Little and Least Breedingon Midway Atoll: Identification,Range Extensions, andAssortatie Breeding Behavior

Peter Pyle 4990 ShorelineHvvy. StinsonBeach, California 94970 ([email protected])

NancyHoffman and BruceCasler MidwayAtoll National Wildlife Refuge P.O. Box 29460 Honolulu,Hawaii 96820-1860

Tristan McKee P.O. Box 307 Pescadero,California 94060

TheLittle ( albifrons) and (S. antillarum) represent Old World and North Americanforms, respectively, of a complexof smallterns comprising from two to sixspecies (Cramp 1985,Sibley and Monroe 1990,Higgins and Davies1996, Thompson et al. 1997).Other membersof thiscomplex include the Saunder's Tern (S. [albifrons] saun- Figure1. UttleTern, Midway Atoll, eady June 1999. Note the whiterump and tail con- ders•of the MiddleEast, the Yellow-billed (S. [antillarum]superciliaris) trasting crisply with the grayback, and the reducedamount of darkto the innerwebs of and Peruvian (S. [antillarum] Iorata) Terns of SouthAmerica, and the the middleprimaries. Photograph by T.McKee. FairyTern (S. nereis)of .Following work by Massey(1976) on thevocalizations of Littleand Least Terns, the American Ornithologists' Union (AOU 1983)considered these two separatespecies, a beliefthat HermesReef, NWHI, in 1988(Conant et al. 1991).Except for two spec- subsequentlyhas gained widespread acceptance (Sharrock 1993, Olsen imensof LittleTern (Clapp 1989, Conant et al. 1991),however, the spe- andLarsson 1995, Higgins and Davies1996, Thompson et al. 1997,AOU cificidentity of theseterns, including breeding individuals, has remained 1998;but see Patten and Erickson 1996). These two taxa do not breed unknown.It has been speculatedthat occurringin the "main" sympatrically,however, and the onlyprevious record of them together (southeastern) Hawaiian Islands have been Least Terns and those involvesa singlebird identifiedas a LeastTern within a colonyof Little observed in the Northwestern Hawaiian Islands have been Little Terns Ternsin Englandbetween 1983 and 1992(Yates and Taffs 1990, Chandler (Pyle 1987,Clapp 1989,Conant et al. 1991),but confirmationbased on andWilds 1994). The BritishOrnithologists' Union (BOU) haswithheld critical identifications is needed. acceptanceof this recordpending a betterunderstanding of variation In the springand summerof 1999we observed5 smallterns on Sand within the Africansubspecies of Little Tern (Scottand Dickson1996; Island,Midway Atoll, NWHI, 2 of whichwe identifiedas Little Terns (S. BOU 1997, 1999). a_sinensis) and 3 asLeast Terns (subspecies unknown). A pair of each The onlyrecords of LittleTern in the areacovered by theAOU (1998) speciessubsequently nested within 5 m of eachother, near the water- involvetwo specimensof theAsiatic subspecies (S. a. sinensis)and sever- catchmentbasin within Midway'srunway system. Here we describethe al sight and photographrecords from the NorthwesternHawaiian identificationand behavior of theseterns and discuss the implicationsof Islands(NWHI; Clapp1989, Conant et al. 1991,Pyle, pets. obs.). In the theirside-by-side breeding on Midway. Hawaiian Islandsoverall, members of this species-pairhave been The first smalltern observedon Midway in 1999was a Little Tern observedannually since at least1976, with up to 6 recordedat onceand recordedby McKeeand Pyleon 15 May at the water-catchmentbasin as manyas 10-12 differentindividuals observed within a year (Clapp (Figure1). Thisindividual was observed daily, or nearlyso, through 14 1989,R. Pyle,pets. comm.). Recordsare from late March to early June,most frequently in thewater-catchment area but alsofeeding just December,with the majorityfrom Junethrough October. Evidence of offshore at severallocalities around Sand Island. On 15 |une the was breedinghas been reported from Oahu in 1984(R. Pylepers. comm.), joinedat the water-catchmentbasin by a LeastTern, on 18June a pair of FrenchFrigate Shoals, NWH1, in 1980 (Pyle 1980), and Pearl and eachspecies was present, and on 20 Junea third LeastTern was observed.

NORTH AMERICAN BIRDS 55:3--6 betweenthe two specieswere very distinct, such that singleindividuals couldbe identifiedby vocalizationsalone.

