MORPHOLOGICAL ADAPTATIONS TO SONG FLIGHT IN PASSERINE BIRDS - A COMPARATIVE-STUDY

Song flight by birds is one of the most energetically expensive results of sexual selection, and behavioural or morphological features which reduce the magnitude of this cost should therefore be selectively advantageous. We use aerodynamical flight theory to predict how changes in morphology affect flight performance during song flight display. If manoeuvrability and gliding flight during aerial displays were important, this should favour reduced body size and increased wing span and wing area. If flapping flight endurance, climb rate, acceleration or maximum speed in gliding dive are important, selection should favour reduced body size and increased wing span, but not increased wing area. Selection should favour increased aspect ratio and decreased wing loading in most cases. These predictions were evaluated by using morphological data on males and females from eight pairs of related passerine species in which males of one species perform song flight but males of the other do not. Song flight was not associated with a reduction in body size, but wing span, wing area and aspect ratio increased, and wing loading decreased in males of species with song flight. This suggests that manoeuvrability and flapping flight performance have been the major selective forces affecting the morphology of males of species with song flight.