear and variable lighting conditions we foundthe colorof the upperparts(usually considered blackish on Townsend'sand dark brown on Audubon's)to be of littleuse in fieldidentification. Audubon'sShearwater, P. lherrninieri(126; 95/4). Fairlycommon 20-35 km off Guerreroover waters 500-1800 m deep;we alsosaw five birds 20-45 km off Jalisco overwaters 500-1000 m deep, northto 19ø51' N, 105ø37' W. Our lackof records betweenGuerrero and Jaliscoprobably reflects our lack of observationsin these waters. 27.6-29.5øC. The A.O.U. (1983) reported Audubon's Shearwater as ranging north in the easternPacific to Oaxaca, and Pitman (1986) reportedup to 2-10 birdsper hour north to waters off Colima. Our reports off Jaliscorepresent the northernmost recordsof Audubon'sShearwater in the region. Leach'sStorm-Petrel, Oceanodroma leucorhoa (125; 75/6). Commonoff Jalisco in waterscooler than 27.5øC; uncommonto rare southfrom therein waterswarmer than 27.5øC, wherewe sawonly 14 birds.Of the 13 dark-morphbirds we saw, 60%

Figure3. Comparisonof Audubon'sShearwater, Puffinus Iherminierisubalaris (upper)and Townsend'sShearwater, P. a. auricularis(loweft. Sketch by Steve N. G. Howell

173 SEABIRD OBSERVATIONS

were over waterswarmer than 27.5øC, while 96% of light-morphbirds were over waters cooler than 27.5øC. All Leach's Storm- were at least 45 km offshore over waters2500-5000 m deep. 25.6-30.2øC. AlthoughLeach's was the commoneststorm-petrel on 6 May both immediately beforeand after a feedingflock that included50+ Galapagosand 90+ Blackstorm- petrels,only three Leach's associated with thisflock. Galapagos(Wedge-rumped) Storm-Petrel, O. tethys (321; 130/9). Commonto fairlycommon throughout the cruise.We recordedit to within9 km of shore,but most were at least 25 km offshore over waters 1000-5000 m deep. Notable concentrationswere 130 birdsrafting with 20 Black Storm-Petrelsat 17ø03' N, 101ø26' W on 3 May, and 50+ rafting and feedingover BottlenoseDolphins (Tursiopstruncatus) on 6 May. In 45 minutes,ten birdscame to a slickof codliver oil laid at 19o42' N, 105ø35' W, 6 May. 25.6-29.5øC. Black Storm-Petrel, O. melania (487; 166/9). Common to fairly common throughout,occurring 2-95 km from shoreover waters 75-5000 m deep.Up to 12 birdsoften followedthe boat and scavengedgalley scraps after meals.A notable concentrationwas 90+ birdsrafting and feedingover Bottlenose Dolphins on 6 May. In 45 minutes, 26 birds came to the same slick of cod liver oil that attractedthe GalapagosStorm-Petrels on 6 May. 25.6-30.2øC. Least Storm-Petrel,O. microsoma(134; 46/6). Common within 35 km (and mostlywithin 25 km) of shoreover watersless than 1000 m deep. The species' abundancein thiszone is maskedin Table1 by the inclusionof hoursfarther offshore when none was seen. The slickof cod liver oil laid on 6 May attractedfive. 27.7- 28.8øC. Red-billedTropicbird, Phaethon aethereus (6; 2/5). Uncommonto rare. Evenon 29 April whenwe passedwithin a few kilometersof PefiaBianca, a largecolony of Red-billedTropicbirds 18 km westof Manzanillo,we sawonly two singletropicbirds. None associatedwith feedingflocks, and we notedonly one in our censusperiods. Apparentlythis species disperses widely and is mostlysolitary at sea;Au andPitman (1986) recordedonly one tropicbird in 134 feedingflocks in the northeasterntropical Pacific. 27.4-28.9øC. Pefia Biancaappears to be a previouslyunreported colony, although it has been knownto localfishermen for manyyears. Howell and S. Webbvisited the rockon 1 January1987 andobserved 900-1000 tropicbirdswith many pairs engaged in noisy courtshipflights and some birds entering crevices.Subsequent observations by Howell are of 50+ birdson 16 March 1989, 100+ on 15 March 1990, and 80-100 on 13 March 1992; on each of thesedates Howell saw somecourtship flights and severalbirds entering and leavingcrevices. At Las RocasPotosi, Guerrero, we sawfour or five pairsof Red-billedTropicbirds engagedin screamingcourtship flights and saw one enter a crevicein the rocks. The A.O.U. (1983) recordedbreeding by Red-billedTropicbirds along Mexico's Pacificcoast south only to Las IslasTres Marias, Nayarit, but clearlythe species' breedingrange is more extensive. MaskedBooby, Sula dactylatra(83; 40/4). Fairlycommon 45-100 km offshore from Michoacansouth, over waters deeper than 2500 m. Mostwere associated with feedingflocks or attractedto the boat.All 70 adultsand subadults we sawclearly had the brightyellow-orange to pinkish-orangebills characteristic of S. d. grantiof the Galapagos,suggesting they originatedthere, although this subspecies also nests in smallnumbers on bas IslasRevillagigedo (Howell and Webb 1990). 27-28.2øC. Blue-lootedBooby, S. nebouxii.We sawone at Zihuatanejoon 3 May andtwo or three at Las lslasBlancas and 11 or 12 at IslaGrande (18 km westof Zihuatanejo)on

