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In the evolutionary drive for efficiency it may seem surprising that genomes appear sometimes to have been pared down to fit energetically demanding lifestyles of their owners. And hummingbirds have less DNA in their cells than any other previously studied birds, reptiles or mammals, according to a new report in the Proceedings of the Royal Society B (published online). Among hummingbird species, however, small variations in genome size don't relate to respiratory or flight demands, suggesting that the bird's DNA was reduced before today's diversification of hummingbirds. Scientists have long noted the link between small genome size and high metabolic rates. Bats and birds have the smallest genomes of vertebrates, and flightless birds tend to have bigger genomes than fliers. The genome is typically present in every cell so carrying around a smaller genome suggests you can have smaller cells, researchers have speculated. Smaller cells mean a larger surface-to-volume ratio and more efficient gas exchange, which would be an advantage for metabolically demanding flight. However, despite the obvious relevance to understanding avian genome size evolution, no hummingbird genome size estimates are currently available, write authors Ryan Gregory of the University of Guelph and colleagues. However, they now report estimates of the genome size in 37 species of hummingbirds by staining DNA in the nucleus and then analysing DNA density. Bird weight, body size, red blood cell concentration and other factors associated with metabolism were also examined in the birds. Hummingbird genomes averaged 1.03 picograms per cell, the researchers report. The average for previously examined birds is 1.42 picograms, for reptiles it is 2.24 picograms and for humans it is 3.5 picograms. Some salamanders have more than 100 picograms. “It is clear that hummingbirds possess the smallest and least variable genome sizes of any bird family studied thus far,” the researchers write. But, they ask, are the small genome sizes of hummingbirds derived or ancestral features? They suggest that hovering flight and reduced genome size evolved together early in the hummingbird lineage. But the researchers note that the four hummingbird species with the largest genomes, which exceeded the mean by more than 10 per cent, all inhabit the upper tropical zone at elevations of 900–1600 metres. As these four species are not close relatives, the authors believe these may represent four separate increases in genome size in relation to their movement into the upper tropical zone. “Metabolic and flight-related factors do not appear to account for the limited genome size variation that does exist in this group. Instead, the clearest pattern relates to habitat with species that have independently specialized on upper tropical zone humid forests exhibiting larger genomes,” they believe. But these results “reinforce the growing recognition of an important evolutionary interplay between features at the genomic, cellular, organismic and ecological levels.”