Genome size and brain cell density in birds

Birds exhibit small, constrained genome sizes relative to many other vertebrates. This has often been attributed to the metabolic constraints of powered flight, which requires small cells for efficient gas exchange. Small cells, in turn, are achieved in part by limiting nucleus size and thus DNA content. However, metabolism is not the only organismal trait that correlates with genome size in birds; relationships have also been reported with body size (positive) and relative brain size (inverse). In this study, a recent data set on cell numbers in the brains of birds was combined with available data on genome sizes to demonstrate a relationship between genome size and density of cells per gram of brain tissue. This suggests that small genomes are relevant not only in the evolution of avian flight but may also be associated with the extraordinary behavioural complexity shown by birds.