174 SEABIRD OBSERVATIONS

4 May. This speciesis notablysedentary about its nestingareas (southin Mexico to Las IslasTres Marietas)and rarely wanderssouth to watersoff Colima (Howell pers. obs.).We are aware of no previousrecords from Guerrero,although there is one hypotheticalrecord from Oaxaca(Binford 1989). Brown Booby,S. leucogaster(5555; 1480'/9). Common to fairlycommon out to 100 km from mainland.The two high countsreflect 1310 birds(99% adult)heading offshorefrom the largecolony at Pefia Bianca,Colima, between0700 and 0730 on 29 April, and probablya largecomponent (1000+ birds)of the colonyat Las Rocas Potosi, Guerrero (100-120 km distant), on 1 May. The Brown Booby was a commonmember of feedingflocks and a conspicuousindicator of themat distances up to 5 km. Adultsconstituted 60-95% of birdsseen on all daysbut 2 May; most were commutingto and from the mainlandbecause usually we saw few or none at dawn when we often saw the more pelagicMasked and Red-lootedboobies. On 1 and 2 May, however, we saw numerous Brown Boobies at dawn, 75-85 km offshore,and on 2 May 90% of the 175 Brown Boobiesseen were juvenilesand immatures. 25.6-30.2øC. At Las Rocas Potosi we counted 6000-6500 birds, includingmany recently fledgedjuveniles, on or aroundthe rocks,1430-1510 on 3 May. The A.O.U. (1983) recordedbreeding by the Brown Booby along Mexico'sPacific coastsouth only to Las IslasTres Marias, Nayarit, thus overlookingthe two coloniesmentioned here, althoughGoldman (1951) photographedthe Guerrerocolony. At Las IslasBlancas (17o40' N, 101o38' W), apparentlynot a breedingsite, we counted200 roosting Brown Boobies. Red-footedBooby, S. sula (203; 120/6). Fairlycommon 35-100 km offshore from Michoacansouth over waters deeper than 1500 m; uncommonnorth to Jalisco.We also saw one brown-plumagedbird roostingat Las IslasBlancas. This specieswas more commonlyattracted to the boat than to feedingflocks, and up to ten birds often accompaniedus, outmaneuveringthe less agile juvenile Brown Boobiesin pursuitof flyingfish flushed by the boat. Of the birdswe saw, 99% were brown-plumagedand at least 80% of these were immatures;we noted only one white-morph adult and three or four brown adults.27.6-29.5øC. MagnificentFrigatebird, Fregata magnificens (40; 16/7). Uncommon. Of the total, we saw 15 on the approach to Manzanilloon 29 April and 16 at sea with feedingflocks on 5 May. Althoughwe notedMagnificent Frigatebirds as far as 95 km offshore, most were lessthan 45 km from land. 26.9-28.6øC.

