Why are tropical mountain passes ‘low’ for some species? Genetic and stable-isotope tests for differentiation, migration, and expansion in elevational generalist songbirds
Most tropical bird species have narrow elevational ranges, likely reflecting climatic specialization. This is consistent with Janzens Rule, the tendency for mountain passes to be effectively higher in the tropics. Hence, those few tropical species that occur across broad elevational gradients raise questions. Are they being sundered by diversifying selection along the gradient? Does elevational movement cause them to resist diversification or specialization? Have they recently expanded, suggesting that elevational generalism is short-lived in geological time? Here we tested for differentiation, movement, and expansion in four elevational generalist songbird species on the Andean west slope. We used morphology and mtDNA to test for genetic differentiation between high- and low-elevation populations. Morphology differed for House Wren (Troglodytes aedon) and Hooded Siskin (Spinus magellanicus), but not for Cinereous Conebill (Conirostrum cinereum) and Rufous-collared Sparrow (Zonotrichia capensis), respectively. mtDNA was structured by elevation only in Z. capensis. To test for elevational movements, we measured hydrogen isotope ({delta}2H) values of metabolically inert feathers and metabolically active liver. {delta}2H data indicated elevational movements by two tree- and shrub-foraging species with moderate-to-high vagility (C. cinereum and S. magellanicus), and sedentary behavior by two terrestrial-foraging species with low-to-moderate vagility (T. aedon and Z. capensis). In S. magellanicus, elevational movements and lack of mtDNA structure contrast with striking morphological divergence, suggesting strong diversifying selection on body proportions across the [~]50 km gradient. All species except C. cinereum exhibited mtDNA-haplotype variation consistent with recent population expansion across the elevational gradient.In sum,three out of four elevational generalist species underwent genetic divergence despite gene flow, two of four make seasonal elevational movements, and three of four have recently expanded. In different ways, each species defies the tendency for tropical birds to have long-term stable distributions and sedentary habits. We conclude that tropical elevational generalism is rare due to evolutionary instability.