The 18'000 bird species thought to exist today, a number which just recently increased from 10'000, belong to the Class Aves. Birds belong to the Class Aves and have the following, among other,
characteristics in common: They are vertebrates carrying a back bone, use feathers for insulation, are warm-blooded, use wings mostly for locomotion and are egg-laying.
There are an estimated 10'000 species worldwide. However, a recent study by Barrowclough based on morphological and genetic differences suggest this numbers has to be almost doubled,
reaching about 18'000 species!