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Birds are the second largest group of vertebrates, being only outnumbered by fish. The elements of this group are distributed all around the world, with about 9 600 known species. The common characteristics birds have are the following:
Their body is covered with feathers; Their bones are lightweight and usually hollow and have a beak; They lay eggs; Their forelimbs are modified as wings and the legs are covered with scales.
The presence of feathers is one of the more distinguish aspects of this group. Beside the insulation function, feathers play a fundamental role in their flight ability.
There are several feeding behaviours amongst birds that can range from carnivores (meat-eating) to frugivorous (fruit-eating). These different food choices influence deeply the form of their beaks, as well as their bodies (legs, wings, feathers, head, etc.). Also very important is the use of their senses order to capture their food - most birds lack a strong sense of smell.
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