922 The Auk 119(4):922–928, 2002 WESTERN SANDPIPERS (CALIDRIS MAURI) DURING THE NONBREEDING SEASON: SPATIAL SEGREGATION ON A HEMISPHERIC SCALE SILKE NEBEL, 1,12 DAVID B. LANK, 1 PATRICK D. O’HARA, 1 GUILLERMO FERNA ´ NDEZ, 1 BEN HAASE, 2 FRANCISCO DELGADO, 3 FELIPE A. ESTELA, 4 LESLEY J. EVANS OGDEN, 1 BRIAN HARRINGTON, 5 BARBARA E. KUS, 6 JAMES E. LYONS, 7 FRANCINE MERCIER, 8 BRENT ORTEGO, 9 JOHN Y. TAKEKAWA, 10 NILS WARNOCK, 11 AND SARAH E. WARNOCK 11 1 Department of BioSciences, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia V5A 1S6, Canada; 2  FEMM, casilla 0901, 11 905 Guayaquil, Ecuador; 3 C. Meliton Martin, N 32-94, Chitre ´, Herrera, Republic of Panama ´; 4 Asociacio ´n Calidris, Kra 24F oeste 3-110, Cali, Colombia; 5 Manomet Center for Conservation Sciences, P.O. Box 1770, Manomet, Massachusetts 02345, USA; 6 U.S. Geological Survey, Western Ecological Research Center, San Diego Field Station, 5745 Kearny Villa Road, Suite M, San Diego, California 92123, USA; 7 Department of Zoology, North Carolina State University, Raleigh, North Carolina 27695-7617, USA; 8 Parks Canada, 24 Eddy Street, Hull, Quebec K1A 0M5, Canada; 9 Texas Parks and Wildlife Department, 2805 North Navarro, Suite 600B, Victoria, Texas 77901, USA; 10 U.S. Geological Survey, Western Ecological Research Center, San Francisco Bay Estuary Field Station, P.O. Box 2012, Vallejo, California94592, USA; and 11 Point Reyes Bird Observatory, 4990 Shoreline Highway, Stinson Beach, California 94970, USA ABSTRACT.—The nonbreeding distribution of Western Sandpipers (Calidris mauri) was doc- umented using 19 data sets from 13 sites along the Pacific and Atlantic coasts of the Amer- icas. Western Sandpipers showed latitudinal segregation with regard to sex and age. Females wintered farther south than males. A ‘‘U’’ shaped pattern was found with respect to age, with juveniles occurring at higher proportions at both the northern and southern ends of the range. Distribution of sexes might be affected by differences in bill length and a latitudinal trend in depth distribution of prey. For age class distribution, two different life-history tac- tics of juveniles might exist that are related to the higher cost of feather wear for juveniles compared to adults. Most juveniles complete three long-distance migrations on one set of flight feathers whereas adults complete two. Juveniles may winter either far north, thereby reducing feather wear induced by ultraviolet light, migration, or both, or far south and spend the summer on the nonbreeding area. Received 30 October 2001, accepted 8 July 2002. RESUMEN.—Determinamos la distribucio ´n de Calidris mauri durante la e ´poca no repro- ductiva utilizando 19 bases de datos de 13 sitios localizados en las costas del Pacı ´fico y At- la ´ntico del Continente Americano. Calidris mauri presento ´ una distribucio ´n latitudinal di- ferencial por sexo y edad. Las hembras invernan ma ´s al sur que los machos. Encontramos un patro ´n en forma de U con respecto a la edad, con una mayor proporcio ´ n de juveniles en los extremos norte y sur del intervalo. La distribucio ´n de los sexos puede estar en funcio ´na la diferencia en la longitud del pico y el patro ´n latitudinal en la profundidad de las presas. La distribucio ´ n de las edades puede estar relacionada a un mayor costo en el desgaste de las plumas en juveniles en relacio ´n a los adultos. Los juveniles que se reproducen ensu primer verano llevan a cabo tres migraciones con un juego de plumas mientras que los adultos solo dos. En juveniles pueden existir dos ta ´cticas de historias de vidas diferentes donde los ju- veniles invernan ma ´s al norte y se reproducen en el primer verano, reduciendo ası ´ el desgaste de las plumas producido por la luz ultravioleta y/o la migracio ´n, o invernan ma ´s al sur y veranear en a ´reas no reproductivas. ‘‘DIFFERENTIAL MIGRATION’’ REFERS to either differences in timing of migration or to spatial 12 E-mail: snebela@sfu.ca distribution of age or sex classes within a spe- cies during the nonbreeding season (Myers 1981a, Gauthreaux 1982, Ketterson and Nolan 1983, Hockey et al. 1998, Turpie 1994, Cristol et al. 1999). According to a recent review, that tac-