Birds classified as Aves have been around since prehistoric times. It is believed that they evolved from dinosaurs. According to Wikipedia they are described as winged, two legged vertebrate animals that lay eggs. There are over 10,000 living species of birds that inhabit the globe. Birds are a vary diverse group with specimens that range in size from 2 inches (the Bee Humming bird) to about 10 ft (the Ostrich)! Modern birds have feathers, a beak and lay hard shelled eggs, a high metabolic rate and a four chambered heart much like ours. Their beaks are as widely diversified as their sizes and range from long narrow spears for getting nectar from flowers to spoon bills for scooping up fishes to the large hooked beak of eagles for grasping and tearing flesh.
The first known prehistoric bird was the Archaeopteryx which was believed to have originated from the Theropod dinosaurs somewhere in the Jurassic period (100-200 million years ago), even though there is no direct evidence to show the progression from one type to the next. Because of this, there remains some controversy over how these birds came to become birds from the otherwise flightless dinosaurs.
From the flightless birds evolved the flighted dinosaurs such as the one we often heard about as children the Pterodactyl and eventually feathered birds such as shown below.
We continue to have living evidence of this progression in our giant flightless birds such as the Cassowary, the Emu and the Ostrich(shown below).
Each adaptation helps the particular bird find and consume the food source for which it depends on.
Nature is amazing in it's diversity of life and the class Aves is the most diverse of all her creations!