Wilson's Phalarope, Phalaropus tricolor. One flew northwest at 17o53' N, 103ø35' W, on 5 May. Red-neckedPhalarope, P. Iobatus(2990; 1500/8). Common to fairly common. We recordedRed-necked Phalaropes up to 95 km offshorebut mostwere lessthan 45 km from land, and large concentrationsoccurred within 15 km of shore.25.6- 29.5øC.

Red Phalarope,P. fulicaria (487; 190/9). Commonto fairlycommon, occurring 9-95 km offshorebut mostly20-55 km from land. 25.6-29.5øC. PomarineJaeger, Stercorarius pomarinus (86; 24/9). Common. We saw43 birds with feedingflocks but the only kleptoparasiticchases noted were singlechases of Masked and Brown boobies and of a Christmas Shearwater. Immatures were com- monerthan adultsat feedingflocks, and mostof the 27 adults(all light-morph) were headingnorthwest. Pomarine Jaegers occurred 4-95 km offshore.25.6-29.4øC. ParasiticJaeger, S. parasiticus(14; 8/6). Uncommon.After eight(including four light-morphadults) on 28 April, this specieswas notablyuncommon, with onesand

175 SEABIRD OBSERVATIONS

twos(mainly immatures) seen mostly at feedingflocks, where we notedtwo chasesof BlackTerns. All ParasiticJaegers were 25-95 km offshore.25.6-29.2øC. Long-tailedJaeger, $. 1ongicaudus(25; 7/8). Fairlycommon 25-95 km offshore, with one noted9 km from land.The 25 birds(18 at feedingflocks) included 17 adults and 6 immatures,all of the lightmorph. We noted 23 chases,all of terns:21 of the BlackTern, and one each of Arctic and Common terns. 27.5-29.4øC. None of the adultshad the longtail streamersof alternateplumage, although one showedslender streamers apparently half-grown; the resthad the shorterand thicker streamersof basic plumage.In contrastto this species'highly variablejuvenile plumage,the immatureswere consistentin appearance,and the availableliterature (e.g., Cramp and Simmons1983, Harrison 1983) suggeststhe immatureLong-tailed Jaegerswe saw were second-summerbirds. Since the immatureplumages of the Long-tailedJaeger are poorlyknown, we describethis plumage briefly. The headand underbodywere whitishwith a smudgydark cap suggestingan adult,a buffy-yellow washon the sidesof the neck,and an obviousdark chest band. The underwingswere darkwith a distinctbut narrowwhite flash along the basesof the primariesand strong pale barringon the coverts.The upperpartsresembled those of the adults,but the brownish-grayupper wing coverts did not contrastas stronglywith the darkremiges, and the upper tail covertswere barred whitish. We were unable to discernany projectingtail streamers.

FrankliftsGull, Larus pipixcan (3; 1/2). Rare. We sawonly three, all in alternate plumage,9-65 km offshore,although this species was fairly common in coastalbays and harbors. 27.5-29.2øC.

