The acceptance of the generalized or unified concept of species (i.e. that species are segments of population lineages) implies that an important task for systematists is to focus on identifying lineages and on testing hypotheses about the acquisition of properties such as phenotypic diagnosability, reciprocal monophyly, or mechanisms of reproductive isolation. However, delimiting species objectively remains one of the most challenging problems faced by biologists. In the present study, we begin to tackle the thorny issue of species delimitation in a complicated group of Neotropical passerine birds (the Arremon torquatus complex, Emberizidae) in which sets of characters vary substantially across space, but do not obviously vary in a concerted fashion. To earlier discussions of species limits in the group, we add a historical perspective offered by a recent molecular phylogeny, present quantitative analyses of morphological and vocal variation, and incorporate ecological niche models as a new tool that aids species delimitation by highlighting cases of ecological distinctiveness and cases where populations appear to be in independent evolutionary trajectories, despite being connected by environments unlikely to represent barriers to gene flow. We demonstrate that at least one pair of taxa (and likely another) currently treated as conspecific are, in fact, distinct lineages that merit species status under essentially any species criterion. However, other pairwise comparisons are not as straightforward owing to nonconcordant patterns of variation in different traits and to the impossibility of distinguishing which characters are causes and which are consequences of reproductive (and evolutionary) isolation. After considering several alternatives, we propose a provisional classification of the complex recognizing eight tentative species-level taxa. Although this classification is likely to change as more detailed work is conducted, it provides a better foundation for studying the biology of these birds and helps to better describe their diversity, which is obscured when all taxa are subsumed into a single species name. The present study highlights several outstanding challenges, both practical and conceptual, for future studies. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99, 152–176.