Speaking as someone who does a lot of bird photography the advise above is quite correct. Let me even suggest a different approach- start with the glass you will need (e.g. 500 mm) & then get the body to meet your needs. The D7000 with the 500mm automatically puts you out at 750mm F/4. I.e. start with the lens then the body. Also consider refurbed or used top glass advertised in Nikonians & Fred Miranda.