Sabine'sGull, L. sabini(112; 38/8). Fairlycommon to common4-75 km (mostly 10-55 km) offshore.Most were seenin transit,but onesand twosoccasionally associatedwith feedingflocks and 11 were with one flock.In this flockwe observed up to two Sabine'sGulls at a time (andat leastfive individualsin all) makingseven kleptoparasiticchases of Black Terns; at least two chaseswere successful.25.8- 29.4øC. Fewerthan 10% of theSabine's Gulls we sawwere fully alternate-plumaged adults, the restapparently being first-year birds with partialhoods or mostlydark hoods fleckedwhitish. Some of thesemay have been adults that had not completedtheir prealternatemolt, although this reportedly occurs before northward migration (Grant 1986). Alternatively,some may havebeen adults that winteredlocally and had not yetcompleted their molt. Although the A.O.U. (1983) describedthe winter range of Sabine'sGull as "Panamasouth to centralChile," the specieswinters, at leastin someyears, north to watersoff westernMexico, as indicatedby the following: Howelland P. Pylesaw four adults between Cabo San Lucas, Baja California Sur, and PuertoVallarta, Jalisco, on 17 December1983; J. C. Arvin(pers. comm., Figure 4) saw200-250 Sabine'sGulls, mainly basioplumaged adults, over the GordaBanks approximately10-20 km southof San Jose del Cabo, Baja CaliforniaSur, on 2 March1984 and28 February1985. Grant(1986) publishedphotographs, taken off El Salvadorin March1980, of adultand first-year Sabine's Gulls showing extensive flightfeather molt. CommonTern, Sterna hitundo(30; 9/6). Uncommonto fairlycommon 2-80 km (mostlyless than 45 kin)offshore. The majoritywere with feeding flocks, and the 30 birdsincluded 15 adultsand 12 first-yearbirds. 27.1-29.5øC. ArcticTern, $. paradisaea(74; 26/6). Fairlycommon 30-95 km offshore,with mostat least55 km fromshore over waters more than 1800 m deep.The majority werewith feeding flocks, and the 74 birdsincluded 62 adultsand two first-year birds. 26.9-29.4øC.

176 SEABIRD OBSERVATIONS

Jehl (1974) stated, "The northward migration route of the Arctic Tern . . is unknown,"and certainlyfor the Pacificthe literatureon this subjectappears to be rather vague. Our observationsindicate that some Arctic Terns move north off westernMexico in springbut that they occurfar offshore. Least Tern, S. antillarum (41; 23/6). Uncommon to fairly common 2-30 km (mostlyless than 18 km) offshoreover watersless than 1500 m deep. Like other terns.this species usually associated with feedingflocks. We notedonly one first-year bird. 27.5-29.5øC. BridledTern S. anaethetus(325: 260/5). Locallycommon. Within 35 km (mostly within18 kin) of landover waters less than 1200 m deepoff Zihuatanejowe saw60 adultBridled Terns on 3 May and 260 adultson 4 May. Uncommonaway from this area, with singleadults off Jaliscoand Colima, and three adultsabout 55 km off Guerrero on 1 May. 27.6-29.2øC. Howell et al. (1990) consideredit "likelythat Zihuatanejowas, and may stillbe, the site of a breedingcolony" (of BridledTerns). K. L. Garrett(pers. comm.) recently foundat the LosAngeles County Museum a specimenof a BridledTern mislabeledas a Sooty Tern. This bird was collectedat "White Friars, Mexico" (17028 ' N, 101031 ' W) on 7 May 1939. and the label reads "nestingfrom surf to summit."From the coordinates,White Friarsappears to be synonymouswith LasRocas Potosi (there are no rocks4 nauticalmiles south of Las RocasPotosi). On 3 May 1992 we circledLas RocasPotosi, 1330-1410, andsaw a fewBridled Terns around the rocksbut no sign of nesting:year-to-year variation in nestingchronology, El Nifio, or simplytime of day couldaccount for the apparentabsence of birdsat the rocks. A previouslyunreported nesting site for this species(and its northernmostin the easternPacific) is lslaIsabel, Nayarit, where we estimated20-30 pairson 9 May

Figure4. Sabine'sGulls 10 km southof SanJose del Cabo,Baja California, Sur, 28 February1985. This speciesoccurs off Mexico duringsome winters perhaps regularly. Photo by John C. Arvin

