9 Biggest Birds in the World

Birds are the only remaining direct descendants of dinosaurs. Although it's very difficult to tell how heavy a fossilized bird might have been, we can tell something about their overall size.

Around 80 million years ago, gigantic birds like Gargantuavis philoinos, which stood 6 to 10 feet (1.8 to 3 meters) tall, roamed the planet. Nobody's really sure whether G. philoinos could fly, but based on what we know from studying the humongous birds like ostriches that live on our planet today, legs like a professional soccer player and the ability eviscerate a human with one powerful kick aren't traits that make flight very easy.

All birds have hollow bones, which is an adaptation birds have made for flight, however lots of birds have evolved to hang out on the ground. And though some of the largest flying birds, like the Andean condor, have enormous wingspans, they're not particularly heavy because they have to glide around on air currents. The largest flying bird is the wandering albatross, with a wingspan measuring up to more than 17 feet (5 meters), but it only weighs about 26 pounds (12 kilograms) — about as much as a small corgi.

It helps flightless birds to be a little physically intimidating because life isn't necessarily easy for a flightless bird. However, flightlessness evolved a lot more frequently in the past than you'd think, considering the number of species that exist today. Scientists estimate there would be about four times as many flightless bird species on Earth if they hadn't been sharing a planet with humans. Both flightlessness (in birds) and the co-presence of humans make a species more vulnerable to extinction.

For instance, elephant birds (Aepyornis), native to Madagascar and considered to be the heaviest and tallest bird that ever lived, were similar to ostriches, but 10 feet (3 meters) tall and weighed 1,000 pounds (454 kilograms). The elephant bird was more closely related to kiwis than ostriches and were hunted to extinction by humans by the 17th century.

Here are the 9 biggest birds by weight starting with the biggest bird in the world, and surprise! All of them are ground-bound.

The largest bird in the world weighs 344 pounds (157 kilograms) and towers over other birds at 9 feet (2.7 meters) tall. But just because the common ostrich is heavy doesn't mean it can't run faster than a person. The tallest birds on the planet have extremely long legs and can cover about 10 feet (3 meters) per stride, achieving top speeds of around 43 miles per hour (69 kilometers per hour) —about as fast as a racehorse.

Common ostriches are native to Africa, south of the Sahara Desert in the east and south of the continent. They eat mostly plant matter like grass, roots and succulents, and occasionally insects and small animals like frogs and mice. Their eggs are huge, weighing 3 pounds (1.3 kilograms) apiece — as much as two dozen chicken eggs.

Ostriches are the biggest members of the group of large, flightless birds called ratites, which lack a keel, which is the structure that attaches flight muscles in other birds. Ostriches, emus, cassowaries, kiwis and rheas are all ratites.

The Somali ostrich is the other species of ostrich, native to the Horn of Africa. Sometimes called the blue-necked ostrich, it is smaller than the common ostrich, but scientists believe the two species were once one. Through the process of allopatric speciation, the one ostrich species was geographically separated, at which point they stopped being able to breed with each other and began evolving separately. They are now two distinct species.

Of all the large birds, the cassowaries are some of the most impressive to behold. With savagely sharp claws, a rhinoceros-like horn (called a casque), eye-catching blue and turquoise skin on its head and two long, red wattles at its throat, the Southern cassowary is thought to be the living bird most reminiscent of a dinosaur.

At 5.5 feet (2 meters) tall, they don't tower over a human like ostriches do, but could probably best you in an intimidating staring contest.

Native to northeastern Australia, New Guinea, Indonesia and Papua New Guinea, Southern cassowaries have a deep, booming call that can be heard for long distances through the dense rainforest. Although you've never personally heard a dinosaur, it's easy to imagine that this bird's call is the closest thing you're going to hear on 21st-century Earth.

They eat mainly fruit, mushrooms and insects off the forest floor. Because they often live in marshy habitat, they're great swimmers.

The slightly smaller Northern cassowary is as fabulous looking as its Southern cousin, with a larger, flared casque, a single bright yellow or red wattle and a full set of razor sharp claws on both feet. It has a very low-frequency, rattling call that sounds like a gigantic boar oinking menacingly.

The Northern cassowary lives in low, marshy places in northern New Guinea and some Indonesian islands. Of all the cassowary species it's the most threatened — its meat and eggs are prized food items, which is resulting in decreasing populations.

Like ostriches, emus are large, long-necked, flightless birds. Lighter and taller than cassowaries, this Australian native isn't picky about where it lives— it's happy in the forest, desert or scrub lands. They eat plant matter and can put on a lot of fat when food is readily available. Although they can do without food, they require daily access to fresh water, which makes them a pest to cattle and sheep farmers in some parts of the continent.

Female emus are larger than males, standing over 6 feet (1.9 meters) tall. They both make low grunting booms that can be heard over long distances.

Emperor penguins are the largest penguins of all, and the first non-ratite on this list, although they're still flightless. About 45 inches (114 centimeters) tall, the emperor penguin lives in large groups on Antarctic ice sheets — useful for huddling together for warmth in a brutally cold environment.

What makes these relatively short birds so beefy is an impressive layer of fat that, along with extremely dense feathers, keeps them warm in the icy landscape. During the winter months the males go on a starvation diet, taking care of the eggs in a pack while females go off to hunt. They can lose half their body weight during this time.

Another ratite, the greater rhea resembles an emu or ostrich, but it's native to South America. They're also much smaller than their cousins at a little under 5 feet (1.5 meters) tall — but they're still the biggest bird in the Americas.

Greater rheas live in groups in the grasslands of Brazil, Bolivia, Uruguay, Paraguay and Argentina, where they eat plant matter, as well as small animals and insects.

The domestic turkey is the only bird on this list that could possibly fly, if it weren't so fat. Turkeys have a keel, unlike ratites and penguins. This means they have all the equipment they need to take off, and their wild counterparts can fly. Domestic turkeys, however, have been commercially bred to put on huge amounts of weight in a short amount of time in order to feed people.

Wild turkeys bear the same scientific name as domestic turkeys, but females weigh a maximum of 11 pounds (5 kilograms) and males max out at 24 pounds (11 kilograms).

Native to eastern Papua New Guinea, the dwarf cassowary is the smallest cassowary species, standing between 39 and 53 inches (99 and 135 centimeters) tall. Unlike their swamp-loving cousins, they live in rocky mountainous terrain. They also lack wattles, but have colorful blue and red heads and necks, and a tall helmet-like casque.