Occurrence of Migrant Shorebirds in the Mariana Islands

Home , Green sandpiper, Ruddy turnstone, Tattler (bird), Wandering tattler

J. Field Ormthol., 68(1):42-55

OCCURRENCE OF MIGRANT SHOREBIRDS IN THE MARIANA ISLANDS

DEREK W. STINSON 1 Division of Fish and Wildlife Departmentof Land and Natural Resources Saipan, NIP 96950

GARYJ. WILES Division of Aquatic and WildlifeResources Guam Departmentof Agriculture PO. Box 2950 Agana, Guam 96910

JAMESD. REICHEL2 Division of Fish and Wildlife Departmentof Land and Natural Resources Saipan, MP 96950

Abstract--We summarizedata on seasonality,and relative abundanceof shorebirdsin the Mariana Islandsbased on bi-weeklysurveys on Saipan (1987-1993) and Guam (1980-1981), and miscellaneouscounts and recordsfrom all 15 islands.Of 46 speciesdocumented (36 Scolopacidae,7 plovers,1 stilt, 1 oystercatcher,1 pratincole), only the PacificGolden-Plover (Pluvialisfulva) is abundant. The Whimbrel (Numeniusphaeopus), Gray-tailed Tattler (Het- eroscelusbrevipes), and Ruddy Turnstone (Arenaria interpres)were common, and the Wood Sandpiper( Tringaglareola), Common Sandpiper (Actitis hypoleucos), Wandering Tattler (Het- eroscelusinterpres), Rufous-necked Stint (Calidris ruficollis),and Mongolian Plover (Charad- rius mongolus)were uncommon migrantsand winter visitors.A greater number and variety of shorebirdswere seenduring autumnalmigration, which coincideswith flooding of seasonalwetlands and higher shorebirdpopulations, than during springmigration. The rarity of certainspecies, radar studies,and comparisonwith studieselsewhere in the region,support the hypothesisthat somespecies normally migrate non-stop between Asia and Australia. The archipelagois not a major winteringor stopoverarea, but shorebirdsmake up a large portion of the avifaunaof the islands.Inter-tidal foragingareas are very limited in the Mar- ianasand the mostimportant sites are threatenedby developmentand human activity.

1NCIDENCIA DE AVES COSTERAS MIGRATORIAS EN LAS ISLAS MARIANAS

Sinopsis.--Resumimosdatos sobre la estacionalidady abundancia relativa de aves costeras en las IslasMarianas basados en muestreosbisemanales 11evados a caboen Saipfin(1987- 1993) yen Gufim (1980-1981), yen conteosy registrosmiscelfineos de todaslas 15 islas.De 46 especiesdocumentadas, solo Pluvialis fulva es abundante.Migrantes comunes incluyen a Numeniusphaeopus, Heteroscelus brevipes y Arenaria interpres,mientras que Tringaglareola, Actitishypoleucos, Heteroscelus interpres, Calidris ruficollis y Charadriusmongolus resultaron visitantesinvernales y migratoriosraros. Durante la migraci6notofial se puedenobservar un mayor nfimero y variedadde avesque durante la migraci6nprimaveral. Esto coincidecon la inundaci6nde anegadosestacionales. La rareza de ciertasespecies, estudios de radar,y

Currentaddress: 12106 SE 314th Pl, Auburn, Washington98092 USA. Currentaddress: Montana Natural HeritageProgram, 1515 E. 6th Ave., Helena, Montana 59620-1800 USA.

42 Vol,68, No. 1 Shorebirdsin theMarianas [43 comparacionescon estudiosen otraspartes de estaregi6n, apoyanla hip6tesisde que algunas especiesnormalmente migran sin detenerseentre Asiay Australia.E1 archipi61ago no es un rrea principalde escalamigratoria o invernal,pero avesde costasforman la porci6nprincipal de la avifaunade las IslasMarianas. Las rireasde forraje entre mareasson muy limitadasen las Marianas,y los lugaresmrls importantes estrin amenazados por el desarrolloy la actividad humana.

