Seabirds in Southeastern Hawaiian Waters

Home , Christmas shearwater, Kermadec petrel, Macauley Island, Skua, Sooty tern, Tahiti petrel

WESTERN BIRDS

Volume 30, Number 1, 1999

SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

LARRY B. SPEAR and DAVID G. AINLEY, H. T. Harvey & Associates,P.O. Box 1180, Alviso, California 95002 PETER PYLE, Point Reyes Bird Observatory,4990 Shoreline Highway, Stinson Beach, California 94970

Waters within 200 nautical miles (370 km) of North America and the Hawaiian Archipelago(the exclusiveeconomic zone) are consideredas withinNorth Americanboundaries by birdrecords committees (e.g., Erickson and Terrill 1996). Seabirdswithin 370 km of the southern Hawaiian Islands (hereafterreferred to as Hawaiian waters)were studiedintensively by the PacificOcean BiologicalSurvey Program (POBSP) during 15 monthsin 1964 and 1965 (King 1970). Theseresearchers replicated a tracklineeach month and providedconsiderable information on the seasonaloccurrence and distributionof seabirds in these waters. The data were primarily qualitative,however, because the POBSP surveyswere not basedon a strip of defined width nor were raw counts corrected for bird movement relative to that of the ship(see Analyses). As a result,estimation of density(birds per unit area) was not possible. From 1984 to 1991, using a more rigoroussurvey protocol, we re- surveyedseabirds in the southeasternpart of the region (Figure1). In this paper we providenew informationon the occurrence,distribution, effect of oceanographicfactors, and behaviorof seabirdsin southeasternHawai- ian waters, includingdensity estimatesof abundant species. We also document the occurrenceof six speciesunrecorded or unconfirmed in thesewaters, the ParasiticJaeger (Stercorarius parasiticus), South Polar Skua (Catharacta maccormicki), Tahiti Petrel (Pterodroma rostrata), Herald Petrel (P. heraldica), Stejneger's Petrel (P. Iongirostris), and Pycroft'sPetrel (P. pycrofti).

STUDY AREA AND SURVEY PROTOCOL

Our studywas a piggybackproject conducted aboard vessels studying the physicaloceanography of the easterntropical Pacific. Our transectswere

Western Birds 30:1-32, 1999 1 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

• I:/ / //

ß o SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS concentratedsouth and southeastof the islandof Hawaii (Figure1). While the ship was underway during daylight, at least two personssurveyed seabirdssimultaneously from the flying bridge. Using the strip-transect surveymethod (see Spear et al. 1992a), they countedall seabirdsseen within a strip of given width off the forequarteroffering the best observation conditions.Strip width, determinedas per Heinemann (1981), was 500 or 600 m dependingon heightof the flyingbridge of the threevessels used (14 and 15 m abovesea level).Alternating between observers, we usedhand- held binocularsto scan the outer portion of the transect strip nearly constantlyfor birdsmissed with the unaidedeye. We useda 25 x 150 mm mountedbinocular to aid speciesidentification. We recordedbehavior and, for birdsflying in a steadydirection, recorded flight direction to the nearest 10 ø. We also recordedspecies and numbersof birdsin feedingflocks (>4 birdspursuing prey) that passedwithin 2000 m. Every 0.5 hr we recordedthe ship'sposition, speed, and course.We conducted289 transects,each of 0.5 hr exceptthose terminated when the ship stopped,and calculatedthe area surveyedas the surveyperiod multi- pliedby stripwidth and ship speed. Our effortcomprised 72.1 hrsof surveys over 971.6 km2 duringspring (18 April through27 June),and 71.9 hrs covering1156.8 km2 duringautumn (7 Octoberthrough 20 November), 1984-1991 (Table 1). Surveyswere not conductedin spring 1985 or autumn 1986 and 1988. Each 0.5 hr we also recorded sea-surfacetemperature and salinity, thermoclinedepth and "slope"(see below),wind direction(nearest 10ø), wiredspeed, and distancefrom land. Thermoclinedepth and slope, indices of mixingin the water column,were monitoredwith expendablebathyther- mographs.Thermocline depth is the point where the warm surfacelayer meets cooler water below; i.e., the shallowestinflection point determined from bathythermographplots of temperaturewith depth.Exceptions were when there was no inflectionpoint, indicatingthat the thermoclinewas at the surface,or there was more than one inflection,when we assumedthat the thermoclinebegan at the depthof the strongestinflection. We measured thermoclineslope as the temperaturedifference between the thermocline and a point 20 m belowthe thermocline.

ANALYSES

To estimate bird densities,we correctedobserved (raw) numbersfor the effectof flight speedand directionof birdsrelative to the ship'sspeed and course(Spear et al. 1992a; flight speedsfrom Spear and Ainley 1997). Without these corrections,densities from at-sea survey data are usually overestimated,particularly for fast fliers. The correctionalso is required becauseany patterns in bird or ship direction will bias analyses.For example,if birdsflew east and west at the samespeed and in equalnumbers, uncorrectedcounts from a shiptraveling west would show greater numbers flyingeast because the observerwould count more that were flyingeast than west. We calculateddensities for each transectby dividingthe correctedcount by the numberof squarekilometers surveyed and report densitiesas birds SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

Table 1 SurveyEffort in Hawaiian Watersby Date

Year Day Hoursof survey Areasurveyed (km 2)

1984 8 May 6.5 85.9 1984 9 May 2.0 25.8 1984 18 May 5.0 55.8 1984 19 May 1.0 12.8 1984 5 Nov 5.5 79.2 1985 16 Oct 8.5 132.6 1985 30 Oct 8.5 120.8 1986 18 Jun 4.5 60.7 1987 31 May 2.5 39.0 1987 7 Oct 5.5 92.9 1988 4 Jun 5.8 85.8 1988 19 Jun 5.0 83.1 1989 2 May 9.0 143.2 1989 27 Jun 8.5 107.8 1989 17 Nov 7.0 122.1 1989 20 Nov 6.5 105.2 1990 21 Apr 10.8 101.0 1990 7 Nov 5.4 91.8 1990 15 Nov 8.5 134.7 1991 18 Apr 4.0 66.9 1991 25 Apr 7.5 106.6 1991 12 Nov 11.0 194.8 1991 19 Nov 5.5 82.7 Total 8 23 144.0 2128.4

per 100 km2 of oceansurface. For predominantspecies (definition given below),unless noted otherwise, all abundanceestimates pertain to corrected counts,with variance given as one standarderror. We report only raw numbers in accountsof less frequent species.The POBSP calculated abundancedifferently; precluding between-study comparisons of absolute abundance. We recognizedthree speciesgroups: "breeding residents," species that breedon the HawaiianIslands, "nonbreeding residents," species that do not breedon the Hawaiian Islandsbut residein the studyarea as prebreedersor duringthe nonbreedingseason, and "migrants,"species that migrateacross the studyarea when travelingbetween breeding and winteringareas. We dividedthe studyarea into four zones(Figure 1): zone 1, 0 to 73 km from Hawaii; zone 2, 74 to 172 km; zone 3, 173 to 271 km; and zone 4, 272 to 370 km. Respectivelyby zone,we surveyed130.8 (10.8 hrs),289.7 (22.0 hrs), 291.4 (22.3 hrs),and 259.7 km2 (17.0 hrs)of oceansurface duringspring, and 140.9 (9.0 hrs),324.1 (19.9 hrs),362.4 (22.5 hrs)and 329.1 (20.5 hrs)km • duringautumn. During April, May, June, October, and November,we surveyed274.5, 359.7, 337.4, 346.3, and810.5 km• of oceansurface, respectively. SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

Using the STATA program (STATACorp. 1995), we used multiple- regressionanalyses and Sidak multiple-comparisontests (an improved versionof the Bonferronitest; SAS Institute1985) to compareseabird densitiesby zone, season,and speciesgroup. The sampleunit was one 0.5- hr transect.We alsouse log-likelihood ratio (G)tests to examineproportional differences. We log-transformeddensities to satisfyassumptions of normality (skew- ness/kurtosistest for normalityof residuals,P > 0.05). Becauseno birds were seenduring 87 (30%) of the 289 transects,densities included values of zero. As a result,transformations were calculatedas the log (density+ 1). Experimentationwith different modifications[e.g., log (density+ 0.5)] showedno appreciableeffect of choiceof modificationson probabilities(?). All analysesof variancewere of the log-transformeddensity values. Normal- ity wasnot achievedin all analyses,but least-squaresregression (ANOVA) is consideredto be very robustwith respectto non-normality(Seber 1977, Kleinbaumet al. 1988). Althoughregression analyses yield the bestlinear unbiasedestimator relating density to independentvariables, even in the absenceof normallydistributed residuals, ? valuesat the lower levelsof significancemust be regardedwith caution (Seber 1977). Therefore, to reducethe chancesof committinga Type I error, we assumedsignificance for ANOVAs at ? < 0.02. We includedtwo- and three-orderpolynomials in regressionanalysis to test for curvilinearity. Unlessnoted otherwise,species accounts pertain only to transectdata; i.e., they do not includefeeding flocks seen outside the transectzone. All referencesto the POBSP referto King (1970). Seasonsare definedas in the northern hemisphere.

