If you came from a 50D, then most likely a D7000 is not going to meet your expectations. I'm going on the assumption that you're on a tight budget based on what you've written so far and what you've bought, so feel free to correct me if I make any incorrect statements.
What was the gear you had previously when using the 50D (i.e., what glass did you have)? My suggestion, along with what others have said, is to invest in good glass. If you're interested in capturing birds, then I'd suggest at least getting the 70-200 - either Sigma or Nikon
models. I believe that both of these will be of more use to you than the 70-300, and definitely more useful than the current lens you have on the D5100.
In order to capture great images/video, it comes down to the following (in order of priority):
1. The person behind the camera (that's you)
2. The lense
3. The camera body
Though I don't have as much photography experience as some of the guys here, I've owned and used my fare share of camera bodies from both Nikon and Canon amongst others. At the end of the day, it wasn't the body that made me a better photographer. It was working with what I have and upgrading the glass when I could no longer use what I had since it kept limiting me. For example, I prefer shooting a lot of available light (even in low light) so naturally I need fast glass (e.g., <= f1.8 ) and that's the approach I've taken. At the end of the day, it's the glass that will help you more than the body.
The D5100 offers a lot of creative avenues and the LCD screen will give you a distinct advantage that the other DSLR bodies won't. Learn to explore with the D5100 first, get decent glass, shoot and capture for at least 6 months and before you know it, you won't even care about the body construction. In essence, go out and take photos and worry less about the body