Morphological variation in birds

The huge diversity of phenotypes and associated geographic patterns has made birds prime examples for studies in speciation. For this purpose, morphological approaches were first choice to assess the degree of relatedness between species and their intraspecific variation for centuries, until molecular genetic studies seriously challenged traditional morphology-based conclusions. However, the current development of multivariate statistics and the ease to blend morphological, phylogenetic, and ecological insight has gradually led to a reconsideration of morphology as a valuable tool for ornithological research. This chapter reviews the most important aspects of morphological variation in birds, how its plasticity can be assessed and to which extent phenotypic variation can be incorporated into a broader evolutionary framework that explains modifications of the avian body in the light of speciation processes.