Large numbers of shorebirdsmigrate annually between their breeding grounds in northeastern Asia and Alaska and wintering areasin southern Asia, Australia,New Zealand, and the islandsof the tropical Pacific.Most migrantspass along routesthrough easternAsia (Lane 1987, Parishet al. 1987, Williams and Williams 1990). However, some birds travel farther to the east acrossthe open expansesof the western Pacific Ocean, thereby taking a more direct flight path betweenJapan and New Guinea. Baker (1951:35) originally proposed the existenceof a Japanese-Mar- ianan Flywayin which shorebirdsflew southwardfrom Japan along a cor- ridor following the Izu, Ogasawara,Iwo, and Mariana archipelagosto reach wintering areas in Micronesia and farther south. However, recent radar studieshave shown that significantnumbers of shorebirdsinstead fly southwardin a broad front through Micronesia(Williams and Williams 1988, 1990). An estimatedhalf million birds passwithin 1000 km of Guam in the Mariana Islands during autumn migration (Williams and Williams 1988). The Mariana and Caroline Islands are potential stopover sites along this route. Yet, general observationsshow that relativelysmall numbers of most speciesof shorebirdsare recorded on these islandsduring passagemigration or in the winter (Baker 1951, Jenkins 1981, Williams and Grout 1985). This suggeststhat most speciesmake non-stopflights across Micronesia (Williams and Williams 1988). In this paper, we describe the speciescomposition, habitat use, and relativeabundance of shorebirdsvisiting the Mariana Islands.Results were determined by regular surveyson the island of Saipan from 1987-1993 and from general observationson the other islandsin the chain, especially Guam, from 1980-1993. This period of data collection is considerably longer than that of previous workers who were able to view shorebirds for only severalmonths to 1.5 yr (Baker 1951;Jenkins 1981; Williams and Grout 1985; Commonwealth Northern Mariana Islands, Div. of Fish & Wildlife-DFW and Guam Div. of Aquatic & Wildlife Resources-GDAWR, unpubl. field notes).

STUDY AREA AND METHODS The Mariana Islandsform an archipelagoof 15 islandsfrom 13014' to 20ø31'Nand 144040' and 146ø05'Ein Micronesia(Fig. 1). The islandslie about 1700 km southof Japan, 2100 km eastof the Philippines,and 1800 km north of New Guinea. Guam is the southernmostand largest island (540 km2) in the Marianas. The other main southern islandsof Saipan, Tinian, and Rota range in size from 85-123 km2. Guam and Saipan provide the best habitat for migrating shorebirds,with intertidal beachesand flats, freshwaterwetlands, and mowed lawnsoccupying much larger areas 44] D. W. Stinsonet al. j. FieldOrnithol. Winter 1997

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FIGURE 1. The Mariana Islands and their location in the western Pacific.

than on Rota and Tinian. The remainingislands in the chain are much smaller(1-48 km2) and, exceptfor rockyshores, a short airstrip,some openfields and two brackish lakes, their steep volcanic terrain offers little appropriatehabitat for shorebirds. Surveysof shorebirdswere conducted primarily on Saipanand Guam, with infrequentobservations made on the otherislands. Shoreline counts were made on foot or from vehicles and were conducted at low tide during daylighthours. Surveys were not madeduring periods of highwinds or moderateto heavyrains. Survey sites were chosenbecause of the rel- ativelylarge numbers of shorebirdspresent at them,and werenot nec- essarilyrepresentative of shorebirdpopulations island-wide. On Saipan,we madeover 450 visitsto shoresites (n = 247 visits), wetlands(n = 182), and the SaipanInternational Airport (n = 28 visits) fromJune 1987 to May1991 and from July 1992 to June 1993. Two beach siteswere sampled twice monthly. Tanapag Beach is a 1-km-longstretch Vol.68, No. I Shorebirdsin the Marianas [45 of beach and sandyintertidal fiats partly vegetatedwith seagrass(Enhalus acoroidesand Haladule uninervis). Birds seen at an old seaplaneramp at the south end of the beach were included in counts at this site. The second location was the Puerto Rico mudfiats, a 17-ha site south of Sai- pan's municipal dump. Counts also were made with lessregularity at 15 freshwaterwetlands, as well as at the airport, which has large grassfields, a rainwater catchment reservoir,and ephemeral rainwater puddles. Sur- veyson Saipan were made in both halvesof each month during most yearsof the study.We calculatedthe mean number of birds present for the years we have data. When two surveyswere conducted during the same half-month period, count resultswere averagedfor the period be- fore calculatinga mean acrossyears. Observationson Guam were made at two primary sites.Dungcas Beach is locatedon the west-centralside of the islandand featuresa 1.5-km-long intertidalsandfiat. Togcha Beach is a shallowintertidal reef fiat on the east side of Guam that is 2.4-km long and 300-400-m wide. Other locations includedlarge mowedfields on severalmilitary bases and a number of sites with freshwater,including natural and human-madewetlands, several aquaculture ponds, and a landfill. Censuseswere made twice monthly from March 1980 to October 1981. However,in the remainingyears, data were taken from irregular counts made each month in autumn, with fewer countsmade in the spring.Very few censuseswere made in the summer.

