A Bird is defined as a worm blooded, egg-laying vertebrate, characterized by a body covered with feathers, scaly legs, a beak and no teeth to chew the food.
The forelimbs of birds have got modified as wings.There are about 10,000 living species of birds-making them the most numerous tetra pod vertebrates.
We always associate the ability to fly with all the birds. But surprisingly many of the birds have lost their ability to fly, as they evolved. Many of the flightless birds have become extinct.
About 40 flightless bird species exist now. They are able to survive merely because they live in isolated areas with minimal predators.
The largest flightless bird is the Ostrich weighing a massive 90 kilograms and the smallest flightless bird the Rail weighs a mere 30 grams!
Flightless birds are called as Ratites and lack the keel, the part of the breastbone to which the flight muscles could attach.
To compensate for the lack of the ability to fly, the large birds have well developed muscular legs, and long, strong claws-enabling them to run, kick and claw in self defense. Their sharp long claws can inflict deadly wounds just like daggers.