Main features

Sight

This is a highly developed sense and is essential if the bird is going to be able to Ily at high speed and/or alti­tude. search for food, escape from predators and Iind a mate.

The eyes till the orbits, leaving room for few eye muscles associated with movement. Birds have to move their heads rather than their eyes to pick up an image. The position of the eyes on the head varies according to feeding habits: predators such as owls and hawks have forward pointing eyes producing binocular vision but a reduced size of visual Held, while seedeaters have laterally placed eyes which pro­vide monocular vision but a wide visual Field in which to locate predators.

The structure of Iheeye (Fig. 1 3.X) is similar to that of mammals but with a few differences:

I. The sclera forms the outermost protective layer and continues in the front of the eve as (he

•r

transparent cornea. At the junction of the cornea and sclera is a ring of small bones known as the sclerotic riιιinflates them, or

- The parabronchi, where gaseous exchange takes place: this air then passes into the cranial air sacs and inflates them.

■ Expiration

- The abdominal muscles contract, squeezing air from the caudal air sacs back into the parabronchi, where further gaseous exchange takes place

- Λir in the cranial air sacs passes straight through the lungs and out.

The circulatory system follows a similar plan to that of the mammal. In order to provide for the high meta­bolic rate of the bird, the heart in particular must be able to pump the blood to deliver oxygen and nutrients to the tissues quickly and efficiently. In a resting chicken blood takes only six seconds to travel around the body.