I recommend The Birder’s Handbook: A Field Guide to the Natural History of North American Birds. It was written by three serious bird experts and it will serve any bird watcher in North America very well. Here’s how you use it. You go bird watching and later, you look up one or two of the birds you saw, read the entries on them, read the entries cross referenced in those first entries, and otherwise explore around this compendium of information about bird ecology and biology.
For instance, you spy a Clapper Rail. So you look that up and see the reference to the essay on rails. There, you learn, among other things, that rails are rare on birder’s lists not because they are rare (they are not) but because they are hiding.
Then, the next time you are out with someone birding, and you see a rail, you can impress your friend with this bit of knowledge about rails. How cool is that?
OK, so you see a Gray Kingbird. You look it up. There, you find a reference to an essay on the evolution of bird nesting behavior, which will blow your mind.
And so on and so forth.
This is one of those books that I don’t have in the field back but that I do keep in the other pack (actually, a Duluth Tote Bag) which gets dragged to and from the car, cabin, etc. One of these days Imma build a trailer that hold books and battery chargers.
Anyway, The Birder’s Handbook: A Field Guide to the Natural History of North American Birds is a must-have.