RESULTS

Seasonal Distributions

We recorded32 species,including 15 speciesof breedingresidents, 11 nonbreedingresidents, and sixmigrants (Table 2). Duringspring, densities of breedingresidents were significantlyhigher than those of nonbreeding resident;those of migrantswere significantly lower (Sidak tests, all ? < 0.01, Figure 2). During autumn, densitiesamong the three groups differed insignificantly(Sidak test, all ? > 0.1). Duringspring, densities of breeding residentswere significantlyhigher, those of migrantssignificantly lower than duringautumn (Sidak tests, df = 287, both? < 0.01, Figure2). Betweenthe two seasons,densities of nonbreedingresidents differed insignificantly (Sidaktest, P = 0.3). During spring,densities were significantlyhigher in zones 1 and 4 than duringautumn (Sidak tests, both ? < 0.01, Figure3), but in zones2 and 3 they did not differ significantly(both ? > 0.6). The lack of a significant differencein the latter two zones resultedin a marginallyinsignificant difference(ANOVA, F[1,287] = 4.88, ? = 0.028) in overalldensity between spring(52.5 + 10.20 birdsper 100 km2, n -- 145 transects)and autumn (32.2 + 6.12 birdsper 100 km2 , n = 144), controlledfor zone.Thus, for a given zone, there'was a marginallyinsignificant trend for densitiesto be greaterduring spr• ,cjthan autumn. SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

Table 2 Raw and Corrected Counts and Status of Seabirds Recorded in SoutheasternHawaiian Waters, 1984-1991

Species Raw Correcteda Statusb

Diomedeidae Black-lootedAlbatross, Phoebastria nigripes 3 3.0 BR Procellariidae Newell'sShearwater, Puffinus newelli 27 17.0 BR ChristmasShearwater, P. nativitatis 1 0.4 BR SootyShearwater, P. griseus 164 105.7 MI Buller'sShearwater, P. bulleri 2 2.0 MI Wedge-tailedShearwater, P. pacificus 202 175.3 BR Juan FernandezPetrel, Pterodroma externa 41 40.1 NO White-neckedPetrel, P. cervicalis 18 23.1 NO Dark-rumpedPetrel, P. phaeopygia 29 24.8 BR Tahiti Petrel, P. rostrata 2 1.2 NO KermadecPetrel, P. neglecta 8 5.6 NO Murphy'sPetrel, P. ultima 1 1.0 MI Herald Petrel, P. heraldica 3 2.3 NO MottledPetrel, P. inexpectata 58 31.7 MI Black-wingedPetrel, P. nigripennis 94 81.2 NO Pycroft'sPetrel, P. pycrofti 5 7.6 NO Stejneger'sPetrel, P. Iongirostris 7 5.0 MI Bulwer'sPetrel, Bulweria bulwerii 59 49.2 BR Oceanitidae Leach'sStorm-Petrel, Oceanodroma leucorhoa 35 41.2 NO Harcourt'sStorm-Petrel, Oceanodroma castro 2 2.2 BR Phaethontidae Red-tailedTropicbird, Phaethon rubricauda 4 4.0 BR White-tailedTropicbird, Phaethon leptu rus 8 6.0 BR Sulidae Red-lootedBooby, Sula sula 1 0.4 BR Fregatidae GreatFrigatebird, Fregata minor 3 1.2 BR Laridae SouthPolar Skua, Catharactamaccormicki 1 0.6 NO PomarineJaeger. Stercorarius pomarinus 9 13.5 NO ParasiticJaeger, $tercorarius parasiticus 2 2.5 NO SootyTern, Sterna fuscata 238 231.6 BR ArcticTern, Sternapamdisaea 7 4.3 MI White Tern, Gygisalba 14 19.1 BR BrownNoddy, Anous stolidus 1 1.0 BR BlackNoddy, Anous minutusc BR aSee Methods,Analyses. bBR,breeding resident; NO, nonbreedingresident; MI, migrant. CSeenin feedingflocks only. SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

50-

E •O- z •

Spring Autumn Figure2. Estimateddensities (mean + standarderror) of breedingresidents (BR), nonbreedingresidents (NO), and migrants (MI) with respect to season.Sample sizes duringspring and autumn were 145 and144 transects,respectively.

I_.20

,- 90 34

45 "-' 60 44 45 18 • 30 t t t 40 ;z:

2I 3I 4I 1I 2I 3I Spring Autumn Figure3. Estimateddensities (mean + SE)of seabirdswith respect to distancefrom Hawaii(see Figure 1 for locationsof zones).Numbers accompanying each error bar are samplesizes (number of transects). SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

PredominantSpecies Predominantspecies (corrected counts > 15 birds,Table 2) were, in order of decreasingabundance, the SootyTern, Wedge-tailedand Sootyshearwa- ters, Black-wingedand Bulwer's petrels, Leach's Storm-Petrel, Juan Fernandez,Mottled, White-necked,and Dark-rumpedpetrels, White Tern, and Newell's Shearwater. Duringspring, the SootyTern wasthe mostabundant species in all zones except zone 2, where it ranked second,and the Wedge-tailedShearwater wassecond most abundant in all zonesexcept 2, whereit rankedfirst (Table 3). The Leach'sStorm-Petrel and Bulwer'sPetrel were third and fourth most abundantin zones 1 and 2, and the Black-wingedand Juan Fernandez petrelsranked among the five most abundantspecies in zones3 and 4. During autumn,the Sooty Tern ranked among the two most abundant speciesin zones2 and 4, the Wedge-tailedShearwater in zones1 and 3, and the MottledPetrel in zones1 and 2 (Table3). The Black-wingedPetrel was secondto fourthmost abundant in all four zones,and the Sooty Shearwater was variablyabundant in all four zonesduring each seasonexcept spring, whenits densitywas <1 birdper 100 km2 in zone4.

BreedingResidents: Predominant Species Sooty Tern. A very abundantbreeder throughout the tropicalPacific (Harrison1983) and the mostabundant seabird breeding on the Hawaiian Islands(Harrison 1990). Harrison (1990) estimatedthat a minimum of 975,750 pairs breed on the northwesternHawaiian Islands(i.e., islands west of Kauai or 160 ø W), 70,000 on the main islands.Egg laying from March to July, fledgingto late October. As did the POBSP, we foundthe Sooty Tern the most abundantspecies duringboth spring and autumn(Table 4). Densitieswere significantlyhigher duringspring than in autumn,a differencedue mostlyto high densitiesin June and especiallyin May (Figure.4). The POBSP also observedpeak numbersin May. A significantinteraction between the effectsof distancefrom Hawaii and seasonreflected a significantdecline in densitiesof Sooty Terns with distanceduring spring, comparedto a significantincrease with distance duringautumn (Table 5). This differenceresulted primarily from the very highdensities in zone 1 duringspring, in markedcontrast to autumn,when theseterns where not seenin zone 1 and their densitiespeaked in zone 4 (Figure5). A quadraticeffect of distancein both springand autumn(Table 5) resultedfrom stable,low densitiesin zones2 and 3, comparedto higher densitiesin zones 1 and 4 (spring)or higher densitiesonly in zone 4 (autumn;Figure 5). Densitiesdecreased with increasein water temperature (Table 6). Adultscomposed 90% of the 54 Sooty Terns whose age we recorded duringspring, 86% of 28 birdsin autumn.In springand autumn,66% of 143 birdsand 55% of 89, respectively,were feeding.Of these,95% (137) werefeeding in flockscapturing prey forced to the surfaceby tuna(Th u n n us spp.• hereafter"feeding over tuna");seven were solitary. SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

