There is limited information on the age and sex composition of shorebirds (Charadrii) at migratory stopover points between the breeding and wintering grounds (Jehl, 1963; Mascher, 1966; Page and Bradstreet, 1968) and on the wintering grounds (Holmes, 1966). There are few comparable data for the Western Sandpiper (Calidris mauri). We have been collecting data on the age and sex composition of this species at different times of the year on Bolinas Lagoon, Marin County, California. Our data show that adult and immature birds migrate at different times in the fall and that males and females migrate at different times in the spring. Further, we found that male and female Western Sandpipers have somewhat different wintering areas; to our knowledge, this is the only sandpiper for which such evidence currently exists.
The Western Sandpiper breeds chiefly in Alaska and winters from California and the southern United States to Venezuela and Peru. Details of the distribution can be found in Bent, 1927; Palmer, 1967; and Holmes, 1971.