SYSTEMATICS AND BIOGEOGRAPHY OF THE TYRANNID GENUS TODIROSTRUM AND RELATED GENERA (AVES) JOHN W. FITZPATRICK 1 Abstract. Of 32 Tody-tyrant species (family Tyrannidae), 14 comprise the genus Todirostrum and 12 are currently placed within Idioptilon. The remaining species represent six monotypic genera. Todirostrum species fall into two major species groups (labelled cinereum and sylvia), and the affinities of one additional, relict species (capi-tate) lie outside the genus. The distributional history of each species group can be approximately traced from the isolation of two ancestral popu-lations. Alternating contraction and expansion of lowland South American forests, corresponding with dry and humid climatic phases, generated new species within each lineage. Their modern compositions and distribution patterns were de-termined by dispersal ability and competitive interactions between related species. Current distributions and plumage patterns appear to re-flect the history of this radiation within Todi-rostrum. Two relict species groups (one includes T. capitate, the other is an intermediate assemblage between Todirostrum and Idioptilon placed in the latter genus) illustrate the potential fate of many taxa in the rapidly radiating South American avi-fauna. As the number of species increases with cyclic production of new forms within lowland forest refuges, each faces increasing competitive pressure as it disperses. Certain species and whole lineages may be secondarily contracted into local distributions by dispersing, competitively superior relatives. The process culminates either in morpho-logic and ecological divergence by the inferior population, or in its eventual extinction. Inter-mediate stages in this "continental taxon cycle" are found in the Tyrannid group examined here. Four specific taxonomic suggestions are offered in addition to comments on affinities within Todirostrum: 1) the genus Ceratotriccus should be merged with Idioptilon; 2) the genera Tacnio-triccus and Poecilotriccus should be moved to 1 Department of Biology, Princeton University, Princeton, New Jersey 0S540 positions immediately preceding that of Todi-rostrum, reflecting their affinities with Todirostrum capitate; 3) Todirostrum and Idioptilon should continue to stand as separate genera; 4) the genera Microcochlearius and Snethlagea are best merged with Idioptilon, while Oncostoma should continue to be generically recognized. A phylogeny of the Tody-tyrants and a revised classification of the group are proposed (Figure 4 and Table 1, re-spectively ) . INTRODUCTION Large complexes of closely related, often morphologically similar species are common in the diverse avifauna of South America. A number of recent studies have focused on ecological relationships between sym-patric members of potentially competing species complexes ( some examples in Cody, 1974). However, the evolutionary history behind modern distributions and ecological interactions is disregarded in many such studies, at the expense of an adequate perspective on the problems examined. The aim of the present study is to reconstruct this evolutionary background for a widely sympatric group of tropical, insectivorous bird species. The Tyrannid subfamily Euscarthminae (Hellmayr, 1926) contains roughly 65 spe-cies, all of which are small and wholly insectivorous. This subfamily forms one of the four major phylogenetic lineages of small Tyrannidae (Smith, 1970), as shown in Figure 1. The core of the subfamily consists of an assemblage of 32 broad-billed species, placed in 10 genera by de Schauen-Bull. Mus. Comp. Zool, 147(10): 435-463, June, 1976 435
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