The relationship between wing kinematics, wing morphology and the brachial index of birds (BI=humerus length/ulna length) was examined. BI was found to differ between three groups of birds, which were classified on the basis of similar wing kinematics. In addition, a comparative analysis of a large dataset, using phylogenetically independent contrasts, suggested a significant, albeit weak, correlation between BI and four measures of wing morphology (wing loading, wing area, wing length and aspect ratio). Although wing kinematics and wing morphology are both correlated with BI in birds, the dominant selective pressure upon this ratio is probably wing kinematics. The previously identified clade specificity of BI within Neornithes is most likely because birds with similar BIs fly with kinematic similarity and closely related birds have similar flight styles. A correlation between BI and wing kinematics means that it may be possible to characterize the wing beat of fossil birds. A more robust relationship between wing morphology and BI may emerge, but only after the relationship between wing kinematics and BI is quantified. A comparative and quantitative study of wing-bone anatomy and wing kinematics is a priority for future studies of avian wing-skeleton evolution and functional morphology.