Sure, you can count birds...when there are 1, 3, 5, 12, or even 22 birds. But how do you count 172 birds when they're in flight? Or 206 birds sitting around where depth of the landscape is involved? I just learned about the "width and depth" problem when simple extrapolation do not work.
If you don't know what I was just writing about, then you really need to read this article.
I just finished reading the article myself on eBird and I thought I'll pass it on. Very interesting and helpful, actually. Check it out here with this link ebird.org/bird-counting-101.
Enjoy, and good birding!