Gonzalez:Plovers and sandpipersin Argentina Habitat partitioning and the distribution and seasonalabundances migratory plovers and sandpipers in Los Alamos, Rio Negro, Argentina Patricia M. Gonzalez Gonzalez,P.M. 1996. Habitat partitioningand the distributionand seasonalabundances of migratoryplovers and sandpipersin LosAlamos, Rio Negro,Argentina. InternationalWader Studies 8: 93-102. BetweenSeptember 1989 and December1990, field studieswere conductedon the population fluctuationsof migratoryplovers and sandpipersand on their useof foraginghabitats along 4 km of coastlinenear Los Alamos, San Antonio Oeste, Rio Negro,in Argentina.Eight species were observed,of which four breedin northernNorth America. The largestflocks (up to 2,550birds) arrivedfrom the NorthernHemisphere in the secondhalf of Novemberand werepresent in significantnumbers until early April. The greatestnumber of birds was recordedin March (8,500 birds),when Calidriscanutus and C.fuscicollis were migratingnorthward. The first monthof the southernwinter was characterizedby the presenceof C. albaand Charadriusfalklandicus, common specieswhose numbers increased substantially thereafter. The mostcommonly used foraging habitatwas rockcovered with patchesof sandysediment and water up to 3 cm deep. Similaritiesin the useof microhabitatsare discussed.The mostsimilar species morphologically (C. albaand C. falklandicus)also showed a positivecorrelation in time. Nestingand the seasonalvariations in the numbersof people,dogs and vehicleswere recorded. The Los Alamoscoast is considereda 'vital site' (Clark1974) for the conservationof migratoryplovers and sandpipers. Entre septiembre1989 y diciembre1990, se estudiaron la fluctuaci6npoblacional y el uso del h&bitatde alimentaci6n de chorlosy playerosmigratorios en 4 km de costacon restinga en las cercanlasde 'Los Alamos',San Antonio Oeste, Rio Negro, Argentina. Se observaron8 especiesde las cuales4 sereproducen en el norte del continenteamericano. Los grupos rmts conspicuos provenientesdes Hemisferio Norte (2 550 ejs.)aparecieron en la 2da. mitad de noviembre registr;indoseabudancias importantes hasta los lros. dias de abril. E1rmtximo de avesocurri6 en marzo,cuando el sistemaalberg6 rmts de 8 500limicolos simult;ineamente: principalmente Calidris canutusy C.fuscicollis en pasomigratorio hacia el HemisferioNorte. E1ler. mesdel inviernoaustral secaracteriz6 por la presenciade C. albay Charadriusfalklandicus,especies frecuentes que entonces exhibieronsu mayor abudancia. El ambientede alimentaci6nrmts utilizado fue la roca cubiertacon peliculasde sedimentosarenosos en machonesy aguahasta 3 cm de profundidad. Se discuten similitudesen el usode microh;ibitats;las especies rmts parecidas estuvieron adem•is correlacionadaspositivamente en el tiempo(C. alba y C.falklandicus). Nidificaci6n y variaci6n temporalen el nfmero de personas,perros y vehiculos,fueron registrados. Se estirna que la costa de LosAlamos representa un sitiovital (Clark 1974)para la conservaci6nde chorlosy playeros migratorios. Entreseptembre 1989 et dEcembre1990, on a •tudiE sur le terrainles fluctuationsde populationsde pluvierset de ScolopacidEsmigrateurs et leur utilisationdes habitats d'alimentation sur une portionlongue de 4 km du littoral du fleuveNegro dansla regionde LosAlamos, San Antonio Oeste,en Argentine.Huit esp•ces,dont quatrese reproduisenten AmEriquedu Nord, ont EtE observEes.Les vols les plus importants-- 2 550oiseaux -- sontarrives de l'hEmisph•reNord au coursdes deux dernitres semaines de novembreet y sontdemeur•s en grandnombre jusqu'au debutd'avril. La plus grandeconcentration a EtEenregistrEe en mars (8 500 oiseaux),au moment o• Calidriscanutus et C.fuscicollis migraient vers le Nord. Le premiermois de l'hiver australa EtE caractEris•par la presencede C. albaet de Charadriusfalklandicus,desespies communesdont le nombrea par la suiteaugmentE. L'habitat d'alimentation le plus frEquemmentutilis• Etait constituEde rochesrecouvertes de plaquesde s•dimentssableux et d'au plus 3 cm d'eau. Les auteurstraitent des similitudes dans l'utilisation des habitats. Lesesp•ces morphologiquement les plus semblables(C. albaet C.falklandicus) ont aussiprEsentE une correlationpositive dans le temps. On a aussienregistrE la densitEde nidificationet les variationssaisonni•res du nombrede personnes,de chienset de vEhicules.On consid•reque la c6te de Los Alamosest une ,,r•gion vitale,, (Clark 1974)pour la conservationdes pluviers et desScolopacidEs. Casillade Correo 84, PedroMoron 384, 8520San Antonio Oeste, Provincial de Rio Negro, Argentina. 