There are thousands of bird pages on the web these days, most are related to bird watching. I have made no attempt to list them all because other excellent lists are already available and because my experience with the insect page says many of them will disappear relatively quickly and become dead links. Those that I consider the best are below, if you are looking for information on a specific bird species, genus, family etc that you can not find, your best bet is to do a global search. Search engines are a lot better these days.
The National Audubon Society
The British Trust for Ornithology
Fat Birder Links to Birds sites around the World
Cornell Lab of Ornithology
American Birding Association
Electronic Resources on Ornithology