—The nonbreeding distribution of Western Sandpipers (Calidris mauri) was documented using 19 data sets from 13 sites along the Pacific and Atlantic coasts of the Americas. 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 tactics 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 distribución de Calidris mauri durante la época no reproductiva utilizando 19 bases de datos de 13 sitios localizados en las costas del Pacı́fico y Atlántico del Continente Americano. Calidris mauri presentó una distribución latitudinal diferencial por sexo y edad. Las hembras invernan más al sur que los machos. Encontramos un patrón en forma de U con respecto a la edad, con una mayor proporción de juveniles en los extremos norte y sur del intervalo. La distribución de los sexos puede estar en función a la diferencia en la longitud del pico y el patrón latitudinal en la profundidad de las presas. La distribución de las edades puede estar relacionada a un mayor costo en el desgaste de las plumas en juveniles en relación 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 tácticas de historias de vidas diferentes donde los juveniles invernan más al norte y se reproducen en el primer verano, reduciendo ası́ el desgaste de las plumas producido por la luz ultravioleta y/o la migración, o invernan más al sur y veranear en áreas no reproductivas. ‘‘DIFFERENTIAL MIGRATION’’ REFERS to either differences in timing of migration or to spatial 12 E-mail: firstname.lastname@example.org distribution of age or sex classes within a species 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 tacOctober 2002] 923 Differential Distribution of Western Sandpipers tic occurs in the majority of migratory bird species (Cristol et al. 1999). The Western Sandpiper (Calidris mauri) is well suited for the study of differential migration. It is one of the most common shorebirds in North America, with the latest total population estimate being 3.5 million (Bishop et al. 2000, Morrison et al. 2001). Most of the population breeds in western Alaska, whereas during the nonbreeding season, its range extends along the American Pacific coast from southern Canada to Peru, and, to a lesser extent, along the east coast of the Americas (Wilson 1994). During winter, the largest concentrations occur in western central Mexico and in Panama (Morrison and Ross 1989; Morrison et al. 1993, 1994, 1998, 2001). No subspecies are recognized (Wilson 1994). Recently, a latitudinal difference in Western Sandpiper life-history tactics was discovered. Most juveniles wintering in California and western Mexico gain fat and migrate north in their first spring (N. Warnock and G. Fernández unpubl. data). In contrast, nearly all juvenile Western Sandpipers wintering at Chitré, Panama, do not undergo hyperphagia, prepare for northward migration, or molt into significant amounts of alternate plumage. Instead, they spend their yearling summer in predominantly basic plumage on the nonbreeding grounds (O’Hara et al. 2002). We assume that the difference between those sites reflects a latitudinal trend within Western Sandpipers, with higher propensity to oversummer the farther south the birds winter. Across shorebird species, a higher propensity to oversummer occurs in those with longer migratory flights (Summers et al. 1995, Hockey et al. 1998). Females are more abundant towards the southern part of the nonbreeding area (Page et al. 1972, Harrington and Haase 1994, Naranjo et al. 1994, Buenrostro et al. 1999), but no information on the distribution of age classes is documented. Here, data are combined from sites across the entire nonbreeding range, allowing us to present an account of the nonbreeding sex and age distribution and to suggest possible explanations for the observed patterns. METHODS AND STUDY SITES We used 19 data sets gathered from 13 locations (Table 1). To restrict comparisons to residents, only birds caught between November and February were included in the analyses. All sandpipers were caught with mist nets, and we assumed capture did not produce a bias towards a sex or age class that would vary among sites. Sex was assigned on the basis of bill length (Page and Fearis 1971). Given that Page and Fearis (1971) analyzed birds collected in California only, the method was verified by internally sexing over 250 Western Sandpipers from Panama and British Columbia, of which only one was misidentified using bill length (C. G. Guglielmo pers. comm.). Juveniles (birds in their first winter) were distinguished from adults on the basis of plumage (Prater et al. 1977) at all sites except Florida, which was not included in the age analysis. Second-winter birds, if recognized, were treated as adults. By comparing bill lengths of adults and juveniles on fall migration in the Fraser River Delta, British Columbia, we demonstrated that juvenile bill lengths were full adult length on southward migration (mean bill length of juvenile males: 22.65 mm, n 5 590, adult males: 22.55 mm, n 5 857, t 5 21.02, df 5 1 and 445, P 5 0.31; juvenile females: 26.75 mm, n 5 847, adult females: 26.80 mm, n 5 342, t 5 0.91, df 5 1 and 187, P 5 0.36). This demonstrates that using bill length to assign sex in juveniles will not create a bias towards males. As a measure of migration distance, we calculated the Great Circle Route (in kilometers, see Acknowledgments) from the breeding grounds, represented by Nome, Alaska (Wilson 1994), to the nonbreeding site. Migration distance correlated tightly with latitude (r2 5 0.92, P , 0.001). Statistical Analysis. Relative proportions of each age and sex class were analyzed as a function of distance from breeding grounds, using analysis of covariance (ANCOVA). Class frequencies were modeled as binomial probabilities, using a linear model with a logit link function. Preliminary analysis indicated overdispersion in the data (reflecting strong heterogeneity in sample size), so we used a quasilikelihood adjustment by rescaling the parameter estimates using model deviance divided by the model degrees of freedom. Relative proportions of all age and sex classes were arcsin transformed prior to analysis to stabilize variance. Analyses of variance were performed using GENMOD, SAS, version 8.1 (SAS Institute 2001).