First, let's start with some interesting numbers:
County with the most species: California's San Diego county (541 species)
County with the fewest species: Hawaii's Kalawao County (28 species)
State with most counties: Texas (254 counties)
State with fewest counties: Delaware (3 counties)
Grand total number of U.S. counties: 3,139
The list of species for each of the United States' more than 3,000 counties that you can get on this website is derived from eBird.org, a website of the Cornell University Lab of Ornithology. Many thousands of birders regularly submit their sightings to eBird.org, which makes the information openly available to all.
The resulting vast store of information about which birds are seen when and where provides extremely valuable information to scientists, birders and others. You can see the progress and routes of migration as birds move to cooler climes in the spring and to warmer places in the fall. EBird information can also show the effect of storms on the distribution of birds. These are just two examples of the many questions eBird data can answer.
Most states have volunteer committees of experts who determine the official state checklist of birds that have ever been seen in that state. Sightings are rigorously vetted to make sure that the bird was actually correctly identified.
Caveat No. 1 is that the eBird lists of sightings are not official state checklists. Unusual eBird sightings are vetted by individual volunteers, but the vetting is not as formal or thorough as that done by official state committees.
Caveat No. 2 is that not all species that have ever been seen in a state or county have been entered into the eBird database. This is another reason that official state checklists do not always agree with eBird checklists.
During most years, new rare sightings are reported and accepted by state committees, so checklists do grow. (Sometimes they even shrink as previous sightings records are re-reviewed.) The county checklists shown on this site were based on eBird lists as of April 2018. Sightings of any new species reported to eBird after that will not be shown on this website's county checklists. However, any such subsequent new species are likely to be rare or uncommon visitors to the county. Otherwise, they would have been previously reported.
Caveat No. 3 is that the eBird sightings gathered for use on this website have been matched against the American Birding Association's United States checklist (Version 8.14), which lists 1,138 species. Any eBird sightings that were not found on the ABA U.S. checklist were removed and not included in these county checklists. The reason is that, generally speaking, such species are birds that are considered extinct, escaped pets, not wild birds, or not a regular or expected species in the U.S. Examples are Ivory-billed Woodpecker, Northern Red Bishop, and Cockatiel.
Most likely, all the birds you will see in a county will be found on this website's county checklist for that county. And if you see a bird that is not shown on that checklist, you probably have an unusual sighting on your hands! Get a photo if at all possible!
Caveat No. 4 is that the county checklists on this website do not indicate how common a species is in the county or at what time of year it's present. Some of the species on the checklist may have been seen only a few times in the county, while other species are common, abundant and present throughout the year. Without further research, you have no way to know the likelihood of seeing any particular bird on the county checklist.
Here are some suggestions for getting more information about species or sightings in a particular location or county: