Ours started doing this too. Its spring, so he is even more excited about the birds and squirrels. I started taking him out on a leash. and release him after he sits and looks at me. This has greatly reduced the barking and growling, he still barks and rushes at the birds after I release him, but I figure hey, a dog has got to have its fun.
I am curious though if anyone more experienced has anything else to say about this.