Table 3 RelativeAbundance of More Common Seabirdsø by Distance From Hawaii

Spring Autumn

Estimated Estimated Rank Species densityb Species density

Zone 1 (0-73 km offshore) 1 SootyTern 32.7 Wedge-tailedShearwater 17.0 2 Wedge-tailedShearwater 22.9 MottledPetrel 6.3 3 Bulwer'sPetrel 18.6 SootyShearwater 6.1 4 Leach'sStorm-Petrel 6.2 Black-wingedPetrel 1.3 5 White-tailedTropicbird 2.2 White-tailedTropicbird 1.2 6 Dark-rumpedPetrel 2.0 7 BlackswingedPetrel 1.6 8 Newell's Shearwater 1.1 9 SootyShearwater 1.0 Zone 2 (74-172 km offshore) 1 Wedge-tailedShearwater 14.4 MottledPetrel 6.4 2 SootyTern 11.2 SootyTern 5.8 3 Leach'sStorm-Petrel 8.1 Black-wingedPetrel 4.4 4 Bulwer'sPetrel 3.5 Wedge-tailedShearwater 3.1 5 SootyShearwater 2.5 SootyShearwater 2.6 6 Black-wingedPetrel 1.9 WhiteTern 1.4 7 Dark-rumpedPetrel 1.0 Zone (173-271 km offshore) 1 SootyTern 7.3 SootyShearwater 16.7 2 Wedge-tailedShearwater 6.2 Wedge-tailedShearwater 5.7 3 Black-wingedPetrel 3.7 Black-wingedPetrel 4.6 4 JuanFernandez Petrel 2.9 SootyTern 2.4 5 SootyShearwater 2.9 6 Newell's Shearwater 2.9 7 Bulwer's Petrel 2.1 8 PomarineJaeger 1.9 9 Leach's Storm-Petrel 1.2 10 ParasiticJaeger 1.0 Zone (272-370 km offshore) 1 SootyTern 13.7 SootyTern 18.5 2 Wedge-tailedShearwater 10.6 Black-wingedPetrel 4.9 3 JuanFernandez Petrel 7.7 Dark-rumpedPetrel 2.4 4 White-necked Petrel 7.4 White Tern 2.2 5 Black-wingedPetrel 4.5 JuanFernandez Petrel 2.0 6 Bulwer'sPetrel 4.5 Wedge-tailedShearwater 1.6 7 NewelrsShearwater 2.2 SootyShearwater 1.0 8 PomarineJaeger 2.1 9 White Tern 2.1 10 Dark-rumpedPetrel 1.5 11 Pycroft'sPetrel 1.5 12 Leach's Storm-Petrel 1.2 aEstimateddensity •_1 birdper 100 km2. bBirdsper 100 kmz, extrapolatedafter correction for shipspeed and direction and species' averageflight speed and direction. SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

Table 4 Estimated Densities with Standard Errors of Pre- dominantSpecies by ResidenceStatus and Season

Densityc

Species Spring Autumn

Breedingresidents SootyTern 13.8 + 4.67 7.7 + 3.93 0.005b Wedge-tailedShearwater 11.7 + 5.06 5.2 + 1.50 0.1 Bulwer'sPetrel 5.6 + 1.39 0.0 0.001b Dark-rompedPetrel 0.8 + 0.43 1.1 + 0.34 0.06 White Tern 0.6 + 0.51 1.0 + 0.68 0.5 Newell'sShearwater 1.7 + 0.58 0.1 + 0.08 0.001b Nonbreedingresidents Black-wingedPetrel 3.0 + 0.96 4.2 + 0.88 0.008• Leach'sStorm-Petrel 4.1 + 1.07 0.5 + 0.23 0.001• JuanFernandez Petrel 2.7 + 0.91 0.8 + 0.33 0.02b White-neckedPetrel 1.9 + 0.96 0.0 0.001b Migrants SootyShearwater 2.0 + 0.52 7.0 + 2.83 0.01b MottledPetrel 0.1 + 0.08 2.7 + 1.05 0.001•

øProbabilitiesfrom a t test comparingthe differencein densities(log-transformed)between seasons. Sample sizes (number of transects)were 145 in spring and 144 in autumn. bDifferencebetween seasons significant at P < 0.02. CBirdsper 100km2.

Wedge-tailedShearwater. Breeds in subtropicalto temperate waters throughoutthe PacificOcean (Harrison 1983, Everettand Pitman 1993). Berger (1972) estimatedthat 98% of the Wedge-tailedShearwaters breeding on the Hawaiian Islandsare of the light phase. Most dark-phasebirds breed in the southernhemisphere (Warham 1990). Harrison (1990) estimated that a minimumof 176,575 pairsbreed on the northwestHawaiian Islands,38,165 pairson the main islands.Egg layingin mid-June,fiedging in November. This was the secondand third most abundantspecies during spring and autumn, respectively(Table 4). There was an insignificanttendency for densitiesto be greaterduring spring than autumn.As foundby the POBSP, abundancepeaked during May (Figure4). Densitiesdecreased significantly with distancefrom Hawaii Island;the effectdiffered little betweenspring and autumn(Table 5, Figure5). There was a quadraticeffect of distance.In spring, this resultedfrom a gradual declinein densityfrom zone 1 to 3, followedby an increasein zone 4. In autumn,density declined abruptly from zone 1 to 2, then stabilizedin zones 2 to 4. Abundanceincreased with decreasein wind speed (Table 5) and increasein water salinity(Table 6). The proportionof dark-phasebirds was 7.8% of 115 in spring,5.0% of 60 birdsin autumn;the differencewas insignificant(G = 0.51, df = 1, P =

10 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

SootyTern Wedge-tailedShearwater 3O 35

2O 21

0 o•r ,•6v nv• u3,Y J• O•T NOV

15 Bulwer'sPetrel 3 Dark-rumpcdPelx½l

0 ocr •ov A• •A¾ • O& •6V

White Tern 4 Newell'sShearwater

•v. M•¾ J• O• N6V A•R •v • och' •q6v

Figure4. Densities(mean + standarderror) by month of predominantspecies of seabirdsbreeding on the Hawaiian Islands.Sample sizes (number of transects)for each month were 45, 52, 48, 45, and 99, respectively.

0.5). In contrast,the POBSP observed17.1% (n -- 6640 birds)dark phase duringspring, a proportionsignificantly greater than ours in spring (G = 8.28, P = 0.005) and a proportionsignificantly greater than theirs in autumn (3.3%, n = 421 birds;G = 75.81, P < 0.001).

11 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

[•1 I • • I I I •. I

12 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

50 35 SootyTern Wedge-tailedShearwater

40 28

3O 21

2O T lO 7

0 2 3 4 0 j. 2 3 4

25 Bulwer's Petrel Dark-rumpedPetrel

•5

0 1 2 3 4 2 3 4

White Tern Newell's Shearwater 1

• 2 3 4 1 2 3 4 Zone

Figure 5. Densities(mean ñ standarderror) by seasonand zone of predominant speciesof seabirdsbreeding on the HawaiianIslands. Light bars, spring; dark bars, autumn.Lines above the barsare standarderrors. See Figure1 for locationof zones, Figure3 for samplesizes.

