Data for Determining Age of Barrow's Goldeneye (Bucephala islandica) from External Characteristics, Alaska and Canada, 2007-2015

We captured Barrow’s goldeneyes in flight with floating mist nets, placed captured birds in small kennels, transported them to shore for processing, and marked each bird with a uniquely numbered metal tarsal band. We determined the sex of each captured bird based on plumage and cloacal characteristics. Because we did not know the ages of captured birds, we measured the bursa of Fabricius (hereafter bursa) of each individual, using these measurements as estimates of age class. For sea ducks, the degree of bursal involution provides an accurate measure of age, especially when used to separate first-year birds (0–1 yr of age) from adults (>1 yr of age). We measured bursal depth (± 1 mm) by inserting a blunt metal probe into the bursal sac and categorized all Barrow’s goldeneyes with bursal depths >10 mm as first-year birds and <10 mm as adults. Additionally, we documented presence or absence of notched tail feathers for each captured bird as a secondary benchmark of age class.

We photographed the head, wing, and belly of each captured bird. From these photographs, as well as previous descriptions of Barrow’s goldeneye plumage, we focused on 5 external characteristics, for both males and females, with binary states that putatively corresponded to age class. For males, we used the following 5 characteristics: (1) Belly color – all white or edged with gray; (2) Head color – Purplish-black or brown; (3) Eye color – yellow or brown; (4) Facial crescent – full or partial; (5) Median secondary coverts – white or black. For females, we used the following 5 characteristics: (1) Belly color – all white or edged with gray; (2) Bill radiance – bright or dull; (3) Bill blackness – <50% black or >50% black; (4) Eye color – yellow or brown; and (5) Median secondary coverts – white or black.

We assessed the ability of 20 novice observers, after receiving training on external markers of age, to estimate age classes of Barrow’s goldeneyes. For our training, we provided observers a manual that highlighted the various external characteristics for each age and sex combination, and observers were given unlimited time to review the manual. We restricted our training to external markers of the head because these markers are most useful for field-based age surveys of Barrow’s goldeneyes, in which observers view birds from a distance through magnifying optics. After completing training, observers viewed a slideshow of 100 photographs of Barrow’s goldeneyes, having 10 seconds per photo to estimate sex and age class. We randomly selected 25 photographs per age-sex cohort for inclusion in the slideshow, and we used only photographs from our capture efforts. Lastly, to assess accuracy of observers’ age classification, we compared their results to bursal-derived age classes.