DISTRIBUTION PATTERNS OF BUARREMON BRUSH-FINCHES (EMBERIZINAE) AND INTERSPECIFIC COMPETITION IN ANDEAN BIRDS

Two species of Andean brush-finches, Buarremon (formerly Atlapetes) to rquatus and B. brunneinuchus, show an interdigitating pattern of distr ibution in the Andes that we interpret as the result of competition be tween them. Plotting 590 specimen localities by latitude and elevation reveals that the two species have complementary, parapatric distribut ions with minimal overlap. More importantly, where only one species oc curs, that species usually occupies the entire elevational range in th at region. Furthermore, the usual relative position of the two species , with B. torquatus at high and B. brunneinuchus at low elevations, is reversed in two regions. These ''natural experiments'' suggest that e levational limits are unlikely to be governed in these two species by autecological factors and that interspecific competition is the proces s most likely to generate such a pattern. However, our re-evaluation o f other data interpreted to support the prevalence of interspecific co mpetition in determining elevational limits of other Andean birds sugg ests that its role has not been properly resolved.