Sizeand tail length:The 2 LittleTerns were 10-15% larger than the 3 LeastTerns and had longerouter rectrices (e.g., compare Figure 2 and Figure4). Within eachspecies there was some size variation (probably sex-specific),but thesmaller of the2 LittleTerns was still approximately 10%larger than the largest Least Tern. This difference would be difficult to appreciatewithout direct comparison.

Plumage:The 2 Little Ternshad white rumpsand tails,contrasting crisplywith the pale gray lower backs (Figure 1). The Least Terns had gray rumpsand tails(except for white on the outer 2-3 pairsof rectrices) whichdid notcontrast with the back color (Figures 4-6). In addition,the 2 LittleTerns showed paler upper and undersurfaces to the primaries than the 3 LeastTerns (compare Figures 1-3 with Figures5-6). There wereno consistentdifferences between the two speciesin headplumage, bill color,or legcolor. These5 birdsin alternateplumage were easily identified by thecom- Figure2. UttleTern, Midway Atoll, 24 June1999. Note the relativelylong outer rectdces binationof thesethree criteria, even when singlebirds were observed andthe paleunder surface to the middleprimaries. This area averaged darker on the alone.Some of theseidentification features, along with other"average fea- LeastTerns. Photograph by PeterPyle. tures';have been discussed by Massey(1976), Cramp (1985), Pratt et al. (1987), Clapp (1989), Yatesand Taffs {1990), Conant et al. (1991), Chandlerand Wilds (1994), Olsen and Larsson (1995), and Higgins and Davies (1996). The differencein the color of the primarieson the Midwaybirds These5 birdswere studied thoroughly, photographed (Figures 2-6), and resultedfrom the moreextensive black shading to the innerwebs of the video-recordedby Pylefrom 20 to 26 June,after which he departed.All middleprimaries (p4-p7) on the LeastTerns (Figures 6-7) thanon the 5 ternswere in definitive-alternateplumage. Little Terns(Figures 2-4). This potentialfield mark hasnot beenmen- From 27 Junethrough 26 JulyHoffman and Caslerobserved breed- tionedfor the separationof thesetwo species,although variation in the ingactivity by bothspecies of tern.On 28 June1999 the Little Terns were colorof the outerprimaries has been used to assignsub-specific status found incubatingeggs near the centerof the water-catchmentbasin amongpopulations of LeastTerns (see Patten and Erickson1996), with which,by thistime, had largely dried up. On 30 Junea rainstormcaused easternsubspecies (S. a. antillarum)averging paler outer primaries than floodingof the basinand the nestwas destroyed. On 11 Julythe Least westernand southernsubspecies (browni and mexicana).The amountof Terns were found on a nest on a berm to the northwest of the basin, and darkon theprimaries in smallSterna is affected by thecombination of a on 14July the LittleTerns had re-nested approximately 5 m to the south complexmolt among these feathers and wear (Cramp 1985, Chandler and of the LeastTern nest.By 18 Julythe LeastTern nesthad failed.On 19 Wilds 1994,Higgins and Davies1996). It is possiblethat the differences Julythe LittleTerns were incubating two eggs,but by 26 July,this nest notedon the Midwaybirds were related to differencesin the previous had failed as well. The terns continued to be observed in or near the molt of the primaries.However, both speciesare reportedto undergo water-catchmentbasin through 6 September1999. similarprimary molt patterns(Cramp 1985,Chandler and Wilds 1994), In May-lune2000 up to 6 smallSterna were recorded in thecatchment andon all of theMidway birds the outer 2-3 primarieswere blackish (see basinby islandpersonnel; the ages (subadult vs. adult) and identifications Figures),indicating retention from the previouspre-basic molt, as of all 6 werenot confirmed. In July,Hoffman, Casler, and McKee observed expectedin thesespecies. Thus, middle primary color per se may be use- up to 3 adultand one subadult Little Terns. A pair of adultsbred and suc- ful to distinguishLeast and Little Terns throughout their ranges. This pos- cessfullyfledged at leasttwo chicksby 21 July,which continued to be sibility,accounting for geographicvariation, molt, and featherwear, observedthrough 20 September.This is the first confirmedsuccessful should be researched further. breedingby smallSterna in theHawaiian Islands, although several obser- Thereis alsovariation in thecolor of the shaftsof the outertwo pri- vationsof juvenileLeast or LittleTerns in August-October,at timeswith maries:typically white to horn in AsianLittle Terns(S. a. sinends)and adults,throughout the NorthwesternHawaiian Islands from 1979-1997 black, concolorouswith the webs of these feathers,in Least Terns and (R. Pyle1980, 1987; P. Pyle 1984; Clapp 1989; Conant et al. 1991)suggest nominateLittle Terns from (Cramp 1985, Clapp 1989, Conant et thatsuccessful breeding by thesesmall terns had occurred previously. al. 1991,Higgins and Davies 1996). This difference could not be appreci- The identificationsof theseterns were based primarily on threecrite- atedon the Midwaybirds in the fieldor on photographs.Note also that ria, as follows: the tailsof sJnensisare longerthan in other subspeciesof Little Tern (Cramp 1985,Higgins and Davies1996); thus, tail lengthmay not be as Vocalizations:The 2 LittleTerns consistently gave single-note nasal chent usefulin the separationof the LeastTern from Europeanand African callnotes. When courting they (or oneof them)gave a drychatter some- forms of the Little Tern. whatreminiscent of the callof a RuddyTurnstone (Arenaria interpres). The contrastbetween the rump colorand back color is likely the best On one occasionone of the Little Ternsgive a soft,warbler-like chwip. plumagefeature to separateLittle Ternsfrom LeastTerns in alternate The LeastTerns consistently gave double-noted che-dic call notesand plumage,although this contrastcan be difficult to discernin harsh, (whencourting) an up-slurred, gutteral rre-it. The differences in thecalls sunnylighting. Certain individualsor populationsof LittleTern (e.g.,

