AGE CLASS SEPARATION OF BLUE-WINGED DUCKS

Accurate determination of age is of fundamental importance to populati on and life history studies of waterfowl and their management. Therefo re, we developed quantitative methods that separate adult and immature blue-winged teal (Anas discors), cinnamon teal (A. cyanoptera), and n orthern shovelers (A. clypeata) during spring and summer. To assess su itability of discriminant models using 9 remigial measurements, we com pared model performance (% agreement between predicted age and age ass igned to birds on the basis of definitive cloacal or rectral feather c haracteristics) in different flyways (Mississippi and Pacific) and bet ween years (1990-91 and 1991-92). We also applied age-classification m odels to wings obtained from U.S. Fish and Wildlife Service harvest su rveys in the Mississippi and Central-Pacific flyways (wing-bees) for w hich age had been determined using qualitative characteristics (i.e., remigial markings, shape, or wear). Except for male northern shovelers , models correctly aged < 90% (range 70-86%) of blue-winged ducks. Mod el performance varied among species and differed between sexes and yea rs. Proportions of individuals that were correctly aged were greater f or males (range 63-86%) than females (range 39-69%). Models for northe rn shovelers performed better in flyway comparisons within year (1991- 92, La. model applied to Calif. birds, and Calif. model applied to La. birds: 90 and 94% for M, and 89 and 76% for F, respectively) than in annual comparisons within the Mississippi Flyway (1991-92 model applie d to 1990-91 data: 79% for M, 50% for Fl. Exclusion of measurements th at varied by flyway or year did not improve model performance. Quantit ative methods appear to be of limited value for age separation of fema le blue-winged ducks. Close agreement between predicted age and age as signed to wings from the wing-bees suggests that qualitative and quant itative methods may be equally accurate for age separation of male blu e-winged ducks. We interpret annual and flyway differences in remigial measurements and reduced performance of age classification models as evidence of high variability in size of blue-winged ducks' remiges. Va riability in remigial size of these and other small-bodied waterfowl m ay be related to nutrition during molt.