Flight feathers evolve fastest near the tip of the wing

Form-function relationships are foundational to evolutionary biology, but we are only beginning to understand the mechanisms that link biomechanical function to morphological evolution. In bird wings, the pace of morphological evolution follows a gradient of aerodynamic force production along the wing, with the fastest rates experienced at the wingtip. We examined how this gradient has manifested in the evolution of the individual flight feathers that comprise the wing surface. Specifically, we studied whether the flight feathers show a pattern of allometric scaling with respect to avian body size, and if the allometric pattern varies along the wing. We also asked whether the pace of evolution of the primary flight feathers follows the same gradient pattern as the overall wing morphology, and if there is modularity within the feather series. We measured the lengths of each of the primary flight feathers from 509 wings representing 514 bird species and calculated morphological disparity and evolutionary rate for each feather in the primary sequence. We found that evolutionary rate and disparity increase significantly toward the wingtip. We also found evidence for modularity within the feather sequence, but we were unable to positively delineate those modules. Our study provides further evidence that the mechanical sensitivity of morphological traits predicts their evolution, with highly sensitive features biased toward higher rates, and that it scales across multiple levels of anatomical organization.