Although variation in microscopic plumulaceous (downy) feather characters is known to be useful in taxonomic identifications of birds, the conserved characters unique to most avian orders remain understudied. We examined plumulaceous feather characters (morphometric and observed pigmentation patterns [qualitative]) within three avian families (Cathartidae, Pandionidae, Accipitridae) occurring in North America that often require taxonomic identification based on incomplete or fragmentary remains. We found significant quantitative differences among these three families for measurements of barbule length, node width, average number of nodes per barbule, and internode length. We observed additional differences in pigmentation patterns and spine distribution at nodes. Differences in pigment patterns and intensity are diagnostic for distinguishing Black Vulture (Coragyps atratus) from Turkey Vulture (Cathartes aura) within Cathartidae. Further observed differences of a higher percentage of spined structures present at node junctions along the length of barbules are diagnostic of Pandionidae. Within Accipitridae, pigmentation patterns and pigment intensity separate Swallow-tailed Kite (Elanoides forficatus) and White-tailed Kite (Elanus leucurus) from all other taxa; and barbule length, together with average nodes per barbule is unique to Northern Harrier (Circus hudsonius) in the families examined in this study. Although significant differences in a combination of microscopic feather characters among species were seldom observed in this study, family level differences were consistently documented. Results support the use of a suite of microscopic characters in combination with macroscopic feather features, geographic distributions, molecular methods, and other circumstantial evidence to aid in the identification of species of birds from feathers.

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