RESULTS A total of 46 speciesof shorebirds has been recorded to date in the Mariana Islands(Reichel and Glass1991, Wiles et al. 1993), althoughfour of these speciesare consideredhypotheticals because of insufficientdoc- umentation. This list includes 36 members of the family Scolopacidae, one oystercatcher,one stilt, sevenplovers, and one pratincole. Information on seasonalityand abundanceis summarizedbelow for the 28 speciesseen most regularly.Information on habitatsregularly used by thesespecies in the Marianasis included in Figure 2. We regard only one shorebird, the Pacific Golden-Plover, as abundant. Three others (Gray- tailed Tattler, Whimbrel, and Ruddy Turnstone) were common, being consistentlypresent during migration and through the winter (Fig. 2). In addition, thesebirds usuallysummered in the Marianasin small numbers. We classifiedfive speciesas uncommon migrants (Mongolian Plover, Wood Sandpiper, Wandering Tattler, Common Sandpiper, and Rufous- necked Stint). Smaller numbers of these birds were recorded annually, and a few individualsof each overwintered.An additional37 specieswere classifiedas rare migrants,vagrants, or accidentals.Species were listedas migrants to the Mariana Islands if the archipelago lies between their breedingand wintering grounds,and listedas vagrants if not. Speciesthat do not migrate in the region and recorded only once were classifiedas accidental.In the following accounts,we defined very small numbers as meaning fewer than 10 birds seen per year and small numbers as 10-50 individualsseen per year throughout the island chain as a whole. Acci- 46] D. W. Stinsonet al. J.Field Ornithol. Winter 1997

UFF LITTLE RINGED PLOVER

FIGURE2. Weeklypresence of annualmigrant shorebird species in the Marianasin order of abundance;/ = singlerecord; X = tworecords; shaded = threeor more records; Habitatcodes: T,F,U,S,R = primaryhabitats; t,f,u = secondaryhabitats; T/t = tidalfiats, beaches;F/f = freshwaterwetlands, flooded fidds, drainedaquaculture ponds; U/u = uplandhabitats, mowed fields; R = rockyshores; S = old concreteseaplane ramp on Saipan.Includes data from: Stophlet(1946), Borror (1947), Kibler (1950), Baker (1951),King (1962);Jenkins (1978, 1981), Kridler (1979, unpubl. notes), Engbring and Owen (1981), Ashman(1983), Beck (1985), Williamsand Grout (1985), Pyleand Eng- bring(1987), Glass et al. (1990),Stinson et al (1991),Downs (1946), DFW files, GDAWR files. dentalsand veryrare migrantswith only a few recordsare not discussed, but listed in Table 1. Taxonomy followsSibley and Monroe (1990) for speciesnot listedin the AOU Checklist(American Ornithologists' Union 198S). Black-belliedPlover (Pluvialissquatarola)mRare migrant. This species was observedeach year in small numbers,with most recordsoccurring from late Augustto late April (Fig. 2). On Saipan,numbers were very low through early autumn, but were somewhathigher from Novemberto April. The birdsstopping in the Marianasoften spentthe winter. Pacific Golden-Plover(Pluvialis fulva)mAbundant migrant. This speciesis regardedas the mostcommon migratory shorebird in Micronesia (Pratt et al. 1987, Williams and Williams 1988). In the Marianas, large numbersof golden-ploversoccur during migration, and we estimated that a few thousandbirds may overwinter.The earliestfall migrantsbegan arrivingin the firstweek of August.Two peaksin autumnmigration were apparent,with the first probablyconsisting of adultsarriving in the second half of Augustand earlySeptember and a secondinflux 3-4 wk later comprisedprimarily of juveniles(Fig. 3; seealso Williams and Williams 1988). This age classpattern during migrationhas been reportedin Ha- waii (Johnsonet al. 1981) and Wake Island (Johnstonand McFarlane 1967), but we did not distinguishage classesduring surveys.Flocks of migratingplovers can be largein the autumn(700-2000 birds). Vo•.68, •o. • Shorebirdsin the Marianas [47

TABLE1. Very rare and accidental shorebird speciesin the Mariana Islands.

