Continuous, age-related plumage variation in male Kirtland's Warblers

ABSTRACT.   The ability to age individual birds visually in the field based on plumage variation could provide important demographic and biogeographical information. We describe an approach to infer ages from a distribution of plumage scores of free-ranging male Kirtland's Warblers ( Dendroica kirtlandii ). We assigned ages to males using a scoring scheme (0–12 points) based on variation in plumage coloration, brightness, and contrast on three dorsal and three ventral body regions presumed to be age-related. The distribution of total additive plumage scores for 875 breeding males was normally distributed, indicating no distinct age classes. Thus, we developed provisional plumage-age classes of second year (SY) and after second-year (ASY), and compared them to the total plumage scores of a smaller subsample of known age ( N = 92) and minimum age ( N = 143) males. Plumage scores of known-age male Kirtland's Warblers increased nonlinearly with age ( r _s= 0.67), but with some overlap. The median plumage score for SY males (median = 5.0) was significantly lower than for third-year (TY) males (median = 7.0) and after third-year (3 year and older) males (median = 8.0), indicating that the plumage of male Kirtland's Warblers becomes more distinctive and brighter with age. Linear discriminant function analysis differentiated ASY male Kirtland's Warbler from SY males with 78.3% accuracy. Investigators could use the distribution of plumage scores and approximate age structures to document changes in male age structure during colonization, use, and abandonment of habitats by Kirtland's Warblers or other species that occupy early successional habitats. Aging free-ranging birds based on a plumage scoring scheme may be especially critical for demographic studies of less-studied species where it is unlikely that a banding program will be initiated, but where plumage-age inferences or management decisions must be made.