There is evidence that nearly 150,000 species of birds once lived on the earth, so what we see today is only a fraction of the world's avifauna over geological time.
Avifaunal diversity varies dramatically throughout the world.
On this page you will find links to extensive databases reflecting that diversity. Some of these checklists are very extensive and cover large geographical areas. Others are more limited. If we want to examine the distribution of a species, the larger databases are usually more appropriate. On the other hand, if you are new to a region, a regional checklist of birds will make birding easier as you will know what to expect.