Species Statusa Referencesb Oriental Pratincole (Glareola maldivarum) vRV 4 Snowy Plover (Charadrius alexand,inus) RV 5, 9, 16 EurasianOystercatcher (Haematopus ostralegus) A 10 Greater Yellowlegs( Tringa melanoleuca) A 4 Nordmann'sGreenshank c (Tringa guttifera) A 7, 8 Common Redshank(Tringa totanus) RV 1, 2, 4, 5 SpottedRedshank c ( Tringa erythropus) A 11 Green Sandpiper(Tringa ochropus) vRV 3, 15 Little Curlew (Numenius minutus) vRM 4, 7 Bristle-thighedCurlew (Numeniustahitiensis) RV 4, 11, 15 Far Eastern Curlew (Numenius madagasgariensis) vRM 3, 13, 14, 15 Eurasian Curlew (Numeniusarquata) vRV 2, 3 Black-tailed Godwit (Limosa limosa) vRM 4, 11, 12 Great Knot ( Calidris tenuirostris) vRM 2, 3 Little Stint ( Calidris minuta) A 4 Temminck's Stint (Calidris minuta) RV 4, 15 Long-toed Stint ( Calidris subminuta) vRM 2, 3, 11 PectoralSandpiper (Calidris melanotos) vRM 4, 6, 7, 16 Dunlin (Calidrisalpina) RV 7, 9, 15 Curlew Sandpiper ( Calidrisferruginea) vRM 5, 16 Dowitcher spp. (Limnodromusspp.) vRV 2, 15

• A = accidental,v = very, R = rare, V = vagrant,M = migrant. b 1. Stinson et al. 1991; 2. Wiles et al. 1993; 3. Stinson et al. 1991; 4. Glass et al. 1990; 5. Wiles et al. 1987; 6. Pyle and Engbring 1987; 7. Williamsand Grout 1985;8. Williams 1987; 9. Engbringand Owen 1981; 10. Maben and Wiles 1981;11.Jenkins 1981; 12. Jenkins 1978; 13. Baker 1951; 14. Hartert 1898; 15. DFW and GDAWR files; 16. G. Wiles, pers. obs. c Hypothetical-uncertainrecord.

Smaller but significantnumbers of golden-ploverspassed through the Marianas from early March to as late as the third week in April during northward migration.A few birdsin non-breedingplumage or incomplete nuptial plumage remained during most summers. Mongolian Plover (Charadrius mongolus)--Uncommon migrant. This specieswas observed annually in low numbers, with Guam typicallyat- tracting more birds than Saipan. Recordsof Mongolian Ploversexist for nearly the entire year (Fig. 2). Data from Saipan suggestthat abundance was greatest from January to mid-April (Fig. 3). Jenkins (1981) also ob- tainedhis highestcounts in March and April 1978,while Maben (GDAWR files) found numbersto be largestin November 1980 and January 1981. A few birds occasionallysummered on Guam. A Mongolian Plover seen at Togcha Beach on 19 Dec. 1991 had been wing-taggedat Shunkunitai, Nemuro-Shi in northeastern Hokkaido, Ja- pan on 25 Aug. 1991 (T. Mundkur, pers. comm.). A similarlymarked bird resided at Togcha Beach from 13 Sep. 1992 to 24 Jan. 1993. Greater Sand Plover (Charadrius leschenaultii)--Rare migrant. Very small numbers of Greater Sand Ploverswere observedin most years,with 48] D. W. Stinsonet al. J.Field Ornithol. Winter 1997 --•TAN.BEACH• P.R.MUDFLAT I lOO

