| Birds have a body plan that shows so | | | | lungs; thus birds receive a supply of |
| many unusual adaptations (mostly aiding | | | | air during both inhalation and |
| flight) that birds have earned their own | | | | exhalation. The gas exchange then takes |
| unique class in the vertebrate phylum. | | | | place in the capillaries. The heart has |
| The skeleton consists of bones which are | | | | four chambers and the right aortic arch |
| very light. They have large pneumatic | | | | gives rise to systemic aorta (unlike in |
| cavities which connect with the | | | | the mammals where the left arch is |
| respiratory system. The skull bones are | | | | involved). Postcava receives blood from |
| fused and do not show sutures. The | | | | the limbs via the renal portal system. |
| orbits are large and separated by a bony | | | | Sound production is achieved using the |
| septum. The spine has cervical, | | | | syrinx, a muscular chamber with several |
| thoracic, lumbar and caudal regions with | | | | tympanic membranes, situated at the |
| the number of cervical (neck) vertebrae | | | | lower end of the trachea where it |
| highly variable and especially flexible, | | | | bifurcates. |
| but movement is reduced in the anterior | | | | The digestive system of the bird is |
| thoracic vertebrae and absent in the | | | | unique and has a crop for storage and a |
| later vertebrae. The last few are fused | | | | gizzard for grinding food. Most are |
| with the pelvis to form the synsacrum. | | | | highly adapted for rapid digestion, an |
| The ribs are flattened and sternum is | | | | adaptation to flight. Migratory birds |
| keeled for the attachment of flight | | | | have the additional ability to reduce |
| muscles, except in the flightless bird | | | | parts of the intestines prior to |
| orders. The forelimbs are modified into | | | | migration. |
| the wings. | | | | The nervous system, relative to the |
| Unlike mammals, birds don't urinate. | | | | bird's size, is actually quite large. |
| Their kidneys extract nitrogenous wastes | | | | The most developed part of the brain is |
| from the bloodstream, but instead of | | | | the one that controls the flight related |
| excreting it as urea dissolved in urine | | | | function while the cerebellum |
| as we do, they excrete it in the form of | | | | coordinates movement and the cerebrum |
| uric acid. They also excrete creatine | | | | controls behaviour patterns, navigation, |
| rather than creatinine as in mammals. | | | | mating and nest building. A bird's eyes |
| Uric acid has a very low solubility in | | | | are developed for taking off, spotting |
| water, so it emerges as a white paste. | | | | landmarks, hunting and feeding. Birds |
| This material, as well as the output of | | | | with eyes on the side of their head have |
| the intestines, emerges from the bird's | | | | a wide vision field while birds with |
| cloaca. The cloaca is a multi-purpose | | | | eyes on the front of their heads like |
| hole for birds: their wastes come out of | | | | owls have binocular vision and can |
| it, they have sex by putting their | | | | measure depth. |
| cloacas together, and females lay eggs | | | | Most birds have poor sense of smell |
| out of it. | | | | except in the Kiwis and vultures. The |
| Birds have one of the most complex lung | | | | visual system is highly developed. Water |
| system of all organisms. Air enters the | | | | birds have special flexible lenses, |
| bird and immediately 75% of the air | | | | allowing accommodation for vision in air |
| bypasses the lungs and flows directly | | | | and water. Some species also have dual |
| into a posterior air sac which extends | | | | fovea. Retina has a fan shaped blood |
| from the lungs and connects with air | | | | supply system called the pecten. The ear |
| spaces in the bones and fills them with | | | | lacks external pinnae but is covered by |
| air. When the bird exhales the air from | | | | feathers. The inner ear has a cochlea |
| the posterior air sac is forced into the | | | | but is not spiral as in mammals |