THE ROLE OF WING LENGTH IN THE EVOLUTION OF AVIAN FLIGHTLESSNESS

Flightlessness has evolved independently in at least 11 extant avian f amilies. A number of hypotheses have been proposed to explain these tr ansitions in individual families, including release from predation on oceanic islands, energetic costs of flight and use of forelimbs for ac tivities other than flying. Few studies have sought to explore factors common to all families containing flightless species, which may expla in the taxonomic distribution of flightlessness. In this study, we fou nd that for all eight avian families which contain both flightless and flighted species, the flighted species have shorter wing lengths rela tive to body mass than their sister families. This result is not biase d by taxon size. Models of avian aerodynamics predict that birds with relatively short wings pay a high energetic cost of flight. We suggest that these increased energetic costs of flying predispose these avian families to evolve flightless species. The various causes for the sho rtening of wings among flighted species of birds and the possibility o f future transitions to flightlessness are discussed.