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Flcuva•3. Bi-weeklymean number of six shorebirdspecies at Tanapagbeach and Puerto Rico mudflatson Saipan,1987-1993. Rufous-neckedStints were regularlyrecorded only at Tanapag beach. the majority of sightingsmade from early September to late December (Fig. 2). Common Ringed Plover (Charadrius hiaticula)---Rare vagrant. Com- mon Ringed Ploverswere found in very smallnumbers in mostyears, with more sightingsnoted in autumn than spring (Fig. 2). A few individuals wintered on Guam. Little Ringed Plover (Charadrius dubius)--Rare migrant. Very small numbers of this specieswere noted in lessthan half of all years.All sightings occurred on Guam and involvedonly one or two birds. Recordswere more common in the autumn from late August to early December, but were scatteredduring the spring (Fig. 2). Black-wingedStilt (Himantopushimantopus)--Rare migrant. This spe- Vol.68, No. 1 Shorebirdsin theMarianas [49 cies was first recorded in the Mariana Islands in 1986, and since then, a few individualshave been seenin mostyears. Nearly all recordswere from late Augustto mid-December(Fig. 2). Stiltscollected on Saipanand Rota were of the Eurasianrace himantopus,as are most stiltsthat visitJapan (Brazil 1991), and not from New Guinea where the Australian race leu- cocephalusis found (Beehler et al. 1986, Hayman et al. 1986). Common Greenshank (Tringa nebularia)mRaremigrant. Sightingsoc- curred from August to April in most years,being somewhatmore common in autumn (Fig. 2). Birds occasionallyoverwintered, such as one bird that was regularly observedat a tidal fiat on Saipan from 19 Oct. 1992 to 27 Mar. 1993. All recent sightingson Guam and Saipan have involvedone or two individuals,although King (1962) reported five birds at a coastalmudflat on 24 Apr. 1960. Marsh Sandpiper (Tringa stag'natilis)mRaremigrant. A few Marsh Sandpipersvisited Saipan and Guam in most years.Records exist from late Augustto late April, but autumn recordspredominated. Wood Sandpiper (Tringa glareola)--Uncommonmigrant. Small numbers of Wood Sandpiperswere observedannually on Saipanand Guam. Nearly all recordswere from late Augustto late April (Fig. 2). Countsat freshwaterwetlands on Saipan were highest from November to January, suggestingthat a fair number of birds spent the winter. Peaksduring migration also occurredin Septemberand March-April. Wandering Tattler (Heteroscelusincanus)--Uncommon migrant. This speciesis typicallyregarded as lesscommon than H. brevipesin the Mar- ianas (Pratt et al. 1987, Williams and Grout 1985). However,Wandering Tattlers tend to selectrocky coastlinesas their preferred habitat and these sitesare difficult to surveybecause of poor accessand rough surf condi- tions. Becausethis type of shoreline is predominant on nearly all islands in the Marianas,it is possiblethat H. incanuswill be found to be at least ascommon as H. brevipesin the archipelago,once better data is obtained. Wandering Tattlers were seen in all months. Birds were most common from mid-Augustto late May, with a few individualssummering. Gray-tailedTattler (Heteroscelusbrevipes)--Common migrant. Because of the difficulty in separatingthis speciesfrom the Wandering Tattler when birds are in winter plumage, we usually identified and recorded individualsonly at the generic level. Gray-tailedTattlers were by far the more common speciesat the sites regularly censusedon Saipan and Guam, and they ranked with Pacific Golden-Ploversas the most consis- tently seen speciesat shore sites.This may wrongly give the impression that H. brevipesis considerablymore numerousthan H. incanus. Modest numbers of Gray-tailedTattlers were recorded on Saipan and Guam each year.Survey results from beacheson Saipanrevealed that fall migration beginsin July and peaksin Septemberand October (Fig. 3). Baker (1951) also noted that migration began in about mid-July.Small numbers of birds summered on Saipan and Guam. Common Sandpiper (Actitis hypoleucos)--Uncommonmigrant. This specieswas recorded annually in small numbers. Common Sandpipers 50] D. W. Stinsonet al. J.Field Ornithol. Winter 1997 were an early fall migrant, with sightingsbeginning in July and peaking in October and November. Abundance reached similar levels during spring migration, with the highest numbersrecorded from February to early May (Fig. 3). Some birds overwintered in the Marianas. Terek Sandpiper (Xenus c/nereus)--Raremigrant. Only a few individuals of this specieswere observedin the Marianas. Recordsshow that it occursalmost exclusivelyas a passagemigrant from late August through November (Fig. 2). The only spring sighting was from Pagan on 26 Apr. 1989 (Stinson et al. 1991). Whimbrel (Numeniusphaeopus)--Common migrant. Whimbrelswere present in the southern Marianas in all seasons.Survey data from Saipan showed that abundance was highest from January to early March, with lower but fairly stablenumbers noted during the rest of the year (Fig. 3). Observations on Guam indicated that numbers are often much smaller in the summer,although Jenkins (1981) did record a sizablesummering populationin 1978.Flocks sometimes roosted on largeopen lawnsat high tide, especiallyat the Saipan airport. An unusuallylarge aggregationof 100 birds roosted on the roof of a large warehousenext to inner Apra Harbor, Guam, in December 1989. Two individuals collected recently on Saipan belong to the subspeciesN. p. variegatus(R. E. Johnson, pers. comm.), which breedsin easternRussia (Hayman et al. 1986, Lane 1987). Bar-tailed Godwit (Limosa lapponica)--Rare migrant. This specieswas much more common than the Black-tailed Godwit, with a few individuals noted in most years. Most records were from mid-September to early De- cember and from mid-March to early May, but a few birds occasionally overwinter (Fig. 2). RuddyTurnstone (Arenaria interpres)•Common migrant. Williamsand Williams (1988) considered Ruddy Turnstones to be the second most common speciesof shorebird in the Marianas, after the Pacific Golden- Plover. Our records indicate that Gray-tailedTattlers are more common at shore sites than turnstonesand that turnstone abundance is usually much lower than the numbers they observedin the fall of 1983. Although Williams and Williams (1988) tallied a maximum of 235 turnstones on a single day on Guam in 1983, we estimated that in most years, only 300- 500 birds may normally stopoverin the Marianas in the autumn. Ruddy Turnstoneswere present in modestnumbers each year during migration and throughout the winter. A few birds often spent the summer on Guam (Jenkins1981). On Saipan,turnstones used the shoresites unpredictably and sporadically.We do not present bi-weeklymeans becausethey were heavily skewedby rare records of flocks of 10 and 25. Turnstoneswere more consistentlyseen at large fields, and data from airports on Saipan and Rota (counts in 30 months) indicated they are more abundant in fall. This supportsThompson's (1973) theory that the turnstone'sspring migration is generallynon-stop through the western Pacific. Sanderling (Calidris alba)--Rare migrant. One to five Sanderlingswere seen in most years on Saipan and Guam. They were present from mid- Augustthrough April, with a few individualsrarely overwintering (Fig. 2). Vol.68, •o. 1 Shoretn'rdsin the Marianas [51