13 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

Table 6 RegressionAnalyses of Relationshipsbetween Estimated Bird Density • and Oceano- graphicVariables b

SST SSTzc SAL TDPT TDPT• TSLP TSLP•

Breedingresidents SootyTern (_)a0.02 e ns• ns ns ns ns ns Wedge-tailedShearwater ns ns (+) 0.01 ns ns ns ns Bulwer'sPetrel (-) 0.001 ns (-) 0.001 (+)0.002 ns (-) 0.005 ns Dark-rumpedPetrel ns ns ns (-) 0.003 ns ns ns White Tern ns ns ns ns ns gs ns NewelFsShearwater (-) 0.001 ns (-) 0.02 ns ns ns ns Nonbreedingresidents Black-wingedPetrel (+)0.001 ns (+) 0.02 (-) 0.001 (+) 0.01 ns ns Leach'sStorm-Petrel (-) 0.001 ns (-) 0.001 ns ns ns ns JuanFernandez Petrel ns (-) 0.05 ns (-)0.001 (+)0.001 (-)0.003 ns White-neckedPetrel ns ns ns (-) 0.001 (+) 0.001 (-) 0.001 (+) 0.002 Migrants SootyShearwater ns ns ns ns ns (-) 0.01 ns MottledPetrel ns ns (+) 0.01 ns ns ns ns

øTransformedas1og(birds/100km2). Thesample size for each species was 289 transects. bSST,sea-surface temperature: SAL, sea-surface salinity; TDPT, thermocline depth; TSLP, thermocline slope. All independent termsanalyzed as continuous variables. CSquaredterms denote quadratic effects (no quadratic effects for salinity). aSignsgiven in parenthesesdenote signs of theregression coefficient, i.e.. a positiveor negativeslope. A positiveslope indicatesan increase in densitywith increase invalue of thevariable. eValuesfollowing coefficients are P values:significance accepted at P _<0.02. fns,not significant.

Duringspring and autumn,83% of 95 birdsand 23% of 14, respectively, were feeding.Of the 109 feedingbirds, 97% were in flocksfeeding over tuna, 3% were scavengingsquid. Bulwer'sPetrel. Breedsthroughout the tropicalPacific (Harrison 1983). Harrison (1990) estimatedthat a minimum of 76,555 pairs breed on the northwesternHawaiian Islands,335 pairs on the main islands.Egg laying from late May to early June, fiedgingin September. Like the POBSP,we observedBulwer's Petrel only duringspring, when it was the third most abundantspecies (Table 4). We observedpeak densities in June (Figure4), the POBSP in May. Densities of Bulwer's Petrels decreased with increase in distance from Hawaii (Table5, Figure5). A quadraticeffect of distanceresulted from an abruptdecline in densityfrom zone 1 to 2, then stabilizationin zones2 to 4. Densitiesincreased with decreasein water temperature,salinity, and thermoclineslope and with increasein thermoclinedepth (Table6). Of the 49 birds,11 were feeding.Eight were solitaryand three were in a feedingflock. Dark-rumped Petrel. An endemic taxon, with an estimated3750 to 4500 pairsbreeding at highelevations on the main HawaiianIslands (Spear

14 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS et al. 1995). Egg layingin early May, fledgingin Octoberand November (Simons1985, Ainley et al. 1995). This was the sixth most abundant speciesduring autumn (Table 4); betweenthe two seasonsdensities differed insignificantly. We observed highestdensities in April (Figure4); the POBSPcounted highest numbers in May. In spring,densities were significantly higher in zone 1 thanin zones2 and 3. In autumn,densities were significantlyhigher in zone4 than in zones1, 2, or 3 (Sidaktests, P < 0.02, Figure5). Densitydecreased with increasein thermoclinedepth (Table6). Sevenof the 25 petrelswere feeding.Five were in flocksfeeding over tuna;two were feedingover tuna but were not in a flock. White Tern. Relativelysmall populations breed on islandsthroughout the tropicalPacific (Harrison 1983). Harrison(1990) estimatedthat 7445 pairs breed on the northwesternHawaiian Islands,50 pairs on Oahu. On Oahu the breedingseason is protractedand variesmuch from year to year (Berger 1972, Miles 1986). This was the seventhmost abundantspecies during autumn (Table 4). Densitiesdiffered insignificantly by seasonand month (Table4; Sidaktests for monthlycomparisons, all P > 0.02, Figure4). The POBSP likewise recordedsimilar numbers throughout the year. These terns were seen only in zones2 and 4 (Figure5). There were no relationshipsbetween densities and distancefrom Hawaii or oceanographic variables(Tables 4 and5). Of the 19 ternsrecorded, 52% werefeeding. Nine were feedingin flocksand one wassolitary. Newell's Shearwater.An estimated18,000 to 19,000 pairsbreed on the main Hawaiian Islands(Spear et al. 1995), to whichthe bird is endemic.Egg layingin late May or earlyJune (Harrison1990), fledgingin Octoberand November(Berger 1972). This was the ninth most abundant speciesduring spring (Table 4). Densitieswere significantlyhigher in springthan in autumn.As did the POBSP, we recordedthe majorityin April and May (Figure4). Densitiesincreased with distancefrom Hawaii and wind speed(Table 5, Figure5; seealso Spear et al. 1995). Densitiesalso increased with decrease in water temperatureand salinity(Table 6). Four of the 17 birds(24%) were feeding,all in flocksover tuna.

NonbreedingResidents: Predominant Species Black-wingedPetrel. Breedsin abundancein the temperateSouth Pacific on islandsoff New Zealandand Australia(Falla et al. 1967). Egg layingin Decemberand January,fledging in late April. This was the fifth and fourth most abundantspecies during spring and autumn,respectively (Table 4); densitieswere significantly higher in autumn. We observedit duringeach of the five monthsexcept April; densitieswere highestin June(Figure 6). The POBSPobserved highest numbers from May to November,with a peak in October. Densitiesincreased significantly with increasein distancefrom Hawaii at bothseasons (Table 5, Figure7); densitiesdiffered insignificantly by season

15 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

12 Black-wingedPcU'cl Lcach'sStorm-Pea'el

o-t APR M•Y • OC• NOV A•R M•Y /• OC'T N•)V

JuanFernandez Petrel Whitc-ncckcd Pctrel

t 0 APR M•Y •N O(•T NbV APR MAY •dN OCT NOV

12 SootyShearwater 12 Mo•lcd Pc•el

0 •' A•R M•Y • O/•'r N6V A•R MAY • O•T NOV

Figure6. Densities(mean + standarderror) by monthof predominantspecies of seabirdsnot breeding in theHawaiian Islands. Sample sizes (number of transects)for each month were 45, 52, 48, 45, and 99, respectively.

16 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

Black-wingedPetrel Leach's Storm-Petrel

i 2 3 4 2 3

1o Juan Fernandez Petrel White-necked Petrel

o I 2 3 4 I 2 3

25 Sooty Shearwater 1o Mottled Petrel

20 @

15 6

5 2

o i 2 3 4 0 I 2 3 Zone

Figure7. Densities(mean + standarderror) by seasonand zoneof predominant speciesof seabirdsnot breeding on the Hawaiian Islands. Light bars, spring; dark bars, autumn.Lines above the barsare standarderrors. See Figure1 for locationof zones, Figure3 for samplesizes.

17 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS in each of the four zones (Sidak tests, all P > 0.02). Densities increased significantlywith increase in water temperature and salinity and with decreasein thermoclinedepth (Table6). The effect of thermoclinedepth wasquadratic because of stabilizationof densityat depths>50 m (Figure8). Duringspring and autumn, 21% of 33 birdsand 10% of 49, respectively, were foraging.Of the 12 foragingpetrels, five were feedingin flocksover tuna; sevenfed alone on the water. Leach'sStorm-Petrel. A veryabundant breeder around the North Pacific (Crossin1974, Ainley1980); wintersthroughout the easterntropical Pacific (Pitman 1986). This was the fourth mostabundant species during spring, when the only speciesin higher densitieswere those breedingon the main Hawaiian Islands(Table 4). Densitiesof thesestorm-petrels were significantly higher in springthan autumn.We observedthem each month (Figure6), although densitieswere highestduring April. The POBSP also observedhighest numbersin April. A significantinteraction between the effectsof distancefrom Hawaiiand seasonon densitywas due to a significantdecrease in densitywith increase in distanceduring spring, compared to an insignificanteffect of distanceduring autumn(Table 5, Figure7). Duringspring, most of thesepetrels occurred in zones1 and 2. A cubiceffect of distancein spring(P < 0.02) resultedfrom stabilizationof densityin zones1 and2, followedby an abruptdecline in zone 3 and stabilizationin 4. Densitiesincreased with increasein wind speed(Table 5) and decreasein water temperatureand salinity(Table 6). Of the 41 birdsrecorded, 31% were feeding,all alone. In spring,flight directionwas mostlynorthwest to north (75%). Juan Fernandez Petrel. An estimated 1 million pairs breed on Mas AfueraIsland, Chile; egg layingin late Decemberand earlyJanuary (Brooke 1987). Fledgingis probablyin May, if the chicksdevelop like thoseof the closelyrelated Dark-rumped Petrel (Harris 1970). This was the sixth most abundant speciesduring spring (Table 4). Densitieswere higher in spring than in autumn becauseof very high densitiesin June (Figure6; it was not abundantearlier in the spring).In contrast,the POBSPobserved highest numbers in Octoberand about half as many in June. A significantinteraction between the effectsof distancefrom Hawaiiand seasonon densitywas due to a significantincrease in densitywith increase in distancein spring;during autumn the effectwas insignificant(Table 5, Figure 7). In spring,densities were significantlyhigher in zone 4 than in other zones(Sidak tests, all P < 0.02, Figure7). Thesepetrels were not seen in zone 2 in springor in zone 1 at either season. Densitiesincreased significantly with decreasein thermoclinedepth and slope(Table 6). A quadraticeffect of watertemperature was due to higher densitiesat temperaturesof 26 øto 27øC andlower densities at temperatures of 24 øand 28øC (Figure8). A quadraticeffect of thermoclinedepth reflected a drop in densityat depths greater than 50 m, followedby density stabilizationat greaterthermocline depths. Of the 40 birdsrecorded, 43% wereforaging. Of the 17 feedingbirds, 15 (88%) were in flocksfeeding over tuna, and two were scavengingsquid.

