There are an estimated 18,000 species of birds in the world — some of them are very small, like the bee hummingbird (Mellisuga helenae) , which weighs around .06 ounces (1.7 grams). But some birds weigh more than an American black bear ( Ursus americanus ). Of course, small birds are pretty much always able to fly, while the largest birds can't.

Birds Used to Be Bigger

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 kno

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