While banding Anna’s Hummingbirds (Calypte anna) on the Palos Verdes Peninsula, southern California, between 1970 and 1976 (Wells et al. 1978), Wells noted three predominant color patterns on the outermost rectrices of the birds handled (Figure 1). By pulling the outermost rectrix on one side of fledglings and then subsequently recapturing these marked individuals and noting the shape, color, and pattern of the regrown rectrices, she confirmed that these patterns were age- and sex-specific. This method of sexing may be applied to nestlings as well as fledglings and adults. We describe these differences for banders and population biologists interested in sex ratios or life tables to enable them to determine the sex and age of Anna’s Hummingbirds. Our confirming data are based on an examination of 121 specimens in the collections of the California Academy of Sciences (CAS), San Francisco, California, and the Museum of Vertebrate Zoology (MVZ), Berkeley, California. These included 79 females and 42 males.