Genome size vary remarkably across metazoans, and is known to be a poor predictor of organismal complexity. We know for example that the freshwater Amoeba Polychaos dubium has a genome more than 100 times larger than the human genome, yet humans are incredibly more complex. From a decade of genome sequencing we know that much of differences in genome size can be explained by contraction or expansion of transposable elements. The mechanisms for genome expansion does not seem to coincide with processes where natural selection is strong. In a new study, Kapusta et al. analyse mammalian and avian genome sizes starting from the observation that there is relatively little inter-specific (between species) genome sizes differences in mammalian and avian genomes. Their study was based on 10 genomes of placental mammals and 24 genomes of avian species (all public data). Their analysis debunk the suggestion of limited DNA loss/gain during the evolution of mammals and birds. However, birds appear to have undergone more extensive DNA loss than mammals, perhaps linked to lowering metabolic cost of flight. This statement was for example tested in penguins and ostrich (species that have lost the ability to fly), and these were found to have lower rates of DNA deletions.