177 SEABIRD OBSERVATIONS

1992, includingbirds displaying, copulating, and enteringand leavingcrevices in the rockystacks off the easternside of the island. SootyTern, S. fuscata(8; 5/3). Uncommon.We sawonly four adult and four first- yearSooty Terns, all with feeding flocks 40-55 km offshoreover waters 1300-3600 m deep. 27.9-29.2øC. BlackTern, Chlidonias niger (2061; 522/9). Commonto fairlycommon. We saw BlackTerns up to 80 km offshore,but most were within 35 km of landover waters lessthan 2000 m deep. BlackTerns were characteristicof inshorewaters, where theywere a dominantcomponent of feedingflocks. In feedingflocks off Guerrero, 80-90% of the BlackTerns were immatures,and only 10% were alternate-plumaged adults;off Jaliscoand Colima, 60-90% were alternate-plumagedadults. 26.0- 29.5øC. BrownNoddy, Anous stolidus (41; 25/3). Fairlycommon off Zihuatanejowithin 35 km of shoreover waters less than 1000 m deep.We sawonly one noddyaway from this area. 27.5-29.4øC. BrownNoddies have not been reported breeding along the Mexicancoast south of Las IslasTres Marietas,Nayarit (A.O.U. 1983), but they may nest at Las Rocas Potosi,where we counted200-250 adultson 3 May. Brewer'sBlackbird, Euphagus cyanocephalus. A male circledthe boat at 19o36' N, 106ø05' W on 28 April. Brown-headedCowbird, Molothrus ater. A male circledthe boat at 17o27' N, 103ø20'W on 30 April, and a femalecircled the boat at 19ø15'N, 105o28' W on 6 May. SpeciesNot Seen Pterodromapetrels were notableby their absence.At leastat this season few or none apparentlyoccur within 75-95 km of shorein the region, despitethe deep waters of the Middle AmericanTrench. Pitman (1986) mapped severalspecies of Pterodroma as occurringoff western Mexico (mostlybetween May and October;R. L. Pitmanpers. comm.),but, given the scaleand format employed,none may havebeen within 95 km of the mainland.

FeedingFlocks Feedingflocks, predominately of shearwaters,boobies, and terns, were conspicuousduring our cruise.Up to 16 jaegers(three species)occurred, but only Long-tailedJaegers made regular kleptoparasiticchases. Small numbersof storm-petrels(particularly the Black Storm-Petrel)associated with 16 of the flocksbut rarelywere noted feeding. Seventy percent of one feedingflock, however,consisted of Black and Galapagosstorm-petrels; thiswas the only feedingflock associated with BottlenoseDolphins. Seventeenof the 34 flocksobserved, including the mostdiverse (average 10.8 speciesper flock),occurred between the inner edgeof the trenchand 25 km of shore over depthsof 1000-4500 m; 16 of these flockswere associatedwith dolphins.The commonerspecies at these flockswere the Pink-footed,Wedge-tailed, and Townsend'sshearwaters, Brown Booby, PomarineJaeger, and BlackTern, sometimeswith with smallernumbers of Christmas and Audubon's shearwaters and Masked and Red-looted boo- bies.

178 SEABIRD OBSERVATIONS

Five feedingflocks (average 7.2 speciesper flock)occurred over the trenchover water 4200-5200 m deep;four of theseflocks were associated with dolphins.The Pink-lootedand Townsend'sshearwaters and Brown Boobywere the commonerspecies in theseflocks. The remaining 12 flocks (average3.8 speciesper flock) were over inshorewaters less than 1000 m deep. Only one of these flockswas associatedwith dolphins,but nine otherswere over schoolingfish appar- entlyfeeding at the surface.Terns, particularly the Black,dominated these flocks, with smaller and/or more local numbersof the Brown Booby, Sabine'sGull, Leastand Bridledterns, and Brown Noddy. Severalspecies, notably terns and shearwaters, were detected commonly at feedingflocks but rarely(or not at all in the caseof SootyTern) during censuses in transit. Au and Pitman (1986) discussedseabird interactions with dolphinsand in the easterntropical Pacific. Both seabirdsand dolphinscommonly

Table 2 Compositionof SeabirdFeeding Flocks Associated with Schoolsof Spottedand Mixed Spottedand SpinnerDolphins in the EasternTropical Pacific

