DISTRIBUTION PATTERNS AND ZOOGEOGRAPHY OF ATLAPETES BRUSH-FINCHES (EMBERIZINAE) OF THE ANDES

We attempted to determine why the distribution of Atlapetes rufinucha (Rufousnaped Brush-finch; Emberizinae) is so patchy. This common, sede ntary species is found in several discrete areas of the humid Andes se parated by distances of hundreds of kilometers, yet the gaps contain s eemingly 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 relat ed to rufinucha, a species with yellow-and-green plumage), which in tu rn also show complementary, patchy distributions. Where rufinucha is t he only species found, it occupies the entire elevational gradient. Wh ere two or more species occur, their elevational distributions are res tricted and often complementary. We attribute these patterns to inters pecific competition. Hypothetical reconstructions of the sequence of h istorical events that would generate such a checkerboard distribution pattern in these sedentary taxa are complex. A novel hypothesis that w ould greatly simplify historical reconstructions is that rufinucha, sc histaceus, and perhaps tricolor refer only to recurring color patterns characterized by differing amounts of pigment in the feathers and, th erefore, adjacent populations (regardless of current taxonomic designa tion) are more closely related than either is to more distant populati ons of the same ''species.'' Therefore, rufinucha and schistaceus popu lations would be merely allopatric forms of the same lineage that alte rnate in color pattern (yellow or gray) between adjacent populations, as known for three other lineages of Andean birds. We found limited su pport for such a hypothesis. Even if rufinucha and schistaceus are dis tinct species, we predict that they are much more closely related than currently believed and that they differ primarily in pigment concentr ation. The dramatic differences in phenotype created by differences in pigment concentration in the plumage may frequently cause problems fo r phenotype-based taxonomic hypotheses.