Small clades at the periphery of passerine morphological space

Although large evolutionary radiations producing many species have captured the attention of biologists, comparison of the sizes of evolutionary lineages show that unusually small groups with few species are more frequent than one would expect from a model of random speciation and extinction.

Among songbirds (Passeriformes), more than 30 of 106 tribe-level taxonomic groups have five or fewer species, compared to the average size of 54 species and a maximum of more than 10 times that number. The existence of so many small taxa suggests that these lineages must have unusually low rates of speciation and extinction compared to other songbirds. In an earlier analysis, Robert E. Ricklefs suggested that species in these small groups might avoid extinction by competitive exclusion because they are marginal both geographically and ecologically. In this new study, which will appear in the June 2005 issue of The American Naturalist, Ricklefs shows that species in small clades tend to have extreme morphology, in particular relatively long toes, sometimes in contrast with short legs, and small beaks. Such traits are associated with foraging on bark or rock surfaces or feeding from perched positions or in dense, shrubby vegetation. These are unusual habits for typically more active songbirds. How such marginal ecology slows the rates of species formation and extinction remains an open question.