Why did flightless birds evolve?

Why did some species of birds evolve flightlessness? According to Kyle Elliott from the University of Manitoba and his colleagues, sea birds probably traded flying for diving. Okay, that seems pretty obvious so far, until you ask why birds can't do both. In fact, there are species of birds that can both fly and dive, notably the murres and cormorants examined in this study. So why give up flight? The answer may be because of energy expenditure and efficiency.

We’re so used to the fact that penguins are flightless that it seems like the natural state for these birds. However, the ability to fly was an advantage not to be discarded lightly. After all, being able to fly would actually be quite handy for birds that migrate vast distances, not to mention for quick escapes from sharks and orcas. Unfortunately, the better adapted your body is to swimming and diving, in terms of biomechanics and energy costs, the less adapted it is for flying.

The researchers compared the flying and diving energy costs in murres and penguins, which dive using their wings to propel themselves, and in cormorants, which dive using their feet to propel themselves. Of the three groups, only penguins are flightless. 
























Pelagic cormorant (Phalacrocorax pelagicus)

By Ingrid Taylar, 9/8/2011.

During flight, murres expended the most energy, far more than similarly sized birds, and more than cormorants. Remember, murres use their wings for both flying and swimming. During dives, penguins expended the least amount of energy, followed by murres and then cormorants. 

Thick-billed Murres (Uria lomvia)

These data suggest that wings can be great for flying or swimming, but not both. Murres, which do use their wings for both flying and diving, are mediocre at both. Cormorants reserve their wings for flying and so expend less energy flying and more energy diving. Penguins, which have devoted their wing structure solely to swimming, expend the least amount of energy doing so. 

I guess the lesson here is that if you really want to do something well, you have to specialize.

Elliott, K., Ricklefs, R., Gaston, A., Hatch, S., Speakman, J., & Davoren, G. (2013). High flight costs, but low dive costs, in auks support the biomechanical hypothesis for flightlessness in penguins Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1304838110.