Many biological studies rely on accurate sex determination of individuals to investigate sex-specific differences in wild populations. Numerous species can be reliably sexed in the field using external characteristics such as plumage color and ornamentation or in the hand using breeding characteristics such as a brood patch (female) or cloacal protuberance (male). However, plumage differences may not be present year-round and breeding characteristics are not present outside of the breeding season. This is a problem for researchers as they may not be able to reliably sex individuals in the field outside of the breeding season. However, monochromatic species often exhibit sex differences in morphometrics including wing chord length and tarsus length. Recent studies have found that the Eastern (leucophrys) and Puget Sound (pugetensis) subspecies of the White-crowned Sparrow (Zonotrichia leucophrys) can be reliably sexed using wing chord length. The Gambel’s (gambelii) subspecies was last studied in 1985 and the Nuttall’s (nuttalli) subspecies has not been tested. We combined wing chord length, tarsus length, and sex data from adult Gambel’s and Nuttall’s White-crowned Sparrows to determine if these subspecies could be reliably sexed using wing chord length and/or tarsus length. We also determined if the migratory Gambel’s subspecies and resident Nuttall’s subspecies varied morphologically. We found that the Gambel’s and Nuttall’s subspecies could be reliably sexed using wing chord length outside overlapping ranges of 74–78 and 69–77 mm, respectively. We also found that the Gambel’s subspecies had longer wing chord lengths and shorter tarsus lengths than the Nuttall’s subspecies. Our findings provide a useful tool for sex determination in Gambel’s, but not Nuttall’s, White-crowned Sparrows. Further, our findings show that migratory and resident individuals within the same species vary morphologically, which may relate to the different selective pressures they face.

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