Metabolic capacity and the evolution of biogeographic patterns in oscine and suboscine passerine birds

Biogeographic analyses of passerine birds demonstrate that suboscines are numerically dominant in South America, whereas oscines are dominant elsewhere. This suggests that oscines generally outcompete suboscines and that suboscine dominance likely persists in South America because of its long isolation from other continents, where oscines have diversified. One hypothesis for oscine competitive superiority is that oscines possess higher metabolic capacities than suboscines, and this favors oscines in most habitats. We tested this hypothesis by comparing summit metabolic rates (M(sum), maximum thermoregulatory metabolic rate) between oscines and suboscines using conventional and phylogenetically informed statistical approaches. We predicted that if the metabolic-capacity hypothesis is valid, then oscines should have higher M(sum) than suboscines. Both conventional and phylogenetically informed ANCOVA on regressions of log M(sum) against log mass showed that oscines had higher M(sum) than suboscines: least squares mean M(sum) was 74% greater for oscines. Moreover, conventional and phylogenetically informed multiple regressions identified log mass, winter-range temperature, and clade (oscines vs. suboscines) as significant effectors of log M(sum). Thus, oscines have generally higher M(sum) than suboscines, which is consistent with the metabolic-capacity hypothesis and suggests that metabolic capacity is one factor influencing the evolution of broad biogeographical patterns in passerines.