Au and Pitman (1986) a This cruiseb

n % n %

LaysanAlbatross 0 1 0.02 Pterodrorna petrels 99 0.6 0 Pink-looted Shearwater •c 277 6.3 Wedge-tailedShearwater 5184 31.0 84 1.9 Townsend's Shearwater • 222 5.0 Other shearwaters 237 1.4 99 2.3 Storm-Petrelspp. 29 0.2 60 1.4 MaskedBooby • 371 2.2 44 1.0 Brown Booby 1130 6.8 2632 61.1 Red-lootedBooby + Boobysp. 5583 33.4 85 1.9 Tropicbirdspp. 1 0.01 0 Frigatebirdspp. 617 3.7 19 0.4 Red Phalarope 0 55 1.3 Jaegerspp. 2168 13.0 62 1.4 Franklin's Gull 0 1 0.02 Sabine's Gull 0 25 0.6 Sooty Tern 955 5.7 9 0.2 Black Tern 9 613 14.0 Noddy terns 175 1.0 8 0.2 Other terns 161 1.0 90 2.0 Total 16710 4386

a134 flocks,January-March. b20 flocks,April-May. CSpeciespossibly included in "other"categories by Au and Pitman(1986). dlncludesBlue-looted Booby in Au and Pitman(1986).

179 SEABIRD OBSERVATIONS associatewith yellowfintuna (Thunnus albacares)and feed on smallerfish and other prey forced near the surfaceby the tuna, a relationshipmost pronouncedin the northeasterntropical Pacific. Our observationsagree with Au and Pitman's(1986) in that the Spotted (Stenella attenuata) and Spinner (S. Iongirostris) were the common dolphin speciesassociated with the flocks. Bird speciescomposition of feeding flocks associatedwith dolphins, however, differed strikingly (Table2). Some of thesedifferences can be readilyexplained by our cruise being closerinshore: compare proportionsof the more coastalBrown Booby and BlackTern with those of the more pelagicRed-looted Booby and SootyTern. The seasonalso explains some of the differences:Au and Pitman's flockswere observedfrom Januarythough March when several northboundtransients we notedin April and May (e.g., Sabine'sGull, Arctic Tern) wouldhave been rare or absentand when many winteringPomarine Jaegerswere stillpresent. Both seasonaland inshoreeffects probably apply in some cases, as with the Pink-looted Shearwater and Black Tern, and year-to-yearvariation and E1Nifio may be other factors.

SUMMARY

From 28 April to 6 May 1992 we cruisedalong the coastof southwestern Mexicofrom Jalisco to Guerrero,observing seabirds. This area is important for non-breedingTownsend's Shearwaters. We foundChristmas Shearwa- tersto be fairlycommon. We notedLong-tailed Jaegers and ArcticTerns as fairly commonspring migrants. Feeding flocks of up to 15 specieswere conspicuous,the mostdiverse flocks being in a belt25-55 km offshoreover waters 1100-3250 m deep, inshoreof the MiddleAmerican Trench; most were associatedwith SpottedDolphins. We report previouslyunpublished coastalbreeding colonies of the Brown Booby, Red-billedTropicbird, and Bridled Tern.

ACKNOWLEDGMENTS

We thank the Kelton Foundationfor making possibleour cruise off western Mexico. To captainRichard Kelton and fellow crew membersBob Chappel, Fred Geller, and Mary Nichollswe extend our thanksfor making the trip run smoothly. Robert L. Pitman and Larry B. Spear kindlymade availablecertain unpublished data. KimballL. Garrett provideddata on specimenshoused at the Los AngelesCounty Museum,and Jim Cunninghamand BetseyCutler allowed access to specimensat the CaliforniaAcademy of Sciences.The Secretariade DesarrolloUrbano y Ecolog•a, throughthe officesof Dra. Gracielade la GarzaGarcia, granted us permission to visit lsla Isabel.David G. Ainley and Larry B. Spear provideda thermometerand advice on data collection;they and J. R. Jehl, Jr., made helpfulcomments on the manu- script.This is contribution578 of Point ReyesBird Observatory.

