While murres can both fly and dive, there appears to be a threshold where one activity overtakes the other in evolution. If a bird needs to fly more, it will lose more of its diving and swimming ability. Conversely, if a bird greatly relies upon swimming and diving for its hunting and survival, then it will tend to lose its flight skills. In the case of penguins, those skills completely disappeared, with the wings evolving into marine mammal-type flippers.
The study also sheds light on what prehistoric flying penguins looked and acted like.
"The flying ancestors of penguins were probably not much different in general appearance than murres and their relatives, and probably behaved in much the same way," Ricklefs said.
The findings could help explain how other birds lost their ability to fly. There is a flightless cormorant in the Galapagos Islands, and steamer ducks of the southern oceans are also flightless.
The reasons for flightlessness are different for ostriches and emus, which do not dive. These big birds instead traded flight for running ability. It’s likely that the ancestors of ostriches and emus did not have to migrate. They perhaps lived in the southern continents with relative few predators. Running with their powerful legs sufficed, versus needing to rely upon flight to take them up and away.
Tony Diamond of the University of New Brunswick, James Lovvorn at Southern Illinois University, and Daniel Roby of the Department of Fisheries and Wildlife all told Discovery News that they agree with the conclusions of the new study.
Diamond said the study "draws on the unique diversity of mobility-modes in birds -- walking, running, swimming, flying -- to clarify and explain evolutionary patterns that are otherwise puzzling."
Rory Wilson of Swansea University, had a more measured response, saying the authors of the study "are probably right, but the result would be more definitive if they compared auks with diving ducks." He explained that some birds have very different types of plumage that can affect heat loss. Heat loss, in turn, can affect the bird’s energy costs when flying and diving.