Bird anatomy

Birds have a body plan that shows so many unusual adaptations (mostly aiding flight) that birds have earned their own unique class in the vertebrate phylum.

The skeleton consists of bones which are very light. They have large pneumatic cavities which connect with the respiratory system. The skull bones are fused and do not show cranial sutures. The orbits are large and separated by a bony septum. The spine has cervical, thoracic, lumbar and caudal regions with the number of cervical (neck) vertebrae highly variable and especially flexible, but movement is reduced in the anterior thoracic vertebrae and absent in the later vertebrae. The last few are fused with the pelvis to form the synsacrum. The ribs are flattened and the sternum is keeled for the attachment of flight muscles, except in the flightless bird orders. The forelimbs are modified into the wings.

Unlike mammals, birds do not urinate. Their kidneys extract nitrogenous wastes from the bloodstream, but instead of excreting it as urea dissolved in urine as we do, they excrete it in the form of uric acid. They also excrete creatine rather than creatinine as in mammals. Uric acid has a very low solubility in water, so it emerges as a white paste. This material, as well as the output of the intestines, emerges from the bird's cloaca. The cloaca is a multi-purpose hole for birds: their wastes come out of it, they have sex by putting their cloacas together, and females lay eggs out of it.

Birds have one of the most complex lung systems of all organisms. Air enters the bird and immediately 75% of the air bypasses the lungs and flows directly into a posterior air sac which extends from the lungs and connects with air spaces in the bones and fills them with air. When the bird exhales the air from the posterior air sac is forced into the lungs; thus birds receive a supply of air during both inhalation and exhalation. The gas exchange then takes place in the capillaries. The heart has four chambers and the right aortic arch gives rise to systemic aorta (unlike in the mammals where the left arch is involved). Postcava receives blood from the limbs via the renal portal system. Birds, unlike mammals, have nucleated erythrocytes, i.e. red blood cells which retain a nucleus.

Sound production is achieved using the syrinx, a muscular chamber with several tympanic membranes, situated at the lower end of the trachea where it bifurcates.

The digestive system of the bird is unique and has a crop for storage and a gizzard for grinding food. Most are highly adapted for rapid digestion, an adaptation to flight. Migratory birds have the additional ability to reduce parts of the intestines prior to migration.

The nervous system, relative to the bird's size, is actually quite large. The most developed part of the brain is the one that controls the flight related function while the cerebellum coordinates movement and the cerebrum controls behaviour patterns, navigation, mating and nest building. A bird’s eyes are developed for taking off, spotting landmarks, hunting and feeding. Birds with eyes on the side of their head have a wide visual field while birds with eyes on the front of their heads like owls have binocular vision and can measure depth.

Most birds have a poor sense of smell except for the kiwis and vultures. The visual system is highly developed. Water birds have special flexible lenses, allowing accommodation for vision in air and water. Some species also have dual fovea. The bird retina has a fan shaped blood supply system called the pecten. The avian ear lacks external pinnae but is covered by feathers. The inner ear has a cochlea but it is not spiral as in mammals.