The energetic cost of using wings for both flying and swimming probably forced penguins out of the air.

Kyle Elliot at the University of Manitoba in Winnipeg, Canada, Robert Ricklefs at the University of Missouri in St Louis and their colleagues attached data recorders to two types of diving bird — 41 thick-billed murres (Uria lomvia) and 22 pelagic cormorants (Phalacrocorax pelagicus). Then the researchers injected the birds with isotopically labelled water, which allowed them to estimate the creatures' metabolic rates. The authors used data from the recorders on the depth and temperature of dives and length of flights to model the energy costs of these activities.

Both birds had the highest recorded flight costs seen in vertebrates, and higher diving costs than penguins. This suggests that adaptations for diving may compromise flight, and that adaptation of wings for optimal diving make flight impossible.

Proc. Natl Acad. Sci. USA http://dx.doi.org/10.1073/pnas.1304838110 (2013)