Eye Colour, Aging, and Decoy Trap Bias in Lesser Scaup, <em>Aythya affinis</em>

Abstract

Researchers routinely assume that samples of trapped or captured animals are representative of the overall population, though these assumptions are not always evaluated. We used decoy-trapped Lesser Scaup (Aythya affinis) to assess the reliability of classifying females as yearlings or adults from a distance, based on documented age-related eye-colour changes, and also to evaluate the presence of sex, condition and age biases in decoy trapping. We compared eye colour of trapped females to photographs of known-age females following a published procedure while females were (1) in traps (by using spotting scopes or binoculars) and (2) in-hand. Assuming in-hand age assessments were correct, we found that adults aged from a distance were frequently misclassified as yearlings, but yearlings were never misclassified as adults. Distance between observer and female, overall observation quality, and cloud cover did not influence age assignment success. A larger proportion of males was captured than observed during a survey of the local breeding population. We also found that decoy-trapped females had lower body mass and were more likely to be yearlings compared to pass-and jump-shot females from the same area. We conclude that female Lesser Scaup cannot be accurately aged from a distance using eye colour and concur with other researchers that possible sex, age and condition biases should be evaluated when using decoy traps.

Annual survival in female white‐winged scoters and lesser scaup

“…For scoters, non‐egg‐producing females were on average lighter than egg‐producing females and tended to have bursa present (S. M. Slattery, unpublished data), indicating that they were younger or reproductively inexperienced birds (Mather and Esler ). For scaup, eye color (Trauger ) indicated that our decoy‐trapped females were on average younger than nest‐trapped birds (S. M. Slattery, unpublished data), consistent with previous findings of decoy trap bias toward younger or poorer quality individuals (Weatherhead and Greenwood , Fast et al ). In addition, we detected evidence of subsequent nesting in only a few decoy‐trapped scaup in the year of trapping (S. M. Slattery, unpublished data), much like Martin et al ().…”