The Furnarii is an assemblage of approximately 500 species of passerine birds in South and Central America that live in many habitats and have a variety of life styles. The group shares a common ancestry and exemplifies the historical process of adaptive radiation, in which a single group evolves into a diversity of ecological niches. This general process and its results will be studied through the comparative anatomy of the limb muscles of representative species. These will be dissected under the microscope and the anatomical variations will be described and recorded. These variations will then be used in a computer analysis to reconstruct the evolutionary history of the group. This phylogeny, in turn, will be used as the basis for a new classification of the group, and of an explanation of its interrelationships. In addition, the historical pattern thus developed will be analyzed in terms of the locomotor specializations of the various species so as to reconstruct the historical process of adaptive evolution. Thus, the study will be significant at two levels of analysis. The first is as a specific study of a particular group of organisms, and the second is as a general study of the widespread phenomenon of adaptive radiation.