How did they get the numbers? Did someone count every bird?
Maybe Audubon’s annual bird counts.
I do it every year.
All you do is identify every and all birds you see on certain days. Then type it into their web page.
Audubon runs lots of bird counts — Christmas, Coastal, Western Rivers, Waterfowl, and many more. Lots of bird counts are run in states, too. Google “bird counts and surveys” for more info. You can volunteer, too. Doesn’t take much time.
The Cornell University Ornithology Lab’s terrific eBird app allows you to count the species you spot and the data is shared immediately in a worldwide database. You can find out what species have been seen in areas near you. It’s really useful if you are on annual migratory routes such as the Pacific Flyway where millions of birds pass through every fall and again in spring.