British researchers say they've solved the puzzle of why penguins can't fly: Their survival hinged on swimming underwater not soaring in the air.

A study of seabirds closely related to the penguin has convinced the researchers a wing designed for flight cannot also be good for diving and swimming, they said.

They examined a close relative of the penguin, the guillemot, a black-and-white seabird not only looks a lot like a penguin, it can swim nearly as well.

And unlike the penguin, it can also fly -- but poorly, the researchers said.

That constitutes a 'biomechanical hypothesis' of why some birds eventually become flightless, John Speakman from the University of Aberdeen said.

"When the bird is flying and diving it has to use its wings to do two different things," he said. "The biomechanical hypothesis is that you cannot build a wing that is good at doing both."

"These birds [guillemots] have these very short wings and they have to beat them at an incredible speed to stay in the air. It is exhausting for them."

Guillemots represent a tipping point between seabirds that are able to both fly and swim, and those that are flightless, Speakman said.

At some point in penguin evolution, the species would have faced a tradeoff between flying or attaining agility when diving and swimming, he said.

"At some point it became so 'expensive' for them to fly, that it was better to give up flying altogether and make the wings into small flippers," he said.