It is established that genome, nucleus and red blood cell size are all correlated in vertebrates. Since larger cells have lower surface area to volume ratios, they are less efficient at gas exchange; which means that high metabolisms are constrained to small cells, which must therefore have small genomes. Powered flight is metabolically expensive, so birds are predicted to have smaller average genomes compared to all other vertebrates. Saurischian dinosaurs underwent a genome reduction before they evolved powered flight and gave rise to birds; within birds genome size tends to be smaller with increasing flying ability.
This study compares 37 species of hummingbirds in terms of nucleus, red blood cell and genome size, as well as other physiological parameters such as cardiac mass, haemoglobin concentration, body mass, wing loading and elevation, taking phylogeny into account. Genome size of hummingbirds was found to be constrained similarly to other vertebrates. Genome size across hummingbird species was found to have low variation and did not correlate with body size, but was positively correlated with heart size. The results generated two important questions which could be extrapolated to other groups; 1) is genome size a derived or ancestral feature? and 2) why do some species within a clade have larger genomes?
Hummingbirds diverged from nightjars; a group which have larger genome sizes than hummingbirds, so genome reduction must have occurred after this divergence and is therefore not an ancestral feature. Hummingbird species with the largest genomes were all from the upper tropical zone in 900-1600 m humid evergreen forests; these species are not close relatives, so the secondary change in genome size (the primary change being the reduction following divergence from nightjars) makes the relationship between phylogeny and genome size not as clear as the relationship between environment and genome size. Thus this study provides a strong example of evolution influenced by interactions between the genome and the environment.