Is a 300mm @21mp same as a 600mm@10mp?

If I am shooting with lets say a 300mm f/2.8 that is 20+ MP camera is that about the same as shooting with a 10mp camera using a 600mm lens?

The reason I ask is I have a 1D MK III and a 300mm f/2.8 IS lens. IF I were to switch to say a 1Ds MK III at 21MP is it ABOUT equal to a using a 500-600mm Lens on my 10mp 1D MK III?

I find that shooting small birds/animals I have to REALLY crop to get anything worthwhile. I know a longer focal length would solve the problem but would more MP do the same thing?

Other than the 1.3 crop factor what am I missing here?

P.S. I already have a 40D and it is not nearly as good as the MKIII so I don't want to go the 1.6 route.

Thanks,
Ryan