The Western Sandpiper (Calidris mauri) is one of the most abundant shorebirds in northwestern Mexico; however, little is known about its winter ecology in this area. We studied residency patterns of male Western Sandpipers during the winters of 1995–1996 (1995) and 1996–1997 (1996) at Estero Punta Banda in Baja California, Mexico. We resighted 54 birds in 1995 and 56 birds in 1996. Birds arrived later in 1995 (median 1 December) than in 1996 (median 24 October). The median departure dates (6 March 1996 and 20 February 1997) did not differ between years. We observed two patterns of residency. Wintering birds in 1995 arrived on 17 November (median) and departed on 17 March (median), with length of stay of 120.0 ± 4.2 d; and in 1996 arrived on 12 October (median) and departed on 8 March (median), with length of stay of 146.9 ± 4.3 d. Transient birds in 1995 arrived on 7 December (median) and departed on 17 February (median), with stays of 33.7 ± 4.3 d; and in 1996 arrived on 23 October (median) and departed on 3 December (median), with stays of 33.7 ± 4.1 d. Residence time was independent of sighting effort. Both wintering birds and transients exhibited site fidelity between years. Both categories were independent with respect to age, trapping month, or year. The mid-season departures were not correlated with either age or the banding period. Older birds were more likely to depart earlier and switch their residency pattern from wintering to transient. While the ecological significance of variation in residency patterns remains unknown, evidence from this and previous studies suggest that such variation may be relatively common, with important implications for studies of shorebird populations and conservation strategies.
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Vol. 72 • No. 4