The present study investigates the patterns and consequences of avian genome size diversity using new data from 244 species of birds from 163 genera, 46 families, and 17 orders. Compared to other amniotes, the genomes of birds are remarkably constrained in size, ranging from only 0.91 to 2.42 pg of DNA per haploid nucleus. Genome size is positively correlated with cellular areas, and negatively with resting metabolic rate across all birds. Genome size is also linked positively to wing loading index in perching birds and negatively to relative brain volume in parrots. These results lend considerable support to the hypothesis that bird genomes are constrained due to selective pressures favouring small cells, and that this relates to the high metabolic demands of powered flight and other traits such as brain complexity. More broadly, this investigation sheds light on the interaction between the genome and numerous biological, behavioural, and ecological features.