Patterns of Elevational and Latitudinal Distribution, including a "Niche Switch," in Some Guans (Cracidae) of the Andes

Abstract

A recently discovered population of Chamaepetes goudotii (Sickle-winged Guan) in the eastern Andes of southern Peru and northern Bolivia is anomalous in its high-elevation (above 3,000 m) distribution; populations from central Peru to not-them Colombia are found generally below 2,100 m. Several hypotheses are evaluated to account for the origin of such an unusual distribution. No evidence for long-distance dispersal or human introduction was found (and previous claims of introduction by ancient civilizations to account for disjunct distributions of two other species of New World birds are disputed). We propose that the current pattern is the result of fragmentation of a once continuous distribution. Hypotheses concerning the cause of the fragmentation and maintenance of the current pattern are evaluated. The relative consistency of elevational distributions of Andean bird species over broad latitudinal ranges (and dramatic differences in species composition with changes in elevation) provides circumstantial evidence against an autecological hypothesis that would propose that ecological requirements are met for C. goudotii only in the two disjunct regions. If potential competitors are restricted to congeners, no support can be found for a hypothesis based on interspecific competition. However, if the field of potential competitors is expanded to include morphologically similar confamilials, a striking pattern of generally nonoverlap- ping distributions in terms of elevation and latitude is revealed. Most striking is the reversal in elevations occupied by C. goudotii and Penelope montagnii. The reliance on interpretation for evaluating competing hypotheses makes such analyses inherently unsatisfying.