Bird photography

The Pentax K-5 II is the next generation of the K-5. You can purchase the same 150-500mm lens for Pentax. It is great in low-light and a great value for the price. This is the camera I am going to purchase over my Pentax K-r. I shoot nature as well. However, if this is absolutley not possible in your case, I would purchase the D7000 with the 150-500mm. There is no need for the Canon 400mm if you have the 150-500, because you should be able to shoot at 5.6 at 400mm anyway.

bird photography depends on the lens you are using. so if you choose sigma lens you really dont need to decide on what camera body you use, since it ia a same lens. but if you are using nikon lens i will recommend you the 200-400mm vr. this lens is the best for bird photography is you dont mind the price. you can also have the 70-300mm lens if you want... cheaper

D7000 is a good choice.

But Pentax K-5, why not consider it also? Better value in my opinion.

D7000 has better low light handling, therefore cleaner image, also focusing is much faster with 39 af points, dual sd slot is handy for shooting series also i believe burst rate is better on d7000