Feathers are one of the most prominent features of a bird's anatomy, and they are unique to birds. Every bird has feathers and everything that has feathers is a bird. Feathers perform a number of functions for a bird. Firstly, they provide insulation, this is very important in a warm blooded animal (body temperature of most birds is maintained at around 40C). It is believed by most scientists that this insulating effect was the primary force driving the evolution of feathers, i.e. ancestral birds developed feathers to keep themselves warm. Feathers also protect birds from UV light.
Birds' bills continue to grow throughout the birds lives, this is necessary to replace the wearing that inevitably occurs at the tips.
One of the most notable things about birds is their jaws and the fact that they have beaks properly called a bills.
The bird's bill is a remarkably adaptable and useful instrument. A good look at a bird guide will show you some of the amazing diversity in bird bills.
A bird's eye is very similar in its basic structure to a human eye. Though with certain modifications and differences.
The eyes make up a much larger percentage of the weight of the head in birds than in man, i.e. 15% for a common Starling but only 1% for man. This is partly because a bird's eyes are larger relative to its skull than a human's and partly because the skull is lighter, i.e. no heavy jaw bone and teeth.
Like us, birds need to breathe air in and out of their lungs in order to fulfil the cycle of bringing oxygen into the body to be used in metabolism and also to take the waste CO2 away from the body. However, unlike us, when a bird breathes the air does not go simply in and out of the lungs in a simple u-shaped path.