Migratory Birds Make

sions about site settlement on both breeding and nonbreeding grounds. Each year a fraction of surviving individuals return to sites occupied in previous years. The accumulation of capture histories, and the widespread distribution of capture–recapture software to analyze such data (Lebreton et al. 1992) have lead to a proliferation of estimates of apparent annual survival rates for migratory birds. Apparent survival rates measure the probability that an individual will both survive over a time period, and if alive, return to a site it previously occupied. In this study, we estimate apparent survival rates of male Western Sandpipers (Calidris mauri) wintering at an isolated estuary in northwestern Baja California, Mexico. Western ABSTRACT.—To estimate annual apparent local survival, we collected capture–resighting data on 256 individually marked male Western Sandpipers (Calidris mauri) wintering at Estero de Punta Banda, Mexico, between 1994–1997. A hierarchical modeling approach was used to address the effect of age class and year on survivorship rates. The best-fi t model included a constant apparent survival probability ( = 0.489; 95% CI = 0.410–0.569), but several models fi t nearly as well, and averaging among the top fi ve, to account for model uncertainty, suggested that adults had somewhat higher values than juveniles ( = 0.490 0.051 vs. 0.450 0.067). Detection probability was substantially higher for adults than for juveniles (p = 0.741 vs. p = 0.537). Those apparent survival estimates are low compared with those from other studies of Western Sandpipers at breeding and other nonbreeding locations, and substantially lower than the true survivorship rates expected for small sandpipers in general. We interpret these results as indicating that this site is of below average quality for nonbreeding male Western Sandpipers. Received 16 October 2001, accepted 12 September 2002.