18 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

1.2 q Black-wingedPetrel Juan Fernandez Petrel

0.4- 0.9-

0.6- 0.2- 0.3- • 0.1-/ 0.0- 0.0- 5'0 7'5 1•0 1•5 J5 2'6 2'7 Thermoclinedepth (m) Sea-surt•eetemperature (øC)

Juan Fernandez Petrel 0.8- 0.8- White-necked Petrel

0.6- 0.6'

0.4- 0.4.

0.2- 0.2-

0.0- 5'0 7'5 1•0 1•5 5'0 7• 160 Thermoclinedepth (m) Thermoclinedepth (m)

White-necked Petrel 0.6 •

0.4-

Thermoclineslope (A in øC) Figure8. Resultsof multiple-regressionanalyses for densi• of seabirds(log-transformed)with oceanographic variables having a nonlineareffect. Shown are the means ofseabird density (log-transformed) + one standard error (vertical lines). Samples sizes for sea-surfacetemperature, from left to right, were 62, 89, 103, and 35, for thermoclinedepth 51, 109, 54. 75, for thermoclineslope 40, 74, 114, 61.

19 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

White-necked Petrel. An estimated50,000 pairs breed on Macauley Island,north of New Zealand(Tennyson et al. 1989), on a schedulesimilar to that of the Juan FernandezPetrel (Fallaet al. 1967, Brooke 1987). A majorwintering area of thesepetrels is in the transitionzone of the western North Pacific(Tanaka and Inaba 1977, Pyle and Eilerts1986). All sightingsoccurred during spring (only in June), when this was the eighthmost abundant species (Table 4, Figure6). We sawthem only during La Nifia in 1988 and in 1989 (13.1 and 0.4 birdsper 100 km2, respectively).We do not compareour resultsfor this specieswith those of the POBSP because the latter sometimes combined counts of White-necked and JuanFernandez petrels (King 1970; seeSpear et al. 1992b for identification problems). Densitiesincreased with increasein distancefrom Hawaii (Table5, Figure 7); most birdswere seen in zone 4. This species'densities increased with decreasein thermoclinedepth and slope (Table6). A quadraticeffect of thermoclinedepth was due to a markeddrop in densityat depths>50 m, followedby densitystabilization (Figure 8). Similarly,a quadraticeffect of thermoclineslope reflecteda gradual drop to very a low density with increasein slopeto 3 ø, followedby levelingof densityat slopesof 4 ø(Figure 8). One of the 23 birdsrecorded was in a flockfeeding over tuna.

Migrants:Predominant Species Sooty Shearwater. Millionsbreed on islandsoff southernNew Zealand and Chile; many winter in the North Pacific(Everett and Pitman 1993, Warhamand Wilson1982) and Peru Current(Murphy 1936). Eggsare laid from mid-Novemberto early December;fledging is in late May and early June. Thiswas the seventhand secondmost abundant species during spring and autumn,respectively (Table 4); densitieswere significantlyhigher in autumn. We saw it in each month except June, with peak densitiesin November (Figure6). The POBSP had highestcounts in April and October. The seasonal difference in abundance reflected densities in zones 1 and 3 beinghigher in springthan in autumn(Sidak tests, both P < 0.002, Figure7). In both seasons,density differed insignificantly with distancefrom Hawaii, althoughdensities in zone 3 were very high in autumn(Table 5, Figure7). Densitydecreased with increasein wind speedand thermoclineslope (Tables 4 and 5). Duringspring and autumn,one of 85 birdsand four of 21, respectively, were foraging.Of these, four were in flocksfeeding over tuna and one appearedto be scavenging.Flight direction was mostlynorthwest in spring, and southwest in autumn. Mottled Petrel. Breedson islandssouth of New Zealand.Egg layingin Decemberand January,fiedging in May (Warhamet al. 1977). Wintersin subpolarwaters of the North and South Pacific(reviewed in Bartle et al. 1993). This was the fifth mostabundant species during autumn, when densities were significantlyhigher than in spring(Table 4). We observed55 in October and three in April (Figure6). This chronologyis similarto that seenby the POBSP, which logged the speciesin October (50 birds),November (2),

2O SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

December(2), April (10), and May (1). Our greatestnumbers were on 16 October,when we estimated15.9 birdsper 100 km2;peak counts by the POBSP were on 18 October. Densitiesdecreased with increasein distancefrom Hawaii in spring,but there was little effect of distancein autumn (Table 5, Figure 7). Nearly all birdswere seenin zones 1 and 2, southeastof Hawaii Island,where densities were significantlyhigher than in zones3 and 4. Densitiesdecreased with increasein wind speed(Table 5) and water salinity(Table 6). We sawnone feeding.Flight direction was southwest to southeastin autumnand northin spring.

BreedingResidents: Non-Predominant Species Our countsof the followingspecies were too lowfor distributionalanalysis (see Table 2). Numbersin parenthesesare Harrison's (1990) estimated minimaof pairsbreeding on the northwesternand main Hawaiian Islands, respectively:Black-looted Albatross (36,260; 0); ChristmasShearwater (2245; 40), Harcourt'sStorm-Petrel (<100 on Kauai; unknownnumber on Hawaii),White-tailed Tropicbird (0; 890), Red-tailedTropicbird (8760; 92), Red-lootedBooby (4540; 1100), GreatFrigatebird (8115; 0), BrownNoddy (76,700; 16,005), and BlackNoddy (6565; 615). The lastwe saw only in feedingflocks near Hawaii (seeFeeding Flocks). We identifiedtwo Harcourt's Storm-Petrels, one each at 18 ø 28 vN, 155 ø 47vW, 50 km off Hawaiion 21 April 1990, the otherat 18 ø25 •N, 155 ø27 • W, 65 km off Hawaii, on 18 April 1991. We distinguishedthe speciesfrom Leach'sby its more square(less forked) tail, darkercolor, narrower band- shapedsharply demarcated white rump-patch.The differencefrom the V- shapedrump patch of the Leach'sis best seen through a 20x binocular whenthe birdis flyingdirectly away. In addition,Harcourt's usually fly with wingsangled back towards the tail more so than do Leach's. NonbreedingResidents: Non-Predominant Species Kermadec Petrel. Breedsin the South Pacificwith a prolongedor year- roundbreeding season (Murphy and Pennoyer1952). We observedit in June (4 birds)and November (4; for identificationcriteria .see Herald and Murphy'spetrels). The POBSP recorded76 KermadecPetrels; numbers were greatestfrom Juneto January.The POBSP recordedthe lightmorph more often than the dark. In June, we saw three dark morphs and one intermediate;in Novemberall were light. KermadecPetrels were seen in zones2 (2 birds)and 3 (6 birds).Three of the eightbirds were in a feeding flockforaging over small tuna. Herald Petrel. Breedsin the SouthPacific with a prolongedor year-round breedingseason (Murphy and Pennoyer1952, Pyle et al. 1990). On the basisof molecularevidence and assortativemating between the light and dark morphs, Brooke and Rowe (1996) split them, recognizingthe light- bellledmorph as the Herald Petrel(P. heraldica), the dark-belliedbirds as the Henderson Petrel (P. atrata), both distinctfrom the polymorphicform breedingon SouthTrinidad in the AtlanticOcean and RoundIsland in the Indian Ocean (P. arrainjoniana).

