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From Citizendium, the Citizens' Compendium
Birds are a class of animals (aves) that can be distinguished from all other living animals by the presence of feathers. They are two-legged, warm-blooded vertebrates with wings and they range greatly in size, the tiniest can fit on the palm of a human hand and the largest can be ridden upon. The largest living bird species (the Ostriches) are flightless. Most bird species exhibit sexual dimorphism to some degree, that is, that male and female birds look different. It is usually the males which are the more brightly coloured.
Birds reproduce by laying eggs, as do reptiles and amphibians. The eggs are usually placed in nests, some of very intricate construction, some very casually put together, and incubated for a period.
Birds are extremely valuable to humans in several respects. Domestic birds are raised for eggs and meat, called poultry, and many wild bird species are hunted for food. Other species are kept as pets. In addition, many birds eat insects that humans find irritating or destructive, and they consume weed seeds. Finally, birds act as useful scavengers around areas of human habitat.
There are more than 9,000 different bird species.