Phylogenetics of primary feather emargination

Phylogenetics of primary feather emargination

Wing-tip slots are a distinct morphological trait observed broadly across the avian clade, yet generally perceived to be unique to soaring terrestrial raptors.  These slots are the result of emarginations on the distal leading and trailing edges of primary feathers, and are thought to allow the feathers to flex independently of one another.  Previous research suggests that these emarginated feathers mitigate induced drag by distributing the wing-tip vortex vertically off the wing in a manner similar to winglets on aircraft.  Here we use a broad phylogenetic sample of 135 species to test for correlations between emargination and independent variables including body mass, behavioral ecology (foraging style and habitat), and whole-wing morphology.  Casual observation suggests soaring marine birds exhibit pointed, high-aspect ratio wings without slots, while large, soaring terrestrial birds exhibit prominent wing-tip slots.  Thus, we hypothesized that 1) emargination would segregate according to habitat, 2) mass of terrestrial species would be positively correlated with emargination, and 3) emargination would vary according to behavioral metrics including flight style (e.g. soaring, flapping, bounding) and foraging style (e.g. skimming, diving).  Our results show that mass is significantly and positively correlated with emargination, which we infer may be the result of mass-specific induced power requirements increasing with body size.  Although emargination varied as predicted with habitat and other behavioral metrics, the results were not statistically significant when accounting for phylogeny because phylogenetic affiliation is confounded with ecology and behavior in our sample.  Thus, our results lend tentative support to a hypothesis that the morphology of primary feathers falls along a spectrum where dynamic-soaring marine species tend to have zero emargination and emargination among terrestrial species scales positively with increasing mass.  The confounding of phylogeny with ecology and behavior in our sample means that future tests of ecomorphological patterns should focus on closely-related species that vary significantly in their ecology and flight behavior.