Our only summer record was a flock of five observedfrom early May to mid-July 1980 at Dungcas Beach by A. Maben (unpubl. data). Groups usually contained one to three birds, and sometimesoccur in close as- sociation with flocks of Rufous-necked Stints. Rufous-neckedStint (Calid•s ruficollis)mUncommonmigrant. This was the most common stint in the Marianas, with small numbers seen annually, usuallybeginning in late Augustand continuing through April or May. Abundance appeared to be greater on Guam than Saipan.Two peaks in fall migration occurred on Saipan (Fig. 3), which may coincidewith the arrival of adultsin Septemberand juvenilesin November and Decem- ber. A similar migration pattern hasbeen documentedin Australia(Paton and Wykes1978). During springmigration, numbers were highest in Feb- ruary and March. Some birds wintered on Guam, but none or very few seem to do so on Saipan. Additionally, there were scatteredrecords of Rufous-neckedStints on Guam during the summer months. Sharp-tailed Sandpiper (Calid•is acuminata)--Rare migrant. Small numbers of Sharp-tailed Sandpiperswere seen annually in the Marianas. This speciesis primarily a fall transient,with mostrecords occurring from mid-Septemberto mid-December.There is a record of an unusuallylarge aggregationof 28 birds from Saipanin January 1979 (Glasset al. 1990). Ruff (Philomachuspug'nax)--Rare migrant. This specieswas noted in very smallnumbers in about half of all years,with the majorityof sightings falling between late August and late November. Glasset al. (1990) reported an unusallylarge flock of 30 Ruffs in October-November 1986. The few spring recordsare from Guam and Rota. Snipe spp. (Gallinago spp.)--Rare migrants.Small numbers of snipe were seen annually in the Marianas. Two species,Swinhoe's Snipe (G. megala) and Common Snipe (G. gallinago), have been collected in the archipelago,but two others, Latham's Snipe (G. hardwickii)and Pintail Snipe (G. stenura),are known only from suspectedsightings. Field identification of Asian Gallinagois extremely difficult (Hayman et al. 1986), thus we were unable to identify most snipesto species.Based on collected individuals,Swinhoes's Snipe is considerablymore common than the other species.Single Common Snipehave been collectedon Saipanand Rota (Takatsukasaand Yamashina1931, Stinson et al. 1991) and this species has been seen twice on Guam. Hypothetical records of Latham's Snipe come from Saipan (Stinson et al. 1991) and Pintail Snipe from Tinian and Rota (Pyleand Engbring 1987). Recordsof all snipewere combined for Fig. 2 and indicate that birds were most common from late September to earlyJanuary. A few individualsprobably wintered in the Marianas.

