The fastest-running birds are those too heavy or bulky to get off the ground. But fear not for them when predators lurk -- flightless birds have powerful legs equipped with sharp claws that can hurt or kill even the largest predators.
The largest bird in the world is also the fastest-running. With the ability to top out at 40 to 50 mph, ostriches are the fourth fastest land animal on Earth, only outrun by cheetahs, pronghorn antelope and the swiftest of horses.
Part of what makes these African birds so fast is their wings. Once thought to be vestiges of an earlier flying variety, wings actually give ostriches amazing stability while they run. These stabilizers, combined with large, muscular legs, also make ostriches the fastest endurance animal on earth. No other running animal can maintain 40-plus-mph for more than a half-hour.
Ostriches have a South American doppelganger called the greater rhea that can run almost as fast. Though Charles Darwin nicknamed greater rheas "the South American ostrich," the two species are actually only distant cousins.
Nevertheless, the similar, leggy build and specialized, flightless wings allow greater rheas to reach speeds up to 40 mph. This is a necessity for outrunning their two chief predators, cougars and jaguars. Greater rheas can even swim to avoid these big cats.
The smaller, Australian cousin of the ostrich, the emu, will also win many a foot race. Like their larger cousins, emus can't fly but use their stubby wings and long sinewy legs to reach swift running speeds, up to 30-plus mph. Emus get a turbo-boost from having something no other bird in the world has -- calf muscles. This also helps give emus the ability to jump as much as 7 feet straight up.
A smaller country mate and cousin of the emu is the compact, fleet-footed cassowary. Though shorter than emus, cassowaries are heavier. In fact, at 130 pounds, they're the second-heaviest bird, behind the ostrich. This keeps cassowaries grounded, but their powerful legs and aerodynamic bodies allow them to break 30 mph in a sprint.
Predators are often more than merely frustrated when chasing this nimble bird. Cassowaries possess deadly sharp claws that can tear open the toughest predator's hide. Like emus, cassowaries can jump as high as 7 feet -- impressive, considering they only stand about 5 feet tall.