Plumage and body size differentiation in Blue-winged teal and Cinnamon teal

Blue-winged Teal (Anas discors) and Cinnamon Teal (A. cyanoptera septentrionalium) are two closely related North American dabbling duck species that are ecological equivalents. Cinnamon Teal are primarily restricted to regions west of the Great Plains, whereas Blue-winged Teal occur primarily in the central and eastern part of the continent, only recently expanding westward. Males of the two species exhibit striking plumage color differences, while females are difficult to differentiate by plumage, and it is unclear if interspecific size differences exist between either sex due to inconsistencies in previous reports. Here we reassess body size differences, and quantify plumage color differences using avian color discrimination modeling. Like previous studies, significant differences were found for bill morphology; mean bill length was 7-10% longer in Cinnamon Teal. Based on avian visual modeling of plumage reflectance data, color differences not visible to human vision were found between species for several female feather patches (e.g. breast coloration) and male wing speculum coloration, potentially representing previously unrecognized interspecific signals to the avian visual system. Although color reflectance data yielded higher accuracy than morphometrics for identifying females, body size measurements (in addition to plumage to a lesser degree) also proved to be reliable in correctly classifying males of each species. In combination, morphometrics and plumage reflectance data represent useful identification tools for avian species that are otherwise difficult to distinguish.