In terms of the photo's orginal area, the image has been cropped about 50%. There is only so much you can do with a 200mm lense when going after birds. I'd love a longer focal length, but a decent lense longer than 200mm costs a small fortune.
So, that is where a good sensor comes in. I'm all for filling the frame and maximising detail, especially for prints, but with wildlife and sports, it isn't always possible. If you have good sensor such as the one found on a Canon 5d Mk III, you can crop in and still retain good picture quality. Just beware however you may run into problems when wanting to print.
Bird Photographing Tips and Tricks
Meter for the bird, not the background: I see many pictures of birds where the exposure has been set for the background and not the bird. This can lead to under exposed birds, especially when dealing with dark birds
Use TV ( or better yet, and if you can, manual): Most of the time when shooting birds, you will want a fast shutter speed to freeze the birds movements. Even when they're not flying birds can be hasty wee things. Dial in your desired shutter speed and take a few snaps. If the animal is too dark, you can open up the aperture. If you're already wide open, bump up the ISO a stop or two and see what happens.
Exposure Compensation: If you're in P, TV or AV mode you can use exposure compensation to increase (most likely) or decrease your exposure. Be aware however, that at high shutter speeds there is a chance that you're lense is wide open already and exposure compensation won't work as it can't do anything to get more light. In that case, you'll have to bump up the ISO.
Practice: Put some bird seed out and you'll most likely find sparrows and tits feasting in no time. Practice on these wee critters. They're fast movers so if you can shoot them, you'll be doing very well!