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 Janzen's Rule, the tendency for mountain passes to be effectively "higher" in the tropics. Hence, those few tropical species that occur across broad elevational gradients (elevational generalists) represent a contradiction to Janzen's Rule. Here, we aim to address the following questions. Are elevational generalists being sundered by diversifying selection along the gradient? Does elevational movement cause these species to resist diversification or specialization? Have they recently expanded, suggesting that elevational generalism is short-lived in geological time? To answer these questions, 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. To test for elevational movements, we measured hydrogen isotope (δ2 H) values of metabolically inert feathers and metabolically active liver. 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. δ2 H data indicated elevational movements by two tree- and shrub-foraging species with moderate-to-high vagility (C. cinereum and S. magellanicus), and sedentary behaviour 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 c. 50 km gradient. All species except C. cinereum exhibited mtDNA haplotype variation consistent with recent population expansion across the elevational gradient, potentially concurrent with Holocene anthropogenic habitat conversion for agriculture. 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.