Birds have evolved numerous adaptations for flight, including wings, powerful muscles, hollow bones, and, most distinctively, feathers. They are a group of endothermic vertebrates, characterised by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. Birds live worldwide. Some species, such as ostriches, penguins and kakapos, have lost the power of flight. All birds reproduce by laying eggs. There are more than 9,800 known species of birds. The smallest is the male bee hummingbird, which is 2.25 inches (5.7 centimeters) long and weighs only 0.056 ounces (1.6 grams). Its body is the size of a large bumblebee! The largest bird is the ostrich, which can weigh up to 340 pounds (154 kilograms). The bird with the most feathers is the tundra, or whistling, swan, which has more than 25,000 feathers. The bird that flies the fastest is the peregrine falcon:117 miles/hour. Know more through this app.