Size of the ulna and taxonomic order affect vane dimensions of secondary remiges in birds

Abstract

The number of secondary feathers varies among orders of birds with some orders exhibiting a positive relationship with ulna length, whereas in other orders secondary number is invariant. This difference has implications for scaling of the width of the feather vane within orders. In those species where the number of secondary remiges is invariant with ulna length, vane width should scale isometrically with ulna size to maintain an aerodynamic flight surface. Where feather count increases with increasing ulna length then vane width should exhibit negative allometry. Vane length should also correlate with ulna length, irrespective of the number of feathers. Data were compiled from an online library of images for the vane length and the width of the vane at 50% of the vane length for the fifth secondary feather for 209 bird species from 24 different orders. The results supported the hypotheses that vane width is a function of ulna size, and the number of secondary feathers as associated with different orders. Vane length was unaffected by the number of secondaries but varied between orders. The results suggest that birds have solved the problem of maintaining the aerodynamic surface of the proximal wing in two ways. Hence as ulna length increases the first solution involves more feathers that exhibit negative allometry for vane width, or in the second where feather count doesn't change, the vane width simply scales isometrically. The implications for the mechanical properties of the vane, and how it affects wing function, have not yet been explored in a range of birds.