Evaluation of bursal depth as an indicator of age class of Harlequin Ducks

We contrasted the estimated age class of recaptured Harlequin Ducks (Histri onicus histrionicus) (n = 255) based on bursal depth with expected age clas s based on bursal depth at first capture and time since first capture. Alth ough neither estimated nor expected ages can be assumed to be correct, rate s of discrepancies between the two for within year recaptures indicate samp ling error, while between-year recaptures test assumptions about rates of b ursal involution. Within-year between year, and overall discrepancy rates w ere 10%, 24%, and 18%, respectively. Most (86%) between-year discrepancies occurred for birds expected to be after-third-year (ATT) but estimated to b e third-year (TY). Of these ATY-TY discrepancies, 22 of 29 (88%) birds had bursal depths of 2 or 3 mm. Further, five of six between year recaptures ch at were known to be ATY but estimated to be TY had 2 mm bursas. Reclassifyi ng birds with 2 or 3 mm bursas as ATY resulted in reduction in between-year (24% to 10%) and overall (18% to 11%) discrepancy rates. We conclude that age determination of Harlequin Ducks based on bursal depth, particularly us ing our modified criteria, is a relatively consistent and reliable techniqu e.