Scaling of muscle composition, wing morphology, and intermittent flight behavior in woodpeckers

Existing theory and empirical evidence suggest that body size should have a profound influence upon the composition of flight muscles and the performance of intermittent flight in birds. I examine the relationships between functional morphology and intermittent flight behavior within a closely-related group of birds using six species of woodpeckers (Picidae): Downy Woodpecker (Picoides pubescens, 27.2 g), Red-naped Sapsucker (Sphyrapicus nuchalis, 47.4 g), Hairy Woodpecker (P. villosus, 70.5 g), Lewis' Woodpecker (Melanerpes lewis, 106.6 g), Northern Flicker (Colaptes auratus, 148.1 g), and Pileated Woodpecker (Dryocopus pileatus, 262.5 g). In woodpeckers with a body mass less than 100 g, the pectoralis muscle was composed mostly of red (R) fibers, with intermediate (I) fibers either absent or restricted to lateral regions. R and I fibers were found in all regions of the pectoralis in the larger species. Positive scaling of the percent and cross-sectional diameter of I fibers may explain why species larger than 100 g flap-bounded when theory predicts they should not have sufficient power. Differences were evident in staining characteristics of I fibers among (but not within) species. Phylogenetic effects were apparent because hypothesized sister taxa shared similarities in muscle composition. Internal and external wing dimensions scaled according to the predictions of geometric similarity, but the Red-naped Sapsucker and Lewis' Woodpecker possessed slightly higher wing-shape indices (narrower, more-pointed wings) than the other species, which may reflect migratory and flycatching behavior as well as phylogeny. All of the species exhibited flap-bounding. The number of flaps in a flapping phase, wingbeat frequency, flapping-phase duration, and bounding-phase duration all scaled negatively with body mass, whereas the percent time spent flapping and flight speed scaled slightly positively. Lewis' Woodpecker was unusual in its intermittent flight behavior by spending more time flapping, using lower wingbeat frequencies and flying slower than would be expected for its body mass. It was the only species to pursue flying insects using highly maneuvered flights including extended glides, although the Red-naped Sapsucker was observed to flycatch without gliding. Slow flight speed, rather than wing design, was related to gliding in the Lewis' Woodpecker. I fibers in the pectoralis of the Lewis' Woodpecker were unique in showing intense glycolytic potential, perhaps reflecting the species' foraging style.