Ecology and caudal skeletal morphology in birds: the convergent evolution of pygostyle shape in underwater foraging taxa

PLoS One. 2014 Feb 26;9(2):e89737. doi: 10.1371/journal.pone.0089737. eCollection 2014.


Birds exhibit a specialized tail that serves as an integral part of the flight apparatus, supplementing the role of the wings in facilitating high performance aerial locomotion. The evolution of this function for the tail contributed to the diversification of birds by allowing them to utilize a wider range of flight behaviors and thus exploit a greater range of ecological niches. The shape of the wings and the tail feathers influence the aerodynamic properties of a bird. Accordingly, taxa that habitually utilize different flight behaviors are characterized by different flight apparatus morphologies. This study explores whether differences in flight behavior are also associated with variation in caudal vertebra and pygostyle morphology. Details of the tail skeleton were characterized in 51 Aequornithes and Charadriiformes species. Free caudal vertebral morphology was measured using linear metrics. Variation in pygostyle morphology was characterized using Elliptical Fourier Analysis, a geometric morphometric method for the analysis of outline shapes. Each taxon was categorized based on flight style (flap, flap-glide, dynamic soar, etc.) and foraging style (aerial, terrestrial, plunge dive, etc.). Phylogenetic MANOVAs and Flexible Discriminant Analyses were used to test whether caudal skeletal morphology can be used to predict flight behavior. Foraging style groups differ significantly in pygostyle shape, and pygostyle shape predicts foraging style with less than 4% misclassification error. Four distinct lineages of underwater foraging birds exhibit an elongate, straight pygostyle, whereas aerial and terrestrial birds are characterized by a short, dorsally deflected pygostyle. Convergent evolution of a common pygostyle phenotype in diving birds suggests that this morphology is related to the mechanical demands of using the tail as a rudder during underwater foraging. Thus, distinct locomotor behaviors influence not only feather attributes but also the underlying caudal skeleton, reinforcing the importance of the entire caudal locomotor module in avian ecological diversification.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Biological Evolution*
  • Birds / anatomy & histology*
  • Bone and Bones / anatomy & histology*
  • Charadriiformes
  • Diving
  • Feathers
  • Flight, Animal*
  • Phylogeny
  • Spine / anatomy & histology

Grant support

Funding was provided by the Sigma Xi Grant-in-Aid of Research, ID number G20130315163195, Ohio University Graduate Student Senate Original Work Grant, and Ohio University Student Enhancement Award. RNF is supported by the Department of Biological Sciences, and RNF and PMO are supported by the Department of Biomedical Sciences and the Heritage College of Osteopathic Medicine at Ohio University. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.