Fig. 1. Phylogenetic tree of birds based on Sibley and Ahlquist (1990) for maximal metabolic rate (MMR). Numbers represent the genetic distance between adjacent taxa according to DNA/DNA hybridization. In some cases, branch lengths were chosen arbitrarily (see Materials and methods). Asterisks indicate species that were not incorporated into the basal metabolic rate phylogenetic tree (see Table 1, where full genus names are given).