There is a fairly wide variety of online species accounts that give information on identification, life history, ecology, population, habitat preference, conservation of avian species. Here are links to just a sampling.
Here in Wisconsin, (of course ours are the best! - are you kidding me?), you can find the WBCI All-Bird Conservation Plan species accounts - just go to this link. Not all species are yet posted, but the Priority Species are.
The Western Great Lakes Bird Conservation site still has some excellent links to species accounts here. The links in the last column on that page no longer appear to be working, however. More, on a related site ("Forest Birds of the Western Great Lakes") can be found here.
The Cornell Lab of course has their All About Birds page, and the USGS Patuxent Wildlife
center has these grassland birds species accounts.
Somewhat more simplified (limited to identification, similar topics) species accounts from the online Sibley guide can be found at this eNature site.
NatureServe Explorer has excellent information - go to this link and fill in a species name in the search box. Click on the link to the species name that results, and there is a table of detailed fields.
You can find Wisconsin DNR accounts to the "Working List" species here.
This Smithsonian page on the Sedge Wren is not a species account in the same sense that the others are, but still worthwhile reading.
There are many more - quite a few states and provinces have their own.