j. Field Ornithol., 63(4):436-444 TERRITORIALITY, HABITAT USE AND ECOLOGICAL DISTINCTNESS OF AN ENDANGERED PACIFIC ISLAND REED-WARBLER ROBERTJ. CRAIG• Department of Natural ResourceManagement and Engineering, U-87 Universityof Connecticut Storrs, Connecticut 06269-d087 USA and Divisionof Fish and Wildlife Commonwealth No. Mariana Islands Saipan,MP 96950 USA Abstract.--Nightingale Reed-Warblers(Acrocephalus luscinia) were studiedon Saipan,Mar- iana Islands from 1988 to 1991 to assessthe territoriality, habitat use and morphologyof this endangered,endemic, island species.Data were comparedto thosefor closelyrelated mainland forms to gain insightsinto differencesevolved in an insular environment.Peak territorial activitywas reachedin January-February, the dry season.Males exhibitedhigh sitefidelity, but femaleshad low mate fidelity betweenyears. Birds inhabitedmostly upland thicket-meadowvegetation, but they also used reed marshes,wetland thickets,and forest openings.Territory size in thicket-meadowcomplexes averaged 9338 _ 3433 (SD) m2. Males were larger than females;mass, wing, tail, and tarsuslength reliably distinguished sexes.Compared to the mainland A. orientalisand A. arundinaceus,A. lusciniahad (1) polygyny largely or entirely absent,(2) much larger territories, (3) upland rather than marshesas the principal breedinghabitat and (4) body size, particularly bill size, greatly increased. The shift to upland habitatsmay accountfor the differencein socialbehavior and territory size. Greater size is typical of island forms that may confrontcompetitively impoverished or food-limited environments. TERRITORIALISMO, USO DE HABITAT Y SlNGULARIDADES ECOL•GICAS DEL AMENAZADO ACROCEPHAL US L USCINIA Sinopsis.--De 1988 a 1991 se estudi6en Saipan (Islas Marianas) el territorialismo,uso de habitat y morfologladel end6micoy amenazadocon desaparecerAcrocephalus luscinia. Se compararon los datos tomadoscon los de especiescontinentales similares, para hacer notar diferenciasevolutivas que se dan en ambientesinsulates. E1 pico de la conductaterritorial se alcanz6en los mesesde enero y febrero que correspondena la temporadaseca. Los machosexhibieron gran fidelidada localidades,pero las hembrasmostrar6n entre aftospoca fidelidad hacia sus parejas. Las aves habitan mayormente en matorrales de alturas, pero ademfisutilizan anegadosde junquillosy matorralesde pantanosy aperturasen bosques. E1 tamafiodel territorio en complejode matorralespromedi6 9338 _+ 3433 (DE) m2. Los machosresultaron de mayor tamafio que las hembras;el peso,largo del ala, cola y tarso es confiablepara distinguir entre los sexos.Comparado con A. orientalisy A. arundinaceus,A. lusciniatiene: (1) territorios de mayor tamafio, (2) areas altas o montafias como habitat principal de anidamientoen vez de anegadoscostaneros, (3) tamafio, y (4) poliginia vir- tualmenteo totalmenteausente. E1 cambioa habitatsde alturas puededeberse a diferencia en la conductasocial y tamafiodel territorio. E1 mayortamafio es tlpicode especiesde islas que puedanconfrontar empobrecimiento competitivo en ambienteslimitados en alimentos. Reed-warblers(Sylviinae: Acrocephalus) are widespreadfrom Europe through Australasia, where many speciesinhabit marshesand are po- lygynous(review in Urano 1985). Members of the genusare strongisland • Currentaddress: Northern Marianas College, Box 1250,Commonwealth No. Mariana Islands, Saipan,MP 96950 USA. 436 Vol.63, No. 4 Reed-Warbler Ecology [437 colonizers,presently represented in the tropical Pacificby as many as 10 species(Pratt et al. 1987). The relativelylarge, long-billedNightingale Reed-Warbler(A. luscirtia)of the Mariana Islands,Micronesia, is among the most distinctive of these. Baker (1951) believed it to be descended from the EasternGreat Reed-Warbler(A. orientalis),a speciesthat has populationsin the Bonin Islands, about 750 km N of the 812-km long Mariana chain (Baker 1951). These two species,along with the Euro- pean-African A. arurtdirtaceus,the AustralasianA. sterttoreusand all other Pacificisland Acrocephalus form a superspeciesthat occursover much of the Old World (Mayr et al. 1986). Three subspeciesof Nightingale Reed-Warbler are currently recog- nized(Mayr et al. 1986). A. l. luscirtia,originally found on Guam, Saipan and Alamagen,has been extinct on Guam sincethe late 1960s(Engbring et al. 1986). A. l. rtijoi of Aguijan, thoughtto be extinct,was rediscovered in May 1992, and A. I. yamashirtaeof Pagan becameextinct ca. 1981 (Glass 1987). Steadman's(1989) findingsof numerous,human-related prehistoricextinctions of Polynesianbirds suggestthat the speciesmay have once inhabited additional islands as well. On Saipan,Nightingale Reed-Warblers are presentlywidespread, with populationsestimated at 4867 in 1982 (Engbringet al. 1986). To provide