Body mass is less important than bird order in determining the molecular rate for bird mitochondrial DNA

A negative relationship between body mass and molecular evolution rates has been suggested, and recently a correlation equation has been published based on mitochondrial genomic data of 475 bird species and their body masses. Here, we re-analysed these data and show that the bird order as a proxy of monophyletic groups was a stronger predictor of the molecular rate than the body mass. We provide order-specific molecular substitution rates. Only three orders (Galliformes, Gruiformes, Pelecaniformes) showed a very clear negative correlation, and specific correlation equations are given for these. The molecular rates of bird orders differed strongly at similar mean body masses, and we suggest that the previously described trend across all birds may arise as smaller species also tend to have characteristic life histories, namely faster turnover of generations, higher fecundity and shorter lifespans.