A quantitative technique was developed to distinguish adult from yearling ring-necked ducks (Aythya collaris) during the breeding season using wing feather criteria. This method requires removal and measurement of the 5th secondary (S 5) and 12th greater secondary covert (GSC 12). Bursal and other plumage characteristics were evaluated for use in the field during the breeding season. Cloacal characteristics are of no value in distinguishing adult and yearling males after arrival on breeding areas (Apr), but bursal depth >10 mm is characteristic of yearling females into their Ist breeding season. Although not all adult females have white head feathers (e.g., flecking), the presence of flecking on mature females strongly indicates adult status. J. WILDL. MANAGE. 50(3):442-445 The ability to distinguish age classes within waterfowl populations is important in understanding aspects of their reproductive biology and population dynamics (Serie et al. 1982). However, determining age classes of waterfowl that reproduce in their 1st year often is difficult, especially during the breeding season. Cloacal (Gower 1939) and rectrix (Pirnie 1935) characteristics are commonly used during the fall and winter to distinguish immatures from adults but are unreliable for birds captured in spring (Hochbaum 1942). Use of qualitative wing feather criteria for determining age class and sex is possible for some species: mallards (Anas platyrhynchos) (Krapu et al. 1979, Gatti 1983), blue-winged teal (A. discors) (Dane 1968), American wigeons (A. americana) (Wishart 1981), canvasbacks (Aythya valisineria) (Serie et al. 1982), and redheads (A. americana) (Dane and Johnson 1975). Unfortunately, these methods do not work well for ring-necked ducks (Carney 1964). Remiges and associated coverts are retained by juvenile waterfowl until the prebasic II molt and are replaced annually thereafter during the prebasic molt (Humphrey and Clark 1961). Multivariate statistical procedures using wing feather measurements have been used to determine age of springand summer-caught mallards (Krapu et al. 1979, Gatti 1983), gadwalls (Anas strepera) (Blohm 1977), American wig ons (Wishart 1981), canvasbacks (Serie et al. 1982), and redheads (Dane and Johnson 1975). Here we describe a quantitative technique for accurate age class separation of male and fema e ring-necked ducks in spring and early summer. Additionally, bursal and other plumage characteristics are evaluated for use in the field during the breeding season. We wish to thank B. D. J. Batt, R. D. Crawford, and M. W. Weller for their helpful comments on earlier drafts of the manuscript. We were assisted in the field or laboratory by J. S. Barzen, D. O. Carlson, L. A. Fluggeman, and J. W. Grimm. Financial support was provided by the North Am. Wildl. Found. through the Delta Waterfowl and Wetland Res. Stn., Univ. Minnesota (Dep. Fish. and Wildl., Comput. Cent., Agric. Exp. Stn., and Graduate School), Minn. Dep. Nat. Resour., and Minn. Waterfowl Assoc. This is J. Pap. 13837 of the Minn. Agric. Exp. Stn., St. Paul, Minn.