The data comes from the North American Landbird Conservation Plan from 2004, which is mostly from Breeding Bird Survey data from the 1990s. You can download whole tables of data or query individual species.
A few examples: Cerulean Warblers are estimated at 560,000 and Henslow’s Sparrow at 80,000. These are two species that are rare but breed in our county in northern Illinois. And then there are Red-winged Blackbirds estimated at 190 million!!
The database is an excellent source of information and a good tool for bird conservationists.