Short Communication, Mitochondrial DNA (mtDNA) is a popular resource used for the study of evolutionary processes in birds, especially to infer divergence times between lineages. In many cases, due to the scarcity of fossils with which to constrain the age of nodes on a phylogenetic topology, dating relies on mtDNA substitution rates that, ideally, are specific to the taxa being studied. As such values are often unavailable, many studies use fixed-rate values from other studies, such as the ‘standard molecular clock’. The validity of this universal rate across all bird lineages and for the different mtDNA loci has been questioned by multiple authors. We here performed the most comprehensive calibration of the mtDNA molecular clock in birds, with the inclusion of complete mitochondrial genomes for 622 bird species and 25 reliable fossil calibrations. The results show variation in the rates among lineages and among markers, confirming the lack of universality of the standard molecular clock. Moreover, we provide specific rates for every mtDNA marker (except D-loop) in each of the sampled avian orders, a valuable tool for molecular clock analyses in the absence of fossil calibrations., This work was supported by grants from the Spanish Ministerio de Economía y Competitividad and FEDER CGL2013-40924-P, CGL2017-89898-R (AEI/FEDER, UE), and a BiodivERsA: EC21C ERANet grant to D.R.V. R.P. was supported by an FPU predoctoral fellowship from the Spanish Ministerio de Educación., Peer reviewed