5: Mega Pixels.
Broadly speaking the more, the better although the quality and size of the sensor is also important. You need to be looking at around 20 MP. This is pretty much the standard at the moment in enthusiast and professional cameras. