Sun 28-Oct-12 05:38 PM | edited Sun 28-Oct-12 05:38 PM by briantilley
Your prime requirement seems to be "reach" - which in this case equates to the number of pixels a given (small) subject will cover.
Let's say that you've got the longest lens you own with a TC on the camera, and the image of the bird is still only 1/4 inch across on the sensor. Using your D7000, it will cover about 1,325 pixels horizontally. If you change to a D600 but keep everything else the same, the bird will cover only 1,060 pixels - 20% fewer.