4 NORTH AMFRI{ AN BIRDb Least and Little Terns

thosefrom the Philippines; Cramp 1985, Clapp 1989) mayhave slightly grayer rumps and tailsthan others; however,a contrastwith the backcolor is usuallyif not alwayspresent (Higgins and Davies1996), unlike in LeastTern. Juvenile Little Ternsaverage less black in theirplumage than juvenile Least Terns, while the separationof adultsin basicplumage requires further study(Chandler and Wilds 1994). Behavioralinteractions between the two species wereconsistent throughout the periodof observa- tion.The LittleTerns (both alone and as a pair) clear- ly dominatedthe LeastTerns, and would not tolerate their presenceat the water-catchmentarea, actively andrepeatedly chasing them away when both species occurredin the vicinity.This aggressive exclusion by the LittleTerns increased through June as the areaof water in the basin, containing"mosquito fish" (Gambusaa•nis) as prey,became smaller and more easilydefended. No aggressiveintra-specific interac- tions were observed, except those related to courtship. Our observationsincrease the known breeding rangesof bothspecies by over 4000 km. Observations of the Midwaybirds by S. Conant(pers. comm.) in June1999 led her,in retrospect,to identifyas Little Ternsthose breeding in 1988on Pearland Hermes Reef (Conant et al. 1991), 120 km to the southeastof Figure3. LittleTern, Midway Atoll, 17 June1999. Notethe reducedareount of darkto the innerwebs of the Midway.Otherwise, the closestknown breedingof middleprimaries. Photograph by PeterPyle. Little Tern occursin Japan,with a singleattempt recordedfrom Saipan, Micronesia (Reichel et al. 1989).Small terns of thiscomplex, likely Little Terns, have also been recorded in Western Samoa, Kiribati, and on severalislands throughout Micronesia (Pyle and Engbring1985, Pratt et al. 1987). The closest known breedingof LeastTern occursin western ,over 4900 km from Midway.Except for birdsthought to be LeastTerns recorded in the main Hawaiian Islands (see above), we are aware of no recordsof migrantsor vagrantsof this species closerto Midwaythan California. The Midway recordsindicate that both of these speciesclearly can travelsuccessfully over large dis- tancesof ocean,and thatvagrants of eachmight be expectedmore frequentlywithin the rangesof the other species.In North America,we suggestthat observerson both coastscarefully examine birds amongLeast Tern colonies and all extralimitalsmall terns,for vagrantLittle Terns.The behaviorof the Midwaybirds is alsoof interest.Although of limited samplesize, we suggestthat the assortativeinterac- tionsand breeding we observedsupport species status for thesetwo closely related taxa.