DISCUSSION Islands in Micronesia offer little habitat for most speciesof migrating and wintering shorebirdsdue to their small sizes,typically narrow intertidal zones, and lack of extensive mudflats (Parish et al. 1987). This is probably the major reasonfor the limited use of theseislands as stopover sitesduring migration by shorebirdsand why many speciesprobably over- 52] D. W. Stinsonet al. J.Field Ornithol. Winter 1997 fly the region. Of the 46 speciesof shorebirdrecorded, 30 breed in the Palearcticor Alaska and regularly winter in Micronesia or further south in Australia, New Guinea, and on other Pacific islands (Hayman et al. 1986). An additional 12 speciesrecorded in the archipelagohave Eur- asianwintering rangesextending as far eastas Japan, the Philippines,or central Indonesia. Two species,the Common Ringed Plover and Little Stint, winter primarily in Africa and southwesternAsia, while two others, the Greater Yellowlegsand a dowitcher sp., normally winter in the New World. Overall abundance.--Turnoverrates among migrant shorebirdsin the Marianas have never been assessedwith marked birds, so it is difficult to estimatethe numbersof birdspassing through or winteringin the islands. It is clear however,that the archipelago is not a major stopoveror wintering area for shorebirds,except for the Pacific Golden-Ploverwhich migrates through in relatively large numbers. We estimate that 5000- 20,000 ploverstransit the archipelagoand that perhapsa few thousand overwinter.All other speciesexhibit much lower abundance.Migrating Whimbrels,Ruddy Turnstones, and Gray-tailedTattlers probably each total no more than severalhundred individualsper season,even though we classifythem as common. For specieswe regard as uncommon, perhaps about 50-200 individuals of each visit the islands. The data summarizedhere may not be indicativeof the numbersand speciescomposition of migrantsaloft (Williamsand Williams 1988). Our few observationsof some shorebirdspecies (e.g., Far EasternCurlew, Bar- tailed Godwit, Great Knot, and Latham's Snipe) support the hypothesis that they migrate nonstopwhen crossingthe Pacificbetween eastern Asia and Australia (Barter and Hou 1990, Parish et al. 1987, Williams and Williams 1988). Regionalcomparisons.--It is difficult to compareshorebird occurrence among Micronesianisland groupsdue to the lack of systematicsurveys. Palau and Yap, the westernmostCarolines are located only 800 km and 1300 km east of the Philippines, respectively.Palau and Yap might be expectedto havegreater shorebird diversity and abundancethan the Mar- ianas becauseof their closer proximity to Asia, but observationssuggest that the number and diversityof shorebirdsare similarfor the three island groups (J. Engbring, pers. comm.). In absolutenumbers, only the Rufous- necked Stint is considerablymore numerous in Palau, with flocks of up to 300 birds present in winter (Engbring 1988). Speciesdiversity appears to decline rapidly in the eastern Carolines,with 25 speciesrecorded in Chuuk, 14 in Pohnpei, and 12 in Kosrae (Baker 1951, Pyle and Engbring 1985). Further observationswill likely increasethese figures somewhat. Census data are not available for these islands, so it is not known if shorebird abundance showsa correspondingdecrease with greater distance from Asia. The Iwo and OgasawaraIslands, to the north of the Marianas do not attract large numbersof shorebirdsbecause these islands lack the wide tidal flats found at a few siteson Guam and Saipan (Brazil 1991; Y. Yamamoto, pers. comm.). Vol.6a, No. ! Shorebirdsin theMarianas [53