LITERATURE CITED

AmericanOrnithologists' Union. 1983. Check-listof North AmericanBirds, 6th ed. Am. Ornithol. Union, Washington,D.C. Au, D. W. K., and Pitman, R. L. 1986. Seabirdinteractions with dolphinsand tuna in the easterntropical Pacific. Condor 88:304-317.

180 SEABIRD OBSERVATIONS

Binford, L. C. 1970. Audubon's Shearwater, Hudsonian Godwit, and Long-tailed Jaegerin Oaxaca,Mexico. Condor 72:366. Binford,L. C. 1989. A distributionalsurvey of the birdsof the Mexicanstate of Oaxaca. Ornithol. Monogr. 43. Cramp,S., and Simmons,K. E. L. (eds.).1983. Handbookof the Birdsof Europe, the MiddleEast, and North Africa,vol. 3. OxfordUniv. Press,Oxford, England. Goldman,E. A. 1951. Biologicalinvestigations in Mexico. SmithsonianMisc. Coll. 115:1-476. Grant, P. J. 1986. Gulls,a Guideto Identification,2nd ed. Buteo Books,Vermillion, SD. Harrison,P. 1983. Seabirds,an IdentificationGuide. Houghton Mifflin, Boston. Harrison,P. 1987. Seabirdsof the World,a PhotographicGuide. Christopher Helm, London. Howell, S. N. G., and Webb, S. 1990. The seabirdsof Las Islas Revillagigedo, Mexico. Wilson Bull. 102:140-146. Howell, S. N. G., and Webb, S. 1992. Changingstatus of the LaysanAlbatross in Mexico. Am. Birds 46:220-223. Howell, S. N. G., Webb, S., and de Montes, B. M. 1990. Notes on tropicalterns in Mexico. Am. Birds 44:381-383. Jehl, J. R., Jr. 1974. The near-shoreavifauna of the Middle Americanwest coast. Auk 91:681-699. Jehl,J. R., Jr. 1982. The biologyand of Townsend'sShearwater. Gerfaut 72:121-135. Loomis, L. M. 1918. Expeditionof the CaliforniaAcademy of Sciencesto the GalapagosIslands, 1905-1906. 12. A reviewof the albatrosses,petrels, and divingpetrels. Proc. Calif. Acad. Sci., 4th ser., 2 (12):1-187. Murphy,R. C. 1936. OceanicBirds of SouthAmerica, vol. 2. Am. Mus. Nat. Hist., New York. Murphy,R. C. 1958. The vertebratesof SCOPE,November 7-December 16, 1956, in Physical,chemical, and biologicaloceanographic observations obtained on expeditionSCOPE in the easterntropical Pacific, November-December 1956 (R. C. Holmes, ed.), pp. 101-111. U.S. Dept. Interior Spec. Sci. Rept., Fisheries 279. NationalGeographic Society. 1987. FieldGuide to the Birdsof NorthAmerica. 2nd ed. Natl. Geogr.Soc., Washington,D.C. Naveen,R. 1983. Audubon'sShearwater, in The AudubonSociety Master Guide to Birding(J. Farrand,Jr., ed.), vol. 1, p. 68. Knopf, New York. Pitman, R. L. 1986. Atlas of seabird distributionand relative abundance in the easterntropical Pacific. Natl. MarineFisheries Service Admin. Rep. LJ 86-02C. SouthwestFisheries Center, P.O. Box 271, La Jolla, CA 92038. Ridgely,R. S., and Gwynne,J. A. 1989. A Guideto the Birdsof Panama,2nd ed. Princeton Univ. Press,Princeton, N.J. Robinson,M. K. 1976. Atlas of North Pacific Ocean monthly mean temperatures andmean salinities of the surfacelayer. Naval Oceanogr. Office Reference Publ. 2. Dept. of the Navy, WashingtonD.C.

Accepted 12 January 1993

181