From Laura Klappenbach,
Your Guide to Animals / Wildlife.
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Birds are an amazing and diverse group of animals. Birds, best known for their ability to fly, are unmatched in their command of the skies. Albatrosses glide long distances over the open sea, hummingbirds hover motionless in mid-air, and eagles swoop down to capture prey with pinpoint accuracy. But not all birds are aerobatic experts. Some species such as kiwis and penguins, lost their ability to fly long ago in favor of lifestyles suited more for land or water.

Birds are vertebrates, which means that they are among those animals that posess a backbone. They range in size from the minute Cuban Bee Hummingbird (Calypte helena) to the grand Ostrich (Struthio camelus). Birds are endothermic and on average, maintain body temperatures in the range of 40°C-44°C (104°F-111°F), though this varies among species and depends on the activity level of the individual bird (Sibley 2001, 30).

Birds are the only group of animals to posess feathers. Feathers are used in flight but also provide birds with other benefits such as temperature regulation and coloration (for display and camouflage purposes). Feathers are made of a protein called keratin, a protein that is also found in mammalian hair and reptilian scales (Sibley 2001, 16).

The digestive system in birds is simple but efficient (enabling them to pass food through their system quickly to minimize the extra weight of undigested food and the time it takes to extract energy from their food). Food travels through the parts of a bird's digestive system in the following order before it is excreted:

• esophagus - narrow tube that carries food to the crop
• crop - a sack-like widening of the digestive tract where food can be stored temporarily
• proventriculus - the first chamber of a bird's stomach where food is broken down by digestive enzymes
• gizzard - the second chamber of a bird's stomach where food is ground up by muscular action and small stones or grit (ingested by the birds)
• intestines - tubes that continue to extract nutrients from food after it has passed through the gizzard (Sibley 2001, 26)

Sources

• Attenborough, David. 1998. The Life of Birds. London: BBC Books.
• Sibley, David Allen. 2001. The Sibley Guide to Bird Life & Behavior. New York: Alfred A. Knopf.
• The University of California, Berkely. 2006 (Accessed Online). Museum of Paleontology.