21 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

We saw three Herald Petrelsat 16 ø40' N, 154 ø 08' W, at 16 ø 53 •N, 154 ø 19 • W, and at 15 ø 56' N, 154 ø 28•W on 12 November 1990 (first two petrels)and 15 November 1991. The birdswere 311,281, and 364 km, respectively,from Hawaii. We distinguishedthem from the light-morph KermadecPetrel by the lack of white primaryshafts on the dorsalsurface andthe longer,wedge shaped tail, differingfrom the short,square tail of the Kermadec.These birds had indistinct M-patterns on the uppersurface of the wingsand back,a featureabsent in the KermadecPetrel. The HeraldPetrels alsohad a slimmerappearance than the Kermadec.Relative to body size, they had longer,more slenderwings (see Spear and Ainley 1998) and a less robustbody. The POBSP may have seen it but confusedit with other similarspecies, and color morph was not reported. Gould (1971) statedthat this specieswas an "uncommon,winter, nonbreeding visitor," but did not giveidentification criteriaor reportcolor. Finally, Amerson (1971) reporteda specimenin the U.S. NationalMuseum (USNM 543342) collectedby R. B. Clapp, 14 March 1968, as it flew over Tern Islandin the northwesternHawaiian Islands(this is the only recordlisted by the AOU 1983), but did not report color. To furtherconfound the problem,the USNM specimenwas, as we write this, unavailablefor examinationbecause of museumremodeling. Tahiti Petrel. Breedson islandsin the tropicalSouth Pacific(Harrison 1983). The POBSP observed12 birds identified as Tahiti or Phoenix (P. alba) petrels, and Gould (1971) reported one Tahiti Petrel but gave no identificationcriteria. The AOU (1983; Appendix A) liststhe Tahiti under "sightrecords" based on informationfrom W. B. King (no referencegiven) for a bird reportedfrom Hawaiianwaters in 1964, but identificationcriteria are not available. The Tahiti and Phoenix petrels can be distinguishedby bill size and pronounceddifferences in the wing profile of flying birds (Spear et al. 1992b). Many Tahiti Petrelsalso have a light rump contrastingwith the darker back and tail, a featureabsent in the Phoenix. We recordedtwo sightingsof the Tahiti Petrel on 5 November1984, at 18 ø 25' N, 159 ø 22' W and 18 ø 37' N, 159 ø 16 • W, 244 and 233 km from Hawaii, respectively.The first petrel was headednorth, the secondsouth. The intervalbetween the two sightingswas 1 hr, so thereis a reasonable chancethat these sightingswere of the same bird. Pycroft'sPetrel. Breedsin relativelylow numberson islandsoff northern New Zealand (Dunned 1985). Egg laying in November and December, fiedgingin May. This petrelhad not beenrecorded away from the breedinggrounds before our study(Spear et. al. 1992b, Howellet al. 1996). We collecteda specimen on 2 May 1987 (Los AngelesCounty Museum[LACM] 103973) on the equator,125 ø W, followedby four more between1988 and 1991 (LACM 104342, USNM 597200, 597201, and 597202). A bird collectedat 9 ø N, 140 ø 00 • W, was about 750 km from the studyarea. We collectedno seabirdsin Hawaiian waters. The POBSP recordedno Pycroft'sPetrels despiteextensive collecting in the centralPacific, but Gouldand Piatt (1993) reportedsightings (without identification criteria) from north of the Hawaiian

22 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

Islandsin the transitionzone (32 ø to 43 ø N) of the central North Pacific. This petrel is regularin the easterntropical Pacific (Spear et al. 1992b, Spear and Ainley unpubl.data). A possiblereason for the lack of prior recordsis confusionof this specieswith Cook's(P. cook/)and Stejneger's petrels,which alsooccur in the easterntropical Pacific (Spear et al. 1992b). Cook'sPetrels occur between 125 øand 150 øW in autumnduring migration to their New Zealandbreeding grounds. We have not recordedthem there duringspring. At sea,the moreextensively gray hind neck of Pycroft'sPetrel separatesit from Cook's (Howell et al. 1996); i.e., Cook's Petrelsshow appreciablymore white in the "face."Pycroft's is smallerwith shorterwings and longertail, relativeto bodysize, than Cook's(Spear et al. 1992b). The lightergray crown and nape of Pycroft'sPetrel distinguish it fromStejneger's Petrel, in which these featuresare sootygray (see Spear et al. 1992b, Howellet al. 1996). Nevertheless,the lastdifference may not be apparentin poor light.This, in combinationwith the nearlyidentical body size and shape of Stejneger'sand Pycroft's,make thesetwo speciesthe more likelyof the trio to be confusedin suchconditions (Spear and S. N. G. Howellpers. obs.). We sightedfive Pycroft'sPetrels, all in Juneand only duringLa Nifia of 1988 and in 1989. On 19 June 1988, we sawthree Pycroft's Petrels at 16 ø 45'N, 153 ø 22•W, at 16 ø 50'N, 153 ø 27•W, andat 16 ø 56'N, 153 ø 34'W, about 330 to 350 km from Hawaii. On 27 June 1989, we saw two at 15 ø 42 ' N, 154 ø 17'W and 17 ø 17' N, 154 ø 43' W, 361 and 230 km from Hawaii. One of the Pycroft'sPetrels was feedingin a flockover smalltuna. In June 1986, we observedtwo Pycroft's/Cook'sPetrels that, in retro- spect, were probablyPycroft's. Pyle and Eilerts(1986) reported Cook's Petrelsaround the northwesternHawaiian Islandsthat alsomay have been Pycroft's. Pomafine Jaeger. These jaegersbreed in the Arctic (Furness1987) but are present in the eastern tropical Pacific throughoutthe year, most abundantlyin winter (Spearand Ainley 1993). Eightof nine birdswere seenin spring,the other in autumn.We observed them in April (3 birds),May (5), and October(1). Springbirds comprised six adults(five light-phase, one dark)and two light-phasesubadults; the autumn bird was a light-phasesubadult. These birdswere seenin zones1 (2 birds), 2 (2), 3 (4), and 4 (1). Two were in a flockfeeding over tuna. In spring,flight directionwas northwestto northeast.The POBSP noteda similarpattern, and that mostbirds wintering in Hawaiianwaters were within50 km of land. ParasiticJaeger. Breeds in the Arctic(Furness 1987). Winteringchronol- ogy in the easterntropical Pacific is similarto that of the PomarineJaeger (Spearand Ainley 1993, unpubl.data). This specieshad not been recordedin Hawaiianwaters. We recordedtwo, a dark-phasesubadult (second or third year)on 27 June 1989, at 16 ø 361Xl, 154 ø 32' W, and a light-phaseadult on 12 November1991, at 16 ø 46 • N, 154 ø 13'W (latterrecord in Pyle 1992). Thesewere distinguishedfrom the Long-tailedJaeger (S. Iongicaudus)by the distinctiveshape of the central rectrices.We sawthree light-phase first-year jaegers on 2 May 1989, at 17ø 40' N, 156 ø 35'W, that were eitherParasitic or Long-tailedjaegers. All were in flocksfeeding over tuna.

23 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

South Polar Skua. Breedsin Antarcticafrom Decemberto April (Ainley et al. 1990) and wintersin low numbersthroughout the easterntropical Pacific(Spear and Ainley 1993, unpubl.data). The POBSP recordedeight skuas,listing them as the Great (Catharacta skua).Gould (1983) reporteda SouthPolar Skua in Hawaiianwaters at 24 ø 18' N, 158 øW, in November1976 but gaveno identificationcriteria. Thus the South Polar Skua was unconfirmed in Hawaiian waters before we sightedand photographeda flyingbird on 7 October 1987 at 18 ø 05' N, 155 ø 01'W (seePyle 1988). The identificationwas basedon the extensive goldenhackles on the nape, a featureabsent in the GreatSkua and Chilean Skua (C. chilensis;Harrison 1983, Furness1987, Ainley and Spear pers. obs).We suspectthat the skuasseen by the POBSPwere SouthPolar Skuas. All skuasthat we haveidentified in the easterntropical Pacific have been of this species.