93 International Wader Studies 8:93-102 Introduction .exceptin front of ITMAS, whereit is about100 m wide. There are sedimentdeposits ranging from fine sandsto smooth rocks (20 cm diameter) and Despiteits interestingabundance and diversityof mollusc shells. birds,no studieshave yet beenconducted on the avifauna of the coastal area of Bahia de San Antonio and the coastalzone bordering the Gulf of San For moreinformation, see Angulo et al. (1981), Mafias,with the exceptionof censusescarried out GonzalezDiaz & Malagnino (1984),Piola & Scasso by Morrison& Ross(1989). (1988) and the San Antonio Oeste Environmental Charter (Underdirectorate of the Environment, Departmentof Economicsof Rio Negro). The objectivesof this projectwere as follows: With respectto continentalvegetation, the regionis (1) to provideinformation on the compositionand includedin the phytogeographicprovince of El abundancesof shorebirdsthroughout the year Monte (Cabrera1976); with respectto ornitho- alongthe coastnear the LosAlamos geographics,it falls under the coastalprovince TechnologicalInstitute of Mining and (Narosky& Yzurieta 1987). Groundwaters (ITMAS) in San Antonio Oeste, Rio Negro, and Material and methods (2) to describethe partitioningof foraginghabitats by the variousspecies of ploversand sand- Observationswere made every two weeksfrom pipersfor purposesof establishing(i) the September1989 to December1990, except for the importanceof thissite for migratoryshore- first two weeksof March,between 3 p.m. and 8 p.m. birds, (ii) the statusof shorebirdpopulations (later in the summer and earlier in the winter) on prior to the full functioningof a Solvay cleardays with wind speedsunder 40 km/h. I manufacturingplant, whoseconstruction has observed the birds with 16 x 50 binoculars. beenunder way for sometime in Punta Delgado,and (iii) the biologicalgrounds for Patterns of distribution and seasonal seekingpolitical action to minimizethe impact abundance of large enterprisesand proposingmanagement plans. The coastwas studiedat high tide when the shoal was floodedand the species,numbers, behaviour and moultingstatus (where possible)of shorebirds Study area presentwere recorded.Pedestrians, dogs, vehicles and otherdisturbances were alsorecorded. Passing The Los Alamos coast is located on the Gulf of San airplaneswere not included. Mafias(64ø55'W, 40ø45'S), close to the city of San Antonio Oeste and the Las Grutas seaside resort in The reliability of the estimateswas substantiatedby the District of SanAntonio, Provinceof Rio Negro, recountingthe birds afterwardsor by using Argentina. It is the coastalsector that extendsfrom ITMAS to the entrance of the Antoine de Saint photographs.The specieswere groupedaccording to wherethey bred, as described in Myers& Myers ExuperyAirport (Figure1). (1979). The zone is arid and mesothermic with a mean tidal amplitudeof between0.64 and 8.26m. On the coast Partitioning of foraging habitat of LosAlamos, the recedingtide uncoversa shoal Observations were made between 5 November 1989 (up to 1 km) perpendicularto the high-waterline. and 24 November 1990. The zone between the It is a horizontal,smooth, slightly convex, greenish- coastaldunes up to 100m of exposedShoal was yellow surface,which has fracture joints running in selectedfor observations,as the birdsgathered variousdirections in the limoliteof the Patagonia there to feed. Formation(marine Miocene) (Angulo et al. 1981). Threehomogeneous macrohabitats were The study area that was selectedowing to its recognized. Thesewere subdivided into micro- abundanceof birdsincluded the non-sloping habitatsthat were temporary because of draining coastalsector of the PatagoniaFormation. Its upper and dryingthrough the tidal cycle. Thespecies and limit is markedby fixed or shiftingdunes 1.5-3 m numbers of birds in each microhabitat were high, which are higher(10 m) and moreunstable in recordedunder differenttidal conditions(given by front of ITMAS. The intertidalcoastal strip, located roman numerals). When possible,notes were made betweenthe dunesand the exposedshoal, of the numberof birdsat rest(rest being considered correspondsto the area includedbetween the any activityother than foraging). high-tidelimits during spring and neap tides (Figure 2). It has a variablewidth of some50 m, 94 Gonzalez:Plovers and sandpipersin Argentina MEDANC•,.•JI=RA I RE,.q'I'. MEDANO$................. SUPRAMAREAL III III Figure1. Map of the Bahiade SanAntonio and Gulf of San Matias hinterland. The dotted area indicates the B IV A Los Alamos coast, and the dotted line shows the low-tide condition.• RESTINGA The macrohabitats were identified with letters and Figure 2. (A) Diagram of the layout of sectionsin which the microhabitats with Arabic numerals. Habitats the habitatdivision project was carriedout. The large circlesindicate high, shiftingor plant-covereddunes. not usedby ploverswere markedwith a dash. The smallercircles indicate low, plant-covereddunes. The dottedlines showthe height of the tide.