I have been hesitant to talk about the association between bird genome size and flight, even though it has been fluttering around in the blogosphere for some time now (e.g., here and here and here). This may seem counterintuitive, since of all the bloggers interested in the issue, I have actually published articles on the subject. The reason is that a colleague and I have ostensibly been working on a paper about this subject (though it has been on the back burner for a long time) and because I have a student doing research on this issue and I did not feel it appropriate to discuss his unpublished study. But today there is another blog discussion (here and here) about how some dinosaurs already had small genomes and therefore that genome reduction was not part of the evolution of flight in the avian descendants of those dinosaurs. I figure one small clarification is useful.
Modern birds have smaller genomes than the dinosaurs are estimated to have had, strong flyers have the smallest, and flightless birds the largest.
The most reasonable interpretation of this is that genomes began to shrink in saurischian dinosaurs, possibly in association with endothermy, and then they shrank more along with the evolution of powered flight.
Gregory, T.R. 2002. A bird’s-eye view of the C-value enigma: genome size, cell size, and metabolic rate in the class Aves. Evolution 56: 121-130.
Gregory, T.R. 2005. Genome size evolution in animals. In: The Evolution of the Genome (ed. T.R. Gregory), pp. 3-87. Elsevier, San Diego.
Organ, C.L., A.M. Shedlock, A. Meade, M. Pagel, and S.V. Edwards. 2007. Origin of avian genome size and structure in non-avian dinosaurs. Nature 446: 180-184.
Zimmer, C. 2007. Jurassic genome. Science 315: 1358-1359.