Birds are the second largest group of vertebrates, outnumbered only 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 usually hollow; They have beaks and lay eggs; Their forelimbs are modified as wings and their legs are covered with scales.
The presence of feathers is one of the more distinguished aspects of this group. Besides 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.). To capture their food they rely on their senses, however most birds lack a strong olfactory system.