Tropical and Subtropical Western Sandpipers (calidris Mauri ) Differ in Life History Strategies

– The Western Sandpiper (Calidris mauri) is one of the most common shorebirds in the western hemisphere, with a global population of about of 3–3.5 million birds. The species breeds primarily in western Alaska, and spends the non-breeding season along the Pacific coast from California to Perú, and the Atlantic coast from North Carolina to Surinam. The largest concentrations occur in northwestern México and in Panamá. Males spend the winter farther north than females and juveniles are disproportionately represented on the northern and southern edges of the distribution. We recently documented a latitudinal difference in life-history tactics of wintering Western Sandpiper. In northwestern México, nearly all sandpipers prepare for northward migration by gaining mass and molting into alternate plumage. In Panamá, most adults, but few, if any, first-year birds do so. Instead, they spend their yearling summer in predominantly basic plumage on the non-breeding grounds. To compensate for not breeding during their first year, we expected more southerly birds to have higher survivorship rates. As predicted, mark-recapture studies show that the apparent annual survival rate of male western sandpipers at Punta Banda, México was lower than that in Chitré, Panamá ______________ Current address: Institute of Ocean Sciences, P.O. Box 6000, 9860 West Saanich Road, Sidney, British Columbia, Canada V8L 4B2. E-mail: [email protected].