basicdata for future conservationand managementof this largely un- known, endangeredspecies, I documentthe social behavior, habitat use and morphologyof the Saipanpopulation. I alsocompare my data with thosefor relatedmainland formsto gain insightsinto differencesevolved in an insular environment. STUDY AREAS AND METHODS I studied Nightingale Reed-Warblers on Saipan in 1988 and 1989 duringJanuary-February, dry seasonmonths that correspondto the time of peak singingand presumablypeak breeding(Glass 1987). Additional observationswere made from November 1990 to September1991. I ob- servedbirds throughoutthe island (Fig. 1) but primarily in the Marpi region. Detailed investigationsof habitat use and territoriality were conducted on about 26 ha in the flat lowlands (about 10-40 m elevation) below the cliffs in the Marpi regionand on the plateau (about 130-200 m elevation) near the top of thesecliffs. These flat areasare vegetatedby tangantangan (Leucaenaleucocephala) thickets and elephant grass(Permiseturn purpu- reum) meadows(Fig. 2). The introducedtangantangan, aerially seeded after World War II to prevent erosion,often occupiesland formerly cultivatedfor sugarcane(Fosberg 1960). I studiedterritoriality by mist-netting birds in ATX (36 mm mesh) 12-m netsand colorbanding them. Birds were attractedto netsby playing tape recordedconspecific songs. Once banded,I confirmedthe identity of territory holdersvisually with binoculars,and recordedtheir movements on field maps. Territory boundarieswere determinedwith a variant of the minimum polygonmethod (Hill and Lein 1989) in which a curved 438] R.J. Craig j. FieldOrnithol. Autumn 1992 I I p146000 Pagan' oAlamagan I oGuguan I '•. • Anatahan O ßG ._(2 16ø00 •Salpan •Tinlan Aguijan •fi•Rota ,14o00 ' I •Guam Mariana Islands I -N- Saipan I •.1 I I I FIGURE1. Principalstudy sites on Saipan,and the geographicrelationship of Saipanto the neighboringMariana Islands,Micronesia. Adapted in part fromJenkins (1983). perimeterwas fitted aroundoutermost perches used by singingor in- traspecificallyaggressive males undisturbed by playbackof tapedcalls. I mapped seven1988 territorieson 1:2000 base maps made from 1:10,000 color aerial photographsusing a Bauschand Lomb Zoom- transferscope. Precise map scalewas determinedfrom taped measure- ments made in the field between distinct landmarks. Field data were transferredto thesebase maps, and territory area was measuredwith a compensatingpolar planimeter. Data on habitat structure from these same seven territories were ob- tainedby dividingthem into eight "blocks"of approximatelyequal size and placinga 30-m transectin eachblock (stratified random sample). On each transect,habitat type (elephantgrass meadow, tangantangan thicket,intergradation between these two types)and plant speciescom- positionwere recordedat 1-m intervals(240 total samplepoints/terri- tory).At 3-m intervals(80 points/territory)I recordedvegetation height in the followingclasses: <2, >2-3, >3-6 and >6 m. Vol.63, No. 4 Reed-WarblerEcology [439 - -...... ' "'"h" $•'- ' '"': ß ,- ; •'t' $'.. "•J• 2, '•" ' '•- • '•,• -•,7.•.••-g'• -,• •'•-',2 •lr• I" •'•,•. • •, ;• • ' •'• ' r'' a• '-' ' .... • /' •-•" I'•' '- '$ -' - . ;: t.- • , ,'"•Fx,,' ,' 'a, • •.•..x ,.' • ß FIGURE2. NightingaleReed-Warbler habitat at Marpi, Saipan,sho•ng elephantgrass meadows(foreground), tanganta•ga• thickets (midground), a•d the Marpi cliffsveg- etatedby native forest. To assessthe morphologyof this population,I made the following measurementson mist-nettedindividuals: mass, wing chord,tail length, tarsuslength, and bill length(from the distal end of thenostrils). Breeding maleNightingale Reed-Warblers were distinguished by their cloacalpro- tuberances,whereas breeding females were sexed by their broodpatches. After colorbanding, sexes were verifiedby observingpairs on their ter- ritories. For comparison,I also made morphologicalmeasurements on museum skins of summeringA. orientalisfrom China. RESULTS Socialbehavior and territoriality.--Male Nightingale Reed-Warblers defendedterritories by singing,and usedeither exposed treetops, interior thicketsor elephantgrass stems as songperches. Males respondedvig- orouslyto playbackof tape recordedsongs, and couldbe inducedto leave their territories in pursuit of taped songs.I never observedfemales to sing,but they usuallyaccompanied their matesduring aggressiveen- counters,such as during response to playback.On no occasiondid I observe any male on the 50 territoriesstudied over three peak breedingseasons (28 individual males) to associatewith more than one female. Of 11 malesbanded in 1988,9 (82%)were present on the sameterritory in 1989. One movedto a new territory 1900 m from its originalone, and one disappeared.Hence, all malesbut one bandedin 1988 (91%) were