What do the Ostrich, Kiwi, penguin, and Giant Coot have in common? Yes, they’re all birds—but none of them can fly. In fact, there are over 60 species of flightless birds, including all the Ratities (the family that includes the Ostrich, Rhea, Emu, and similar species) and all the penguins (in spite of the evidence in this video):

Since the ability to fly gives most birds an obvious evolutionary advantage—making it easier to evade predators, hunt for prey, and migrate long distances, one might wonder why these particular species gave it up. A closer look provides some clues.

For one, the gift of flight comes with some strings attached. Sacrifices must be made. Flying takes a huge amount of energy, which increases with weight. And being a lightweight means you aren’t as tough as you might otherwise be. That limits the size of the prey you catch and the predators you fight off. It’s significant that so many species gave up flight as soon as the pressure of predators was removed.

Most flightless birds live on islands. Islands typically have few or no large predators, so sticking to the ground is less perilous. New Zealand has the largest number of flightless species, and until people arrived with their rats, cats, dogs, and pigs, there were no land predators that could threaten them. Now, sadly, many of those species are either extinct or threatened.

Paleobiologists believe that many flightless species, even those in the same family, such as the Ratites, are the result of convergent evolution, meaning they lost this ability independently of one another. Apparently, losing the ability to fly is fairly straightforward, but gaining it is another matter entirely. In fact, there are no known instances of birds losing flight and then regaining it.

As they lost the ability to fly, many species grew to large sizes with long legs, long necks, and only vestigial wings. Should we encounter many of these, we would do well to keep our distance. Take cassowaries, for example. The third-largest bird (after ostriches and emus), they are considered quite dangerous. Their three-toed feet have sharp claws; the middle one of which grows to five inches in length! When the birds kick with their powerful legs, they can inflict major damage. Who needs to fly when you can defend yourself so capably?

Just because they’re flightless, however, doesn’t mean these birds are also wingless. With the exception of the (now extinct) moa, these birds still have wings—they just use them for other purposes. Wings come in handy when you can top 40 mph, as ostriches do, both for stabilization and for stopping. Penguins use their wings as fins, enabling them to swim and maneuver underwater. Perhaps most importantly, at least to natural selection, is the prominent place wings occupy in mating dances. With much flapping and other displays, the males warn off other males while simultaneously attracting the ladies.

As I browsed the list of flightless birds, I noted how many are now extinct. Perhaps flightlessness is an evolutionary dead end? Ducks and rails (which include coots and cranes) in particular feature many species no longer with us, along with all three game birds, all five species of doves, all three owls, and all five songbirds. The most famous extinct flightless bird is likely the dodo, doomed by its inability to elude hungry sailors eager for fresh meat.

Not too surprisingly, none of the birds on the flightless list show up in North America. While I took these photos in zoos, if I want to add a flightless bird to my life list, I’ll have to travel. Gee darn.

(Photos, from top: Emu, Southern Cassowary, Ostrich)

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The answer to last week’s quiz is the Colorado state bird: Lark Bunting. (It’s an immature male.)