Japan, eastern mainland Asia, and New Guinea, the large land masses lying north and south of the Mariana Islands,are the probable sourceof most shorebirdsvisiting the island chain during migration (Baker 1951, Williams and Williams 1988). Nearly all of the common and uncommon shorebirdsin the Marianas are also common in Japan and New Guinea during the non-breeding season(Beehler et al. 1986, Brazil 1991). Only the Wandering Tattler, which has an oceanicmigration route from Alaska and northwesternCanada to the tropical Pacific (Hayman et al. 1986), is consideredrare in Japan and New Guinea. In contrast,about 17 species that are fairly common to abundant during migration in Japan and/or New Guinea occur only rarely or have not yet been reported in the Mar- ianas. These speciesapparently either migrate predominantly through eastern Asia, or overtly Micronesia. Seasonality.--Shorebirdabundance and variety in the Marianas is greatest from Septemberto April. Observationsindicate that autumn migration, which is presumably enhanced by young of the year, is of longer duration and involveslarger numbers of birds and speciesthan does springmigration. Of the speciesfor which there is sufficientinformation, approximately15 are more common in the fall and first half of winter, at least four (Common Sandpiper,Grey-tailed Tattler, Rufous-neckedStint, Dunlin) appear to be equally common in the autumn and spring, and three (Blackobelliedand Mongolian Plovers,Whimbrel) are more common in the winter and spring.About 11 speciesregularly overwinterin the Marianas in at least small numbers, of which Pacific Golden-Plovers, Mongolian Plovers, Whimbrels, both tattlers, Ruddy Turnstones, and Wood Sandpipers,are present in modest to large numbers. Only four or five species(Pacific-Golden Plovers, Gray-tailed Tattlers, Whimbrels, Rud- dy Turnstones,and possiblyWandering Tattlers) regularlysummer on the islands,with small numbers present.All of these individualsare assumed to be nonbreeders(Johnson 1979). Habitat use.--Habitat use by each speciesof shorebirdin the Marianas generally coincideswith that known from other wintering areas (Hayman et al. 1986, Brazil 1991). Autumn migration coincideswith the rainy season in the Marianas and flooding of seasonalwetlands. Most of the wetlands are covered with dense reed thickets and thus not used by shorebirds, but widely scatteredsites have short grass,mud, and shallowpools of water, that provide habitat. Wet agricultural fields also are frequented by shorebirdsin the fall. Depending on rainfall patternsfor the year,most of these sites dry out in December or January and remain dry in the spring, and birds concentrateat the few siteswhere open water persists. On Guam, several small aquaculture farms occasionallyhave drained ponds that are highly attractiveto shorebirds. Human activityon the major southern Mariana Islandsof Guam, Rota, Tinian, and Saipanhas undoubtedlybenefited someshorebird species by the creationof large open fieldsof short grasslocated at airports,military bases,golf courses,and antenna fields. They are a preferred habitat of 54] D. 14/.Stinson et al. J.Field Ornithol. Winter 1997 migrating and wintering Pacific Golden-Ploversand larger numbers of ploversmay use these islandsnow than in prehistoric times. Conservation.--Browntree snakes(Boiga irregularis)were accidentally introduced to Guam probably from the Admiralty Islands shortly after World War II. Snake predation has resulted in the extirpation of most of the nativeforest birds and residentseabirds of Guam (Savidge1987, Rei- chel 1991), and brown tree snakesmay eventuallycolonize the neighboring southern Mariana Islands.Fortunately, there is no evidence that vis- iting shorebirdshave been affected. Shorebirds are not hunted by island residentsin the Marianas, and shoreline habitats are not immediately threatened by reclamation, as in many locations in neighboring Asian countries. Nonetheless,shorebird habitatsface a variety of problems in the island chain. Economic growth and rapidly expanding tourism have stimulated considerabledevelop- ment along coastlinesduring the last few decades.Four highrise condo- miniums and a hotel have been built at Dungcas Beach in the past 20 years.The area is under increasingdisturbance from jet skiers,fishermen, walkers,and dogs.At Togcha Beach, there are plans to constructa golf coursenearby and the area is increasinglyused by picnickers.The Puerto Rico mudflats are located next to Saipan's municipal dump. Garbage from the dump falls directly onto the reef flat and toxic substancesmay affect lagoon waters. Future military cutbacksmay force the closing of several installations on Guam. This would reduce the extent of large mowed fields available to shorebirds.

ACKNOWLEDGMENTS

This studywas funded by FederalAid in Wildlife Restorationto the Commonwealthof the Northern Mariana Islands,Project W-I-R-6-10 and Federal Aid in Wildlife Restorationto Guam, ProjectsFW-2R. Comments by C. Minton and C. R. Chandler improved the manu- script.We thank the many people who have watched shorebirdsand contributed data over the years,especially R. Beck, P. Glass,T. Lemke, T. Pratt, A. Maben, P. Conry,V. Camacho and the late David T. Aidan. Thanks to C. Rice for adviceon data analysis.

LITERATURE CITED

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