Migrants:Non-Predominant Species Buller's Shearwater. Breeds in New Zealand and winters in the North Pacific (Falla et al. 1967, Wahl 1985). Egg laying in November and December,fledging in April. We observedfour on 13 November 1989 at 17 ø 54' N, 154 ø 53' W, two sittingon the water and two in a feedingflock. Five earlierrecords from Hawaiianwaters include three sightingsby the POBSP in March and April 1964-1965 and two on 3 November 1984 (Pyleand Eilerts1986). Murphy'sPetrel. Breedsduring summer in the subtropicalSouth Pacific (Harrison1983) andmigrates to the NorthPacific (Bartle et al. 1993). Four specimenshave been collectedin Hawaiian waters:Kure Atoll (7 October 1963), French Frigate Shoals(9 September1966), Kauai (25 November 1986), and at sea 13 km off Oahu (29 October 1966; Gould and King 1967, Clapp 1974, R. L. Pyle; SIGHTINGS Data Base, BishopMuseum, Honolulu).On 27 June 1989 we saw another flying southat 16 ø 09' N, 154 ø24 •W. The birdlacked white patches on the undersideof the forewings and whiteprimary shafts, distinguishing it from Solander's(P. solandri) and Kermadecpetrels, and was sootygray with an indistinctM-pattern on the dorsalwings and back, distinguishing it from the uniformdarker (nearly black like the ChristmasShearwater) Henderson Petrel (see Herald Petrel). Stejneger'sPetrel. Breedson Mas Afuera Island,Chile (Brooke 1987) and wintersin the northwesternPacific (Tanaka et al. 1985). Egg layingin Decemberand January(Brooke 1987), fledgingin April and May. This specieswas recorded on Lanaiin 1914, v•hena partiallyeaten bird was found (Clapp 1984). Two possiblesighting were made in Hawaiian watersin the 1990s (R. L. Pyle, SIGHTINGS Data Base, BishopMuseum, Honolulu).We sawseven: 5 November 1984 (1 bird), 16 October 1985 (3), 30 October 1985 (2), and 18 June 1986 (1), three in zone 2 and two each in zones 3 and 4. The bird seen in June was flying northwest;those in autumnwere flying south to southeast.Identification was based on criteriain Spearet al. (1992b); seealso Pycroft's Petrel. Arctic Tern. Breeds in the Arctic and winters in the Antarctic. We observedseven, two on 21 April 1990 and five on 25 April 1991, dates

24 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS consistentwith observationsby the POBSP. None were feeding. Flight direction was northward.

FeedingFlocks We recorded16 flocksof seabirdsforaging over tuna (Figure1); the fish were from 0.3 to about0.6 m in length.Thirteen (81%) of the flockswere seenduring spring. Numerically, they were dominatedby SootyTerns (62% of all individualsrecorded) and Wedge-tailedShearwaters (20%• Table 7), i.e., findingssimilar to thoseof the POBSP. The speciescomposition of flockswas 92.5% breedingresidents, 6.0% nonbreedingresidents, and 1.4% migrants. The ratio of the numberof feedingbirds to the distancethe shiptraveled (-- surveyeffort for flocks)for each zone indicateda higherincidence of feedingin zone 4 than in 1, 2, or 3 (G tests,all P < 0.001, Table 7) and in zones 1 and 2 than in 3 (both P < 0.010). The ratio betweenzones 1 and 2 did not differ(P = 0.4). Thesedifferences resulted mainly from variationin numbersof Sooty Terns. The mean numberof speciesper flock differedinsignificantly among zones(ANOVA, P -- 0.9, Table 7), as did the ratio of numberof speciesto surveyeffort (G = 3.36, df -- 3, P -- 0.3). However, of the 17 species recorded,the numberrecorded in zone 3 (15 species)was over twice that ir• zones 1 and 4 (7), and 1.7 times higher than in zone 2 (9 species).The differenceswere mainlydue to the presenceof four speciesof nonbreeding residentPterodroma seen feedingonly in zone 3. Low numbersof flocks precludedetailed comparisons.

DISCUSSION

Althoughour surveyeffort was not extensive(surveys covered 2128.4 km2 of oceanarea on 23 daysover eight years), we showthat many findings of the POBSP have changedlittle between1964-1965 and 1984-1991. We reportthe firstquantitative estimates for thesewaters of birdabundance basedon a rigoroussurvey protocol and documentthe occurrenceof several speciesunreported or unconfirmedfrom Hawaiian waters.

SpeciesStatus Speciespredominant in southeasternHawaiian watersand breedingon the Hawaiian Islands,were, in decreasingorder of abundance,the Sooty Tern, Wedge-tailedShearwater, Bulwer's Petrel, Dark-rumped Petrel, White Tern, and Newell'sShearwater. Predominant nonbreeding residents were, in decreasingnumerical importance, the Black-wingedPetrel, Leach's Storm-Petrel, Juan Fernandez Petrel, and White-necked Petrel. Predomi- nant migrantscomprised the Sooty Shearwaterand MottledPetrel. Our recordsof the Tahiti,Herald, Pycroft's, and Stejneger's petrels, South Polar Skua, and ParasiticJaeger, species we classifyas nonbreedingresidents,are the firstrecords for Hawaiianwaters, although a deadStejneger's Petrelwas foundon Lanai in 1914 (Clapp 1984) and confirmationof the HeraldPetrel collected on FrenchFrigate Shoals in 1968 (USNM 543342;

25 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

Talkie? Numberof Individuals,Number of ParticipantSpecies, and Species Composition of 16 FeedingFlocks, 1984-1991 ø

Distancefrom Hawaii

Zone 1 Zone 2 Zone 3 Zone4 All zones (n -- 2) (n -- 4) (n = 7) (n = 3) (n = 16) Individualsb 38.0+ 7.1 38.3+_ 16.3 14.4+ 7.7 101.0+ 118.9 39.0+ 50.5 Speciesper flock b 4.5+ 2.1 4.0+ 2.6 4.6+ 1.1 4.3+ 2.5 4.4_+ 1.8

Speciescomposition c Breedingresidents Dark-rumpedPetrel 2 2.6 0 -- 1 1.0 7 Z3 10 1.6 Bulwer'sPetrel 0 -- 2 1.3 1 1.0 0 -- 3 0.5 Wedge-tailedShearwater 20 26.3 38 25.5 35 35.0 33 10.9 126 20.1 Newell'sShearwater 0 -- 19 12.8 1 1.0 0 -- 20 3.2 GreatFrigatebird 2 2.6 1 0.7 1 1.0 0 -- 4 0.6 SootyTern 45 59.2 80 53.7 28 28.0 236 78.1 389 61.9 White Tern 3 3.9 1 0.7 8 8.0 15 5.0 27 4.3 BlackNoddy 3 3.9 0 -- 0 -- 0 -- 3 0.5 Nonbreedingresidents JuanFernandez Petrel 0 -- 0 -- 8 8.0 8 2.6 16 2.5 White-neckedPetrel 0 -- 0 -- 4 4.0 0 -- 4 0.6 KermadecPetrel 0 -- 0 -- 4 4.0 0 -- 4 0.6 Black-wingedPetrel 0 -- 0 -- 1 1.0 0 -- 1 0.2 Pycroft'sPetrel 0 -- 0 -- 1 1.0 0 -- 1 0.2 PomarineJaeger 1 1.3 2 1.3 1 1.0 1 0.3 5 0.8 ParasiticJaeger 0 -- 0 -- 0 -- 2 0.7 2 0.5 jaegerspp. 0 -- 0 -- 3 3.0 0 -- 3 0.6 Migrants Buller'sShearwater 0 -- 4 2.7 0 -- 0 -- 4 0.6 SootyShearwater 0 -- 2 1.3 3 3.0 0 -- 5 0.8 Total 76 99.8 149 100.0 100 100.0 302 99.9 628 100.1

øZonesgiven with respect to distancefrom Hawaii (see Figure 1). Values of n underzones denote number of feedingflocks. bMeanplus or minus standard deviation. CNumberofindividuals. followed by percentage oftotal number of all birds recorded in flocks.

