Despite their status as the most speciose group of terrestrial vertebrates, birds exhibit the smallest and least variable genome sizes among tetrapods. It has been suggested that this is because powered flight imposes metabolic constraints on cell size, and thus on genome size. This notion has been supported by analyses of genome size and cell size versus resting metabolic rate and other parameters across birds, but most previous studies suffer from one or more limitations that have left the question open. The present study provides new insights into this issue through an examination of newly measured genome sizes, nucleus and cell sizes, body masses and wing parameters for 74 species of birds in the order Passeriformes. A positive relationship was found between genome size and nucleus/cell size, as well as between genome size and wing loading index, which is interpreted as an indicator of adaptations for efficient flight. This represents the single largest dataset presented for birds to date, and is the first to analyse a distinctly flight-related parameter along with genome size using phylogenetic comparative analyses. The results lend additional support to the hypothesis that the small genomes of birds are indeed related in some manner to flight, though the mechanistic and historical bases for this association remain an interesting area of investigation.

It has often been suggested that the genome sizes of birds are constrained relative to other tetrapods owing to the high metabolic demands of powered flight and the link between nuclear DNA content and red blood cell size. This hypothesis predicts that hummingbirds, which engage in energy-intensive hovering flight, will display especially constrained genomes even relative to other birds. We report genome size measurements for 37 species of hummingbirds that confirm this prediction. Our results suggest that genome size was reduced before the divergence of extant hummingbird lineages, and that only minimal additional reduction occurred during hummingbird diversification. Unlike in some other avian taxa, the small amount of variation observed within hummingbirds is not explained by variation in respiratory and flight-related parameters. Unexpectedly, genome size appears to have increased in four unrelated hummingbird species whose distributions are centred on humid forests of the upper-tropical elevational zone on the eastern slope of the Andes. This suggests that the secondary expansion of the genome may have been mediated by biogeographical and demographic effects.

Genome size (haploid nuclear DNA content) has been found to correlate positively with cell size and negatively with cell division rate in a variety of taxa. These cytological relationships manifest in various ways at the organism level, for example, in terms of body size, metabolic rate, or developmental rate, depending on the biology of the organisms. In birds, it has been suggested that high metabolic rate and strong flight ability are linked to small genome size. However, it was also hypothesized that the exceptional cognitive abilities of birds may impose additional constraints on genome size through effects on neuron size and differentiation, as has been observed in amphibians. To test this hypothesis, a comparative analysis was made between genome size, cell (erythrocyte) size, and brain size in 54 species of parrots and cockatoos (order Psittaciformes, family Psittacidae). Relative brain volume, which is taken as an indicator of investment in brain tissue and is widely correlated with behavioural and ecological traits, was found to correlate inversely with genome size. Several possible and mutually compatible explanations for this relationship are described.