Small clades at the periphery of passerine morphological space.

Among passerine birds (order Passeriformes), tribe- to family-level clades with five or fewer species are more frequent than one would expect from a homogeneous speciation and extinction process. Previous analyses also suggested that small clades tend to be marginal geographically and/or ecologically. In this study, I use principal component (PC) scores based on eight log-transformed measurements of the wing, tail, leg, and beak to test the hypothesis that small clades (<or=5 species) occupy peripheral positions in morphological space. Species in smaller clades exhibited higher absolute deviations and greater standard deviations on PC axes 4, 6, and 7, reflecting peripheral positions associated with relatively long toes, sometimes in contrast with short legs, and relatively small beaks compared with other birds. These extreme morphological characters are related to foraging on hard substrates, such as rock or bark, or feeding from perched positions or in dense vegetation. The implication that these small clades are protected from extinction at the periphery of morphological and, by inference, ecological space suggests that more detailed studies of the evolution and ecology of species in small clades should be a priority.