The ability of birds to fly and their warm-blooded metabolism has allowed them to colonize and take good advantage of a large variety of habitats. Only parts of the open ocean and the center of Antarctica are typically devoid of birds. More information can be had on the Geography page.
There are about 10,000 species of birds in the world and different parts of the world have their own unique set of bird species; this is the avifauna of the areas.
The countries with the most diverse avifauna are Colombia with 1,821 bird species, Peru with 1,781, Brazil with 1,712, Indonesia with 1,604, and Ecuador with 1,515. You can get the full list here, but notice that rainforest countries have the greatest variety of bird species.
The IOC World Bird List lists the official English names for every bird in the world and is an extremely handy reference.
Fatbirder – links to birders all over the world
International Ornithological Congress Bird Checklist
Sibley-Monroe Checklist of the Bird World
Web-Published Checklists of the Bird World
World Bird Database – Avibase