Distribution Patterns and Zoogeography of Atlapetes Brush-Finches (Emberizinae) of the Andes

Abstract

We attempted to determine why the distribution of Atlapetes rufinucha (Rufous- naped Brush-finch; Emberizinae) is so patchy. This common, sedentary species is found in several discrete areas of the humid Andes separated by distances of hundreds of kilometers, yet the gaps contain seemingly suitable habitat. Mapping of 906 specimen localities by both latitude and elevation shows that these gaps are filled by populations of other Atlapetes species, especially A. tricolor and A. schistaceus (a gray-plumaged species currently thought to be only distantly related to rufinucha, a species with yellow-and-green plumage), which in turn also show complementary, patchy distributions. Where rufinucha is the only species found, it occupies the entire elevational gradient. Where two or more species occur, their elevational distributions are restricted and often complementary. We attribute these patterns to interspecific competition. Hypothetical reconstructions of the sequence of historical events that would generate such a checkerboard distribution pattern in these sedentary taxa are complex. A novel hypothesis that would greatly simplify historical reconstructions is that rufinucha, schistaceus, and perhaps tricolor refer only to recurring color patterns characterized by differing amounts of pigment in the feathers and, therefore, adjacent populations (re- gardless of current taxonomic designation) are more closely related than either is to more distant populations of the same species. Therefore, rufinucha and schistaceus populations would be merely allopatric forms of the same lineage that alternate in color pattern (yellow or gray) between adjacent populations, as known for three other lineages of Andean birds. We found limited support for such a hypothesis. Even if rufinucha and schistaceus are distinct species, we predict that they are much more closely related than currently believed and that they differ primarily in pigment concentration. The dramatic differences in phenotype created by differences in pigment concentration in the plumage may frequently cause problems for phenotype-based taxonomic hypotheses. Received 22 December 1992, accepted 12 May 1993.