Acknowledgments We thankSuzanne Canja, Sheila Conant, Brook Gamble,Norm Garon,Jon King, Keith Larson, Robertand LeilaniPyle, and Susan Riddle for their observationsof ternson Midwayor for helpwith previousrecords. We thankSteve N.G. Howellfor Figure4. LeastTern, Midway Atoll, 24 June1999. Note the relatively short outer rectdces and gray rump and commentson the manuscript. tail,concolorous with the restof the upperparts.Photograph by PeterPyle.

VOLUME 55 (2001), NUMBER 1 5 Literature Cited AmericanOrnithologists' Union. 1983. Check-listof North American Birds.6th edition. American Ornithologists'Union, Lawrence, Kansas. AmericanOrnithologists' Union. 1998. Check-listof NorthAmerican Birds.7th edition.American Ornithologists' Union, Washington D.C. BritishOrnithologists' Union. 1997.Records Committee: 23rd report (July1996). Ibis 139:197-201. BritishOrnithologists' Union. 1999.Records Committee: 25th report (October 1998). Ibis 141:175-180. Chandler, R. ]., and C. Wilds. 1994. Little, Least, and Saunder's Terns. British Birds 87:60-67. Clapp,R.B. 1989.First recordof the Little Tern,Sterna albifrons, from Hawaii. 'Elepaio49:41-46. Conant,S., R. Clapp,L. Hiruki, andB. Choy.1991. A newtern (Sterna) breedingrecord for Hawaii.Pacific Science 45:348-354. Cramp,S., Ed. 1985.The Birds of theWestern Palearctic. Vol. IV. Oxford UniversityPress, Oxford. Higgins,P. J.,and S. J.J. F Davies,Eds. 1996. Handbook of Australian, NewZealand and Antarctic birds. Volume 3. OxfordUniversity Press, Oxford. Massey,B. W. 1976.Vocal differences between American Least Tern and EuropeanLittle Tern.Auk 93:760-773. Olsen,K. M., and H. Larsson.1995. Ternsof Europeand North America. PrincetonUniversity Press, Princeton, New Jersey. Figure5. LeastTern, Midway Atoll, 24 June1999. Note the gray rurnp and tail, concolor- Patten,M. A., and R. A. Erickson.1996. Subspecies of the LeastTern in ouswith the restof the upperparts,and the extensivearnount of darkcoloration to inner Mexico. Condor 98:888-890. websof the rniddleprimaries. Photograph by PeterPyle Pratt, H.D, P. L. Bruner, and D. G. Berrett. 1987. A Field Guide to the Birdsof Hawaii and the TropicalPacific. Princeton University Press, Princeton,New ]ersey. Pyle,P. 1984. Observations of migrantand vagrant birds from Kure and Midwayatolls, 1982-1983. 'Elepaio 44:107-111. Pyle,P., and |. Engbring.1985. Checklist of the birdsof Micronesia. 'Elepaio46:57-68. Pyle,R. L. 1980.Hawaii bird observations,March-July 1979. 'Elepaio 41:47-49. Pyle,R. L. 1987.Hawaiian Islands region. American Birds 41:148-150. Reichel,J. D., D. T. Alden,and P. O. Glass.1989. Range extension for the Little Tern in the Western Pacific. Colonial Waterbirds 12:218-219. Scott, B., and W. Dickson. 1996. News and comment: Do West African Little Ternssqueak? British Birds 89:97. Sharrock,J. T. R. 1993.The "British Birds" list of Englishnames of West Palearctic birds. British Birds 86:1-2. Sibley,C. G., and B. L. Monroe.1990. Distribution and taxonomyof birdsof theworld. Yale University Press, New Haven,Connecticut. Thompson,B.C., J.A. Jackson,J. Burger, L. A. Hill, E. M. Kirsch,and J. L. Atwood. 1997. LeastTern (Sternaantillarum). In The Birdsof NorthAmerica, No. 290 (A. Pooleand E Gill, eds.).The Academyof Natural Sciences,Philadelphia, Pennsylvania, and the American Ornithologists'Union, Washington D.C. Yates,B., and H. Taffs. 1990. Least Tern in East Sussex--a new Western Palearcticbird. BirdingWorm 3:197-199.

--Received6 January2001, Accepted 5 February 2001.

Figure6. LeastTern, Midway Atoll, 24 June1999. Note the gray rump and tail, concolor- ouswith the restof the upperparts,and the extensivearnount of darkcoloration to inner websof the middleprirnaries. Photograph by PeterPyle.

6 NORTH AMERICAN B1RDS