Amerson1971) awaitsre-opening of the specimencollection at the museum.The six specieswere uncommon(Herald, Pycroft's, and Stejneger's petrelsand ParasiticJaeger) or rare (TahitiPetrel and South Polar Skua) duringour study. Particularlynoteworthy was the Pycroft'sPetrel, which is not listedamong speciesoccurring in North America (AOU 1983), and the Black-winged Petrel, which was the fourth and fifth most abundant species (and most abundantPterodroma) during spring and autumn, respectively--seeKing (1970) for similarfindings--but which is consideredby the AOU (1983) as "accidental" in Hawaiian waters. Duringspring, species breeding on the HawaiianIslands were significantly more abundantin southeasternHawaiian waters than nonbreedingresidents or migrants,and migrantswere significantlyless abundantthan

26 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS nonbreedingresidents. In autumn,however, abundance differed little among the three groupsbecause of decreasein densitiesof breedingspecies, stabilizationin densitiesof nonbreedingresidents, and increasein migrants. The decreasein breederswas most marked in the Bulwer'sPetrel, which was the third most abundantspecies during spring but not recordedin autumn [seealso King (1970) for similarresults]. The marked increasein migrantsduring autumn was due to southward movement through Hawaiian waters of Sooty Shearwatersand Mottled Petrels.In spring,these birds migrate to the North Pacificfrom breeding sitesnear New Zealand,returning in autumn (Warham 1996). The lower number in the study area during spring was likely becausethese birds' migration routes follow the predominantwind systems(see Spear and Ainley, in press),which move clockwise in the North Pacific.EvidentYy most SootyShearwaters and MottledPetrels migrate north alongthe westernside of the Pacific(west of Hawaii)in spring,move from west to eastto the northeasternPacific in summer,and follow the southwesterlytrade winds throughthe studyarea in autumn. The concentration of Mottled Petrels in zones 1 and 2 off the eastern side of Hawaii Island was the highestthat we encounteredduring extensive surveysin the easterntropical Pacific. We suspectthat the HawaiianIslands disruptthe migrationfront movingfrom the north or northeasttoward the southwest,deflecting birds southwardaround the islands.The combined data from our studyand the POBSP for the Mottled Petrel suggesta very synchronousmigration across Hawaiian waters during early to mid-October. The consistentflight direction of the SootyShearwater, Stejneger's Petrel, Arctic Tern, Leach's Storm-Petrel,and Pomarine Jaeger (the latter two classifiedas nonbreedingresidents because of the presenceof birdsnear the Hawaiian Islandsin summer; King 1970) also indicate direct migration through Hawaiian waters, as do our observationsof Buller'sShearwaters elsewherein the easterntropical Pacific (Spear and Ainley,unpubl. data). SeabirdAbundance in Relationto OceanographicFactors Lowersea-surface temperature, higher salinity, and a shallow,less strati- fied thermoclineare evidenceof mixingin the water columndue to fronts (upwellingand/or divergences/convergences;reviewed in Owen 1981, Fiedleret al. 1991). Theseconditions increase primary productivity and the food of higher-orderpredators. It was not surprising,therefore, that densitiesof eachpredominant species except the WhiteTern were correlated with one or more of thesevariables and that mostcorrelations indicated prefer- ence for more productivesurface waters. The Wedge-tailedand Newell's shearwaters,BulweFs and Black-wingedpetrels, Leach's Storm-Petrel, and Sooty Tern preferred low temperaturesor high salinity,and the Dark- rumped, Black-winged,Juan Fernandez,White-necked, and Bulwer's petrels preferredshallow and/or lessstratified thermoclines. Consistent with thesefindings, we sawthe White-neckedand Pycroft'spetrels in Hawaiian watersonly in 1989 andespecially 1988. Oceanographically,La Nifia 1988 was a year of unusualconditions in the studyarea, with the coolestsurface water, shallowestthermocline, and most mixed thermal structureencoun- teredduring the 8-year study(Ainley and Spear,unpubl. data).

27 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

The high densitiesof seabirdsduring spring in the southernmostregion (zone4) were due mostlyto the abundanceof "tuna-birds,"the SootyTern, Wedge-tailedShearwater, and Juan Fernandezand White-neckedpetrels (seeKing 1970 for similarresults). High densitiesof thesespecies in these waters in springwere consistentwith the significantlyhigher numberof feedingflocks foraging on prey forcedto the surfaceby tunas.This finding wasprobably related to the proximityof thesewaters to the enrichedsurface layer along the northernboundary of the east-flowingEquatorial Counter- current(Wyrtki 1966, Fiedleret al. 1991, Ballanceet al. 1997). DuringMay and June this currentreaches its northernapex, with divergenceat about 10 ø N, where surfacewaters escape and move north into the west-flowing North EquatorialCurrent (Wyrtki 1966). Consistentwith thisidea, Murphy and Shomura(1972) foundthat the EquatorialCountercurrent has higher densitiesof schoolingsurface-feeding smaller tunas than the North Equato- rial Current.Other studiesalso have noted the importanceof the Equatorial Countercurrentas a seabirdfeeding area (Gould1974, King 1974, Au and Pitman 1986, Spear et al. 1995, Ballanceet. al. 1997). Amongbreeding species, the SootyTern, Wedge-tailedShearwater, and Bulwer'sPetrel decreasedsignificantly in abundancewith increasein distancefrom Hawaii duringthe breedingseason in spring(see King 1970 for similar results).In contrast,in autumn, densitiesof the former two increasedwith distancefrom Hawaii, and the Bulwer'sPetrel disappeared. During autumn,a similarpattern of increaseddensity with distancefrom Hawaiiwas also seen in the Dark-rumpedPetrel (during spring, the distribution of this specieswas bimodal).These resultsindicate that in autumn feedingconditions also were betterin the southernpart of the studyarea and,in turn, suggestthat, to facilitatebreeding, some species may have been constrainedto feed in lessproductive waters near the HawaiianIslands. Additionalevidence indicating better food supplyin the more southern waterswas the densityincrease with increasein distancefrom Hawaiiduring spring in the Black-winged,Juan Fernandez, and White-neckedpetrels (nonbreedingresidents that nest in the South Pacific;see King 1970 for similar results),and Newell's Shearwater, a breeding species. A likely explanationfor useby the shearwaterof more southernwaters during the breedingseason is that it is a veryfast flier, capable of breedingon Hawaii while foragingat greaterdistances (Spear et al. 1995, Spear and Ainley 1997). Thus, of the 10 predominantspecies (excluding migrants), the White Tern was the only breeder with no distributionalpatterns relative to island distance.Similarly, the Leach'sStorm-Petrel, with densitiesdecreasing with increasein distancefrom Hawaii in spring,was the only nonbreedernot conformingto the idea that unconstrainedspecies should forage in more southern waters. The inshore distribution of the Leach's Storm-Petrel (a planktivore)in spring,and moderately high incidence of feedingover tuna by seabirdsin watersnear Hawaii (zones1 and 2), is consistentwith an "island effect" (Murphy and Shomura 1972). These authorsfound that Skipjack Tuna (Katsuwonuspelamis; a smaller,surface-feeding tuna), as well as smallerYellowfin Tuna (Thunnusalbacares), were abundantjust offshore of mid-Pacificarchipelagos.

28 SEABIRDS IN SOUTHEASTERN HAWAIIAN WATERS

ACKNOWLEDGMENTS

We thank the staff of the National Oceanic and AtmosphericAdministration (NOAA) vesselsDiscoverer, Malcolm Baldrige, and Oceanographer.Cruises were madepossible by the PacificMarine Environmental Laboratories and Atlantic Marine OceanographicLaboratories. Many persons assisted at sea;we are especiallygrateful to Ian Gaffney,Steve Howell, Nina Karnovsky,and Sophie Webb for repeatedtrips. Commentsby Craig Harrison,Robert Pyle, Scott Terrill, and PhilipUnitt on earlier draftswere muchappreciated. Funding was by NationalScience Foundation grants OCE8515637 and OCE8911125 and NationalGeographic Society grants 3321-86 and 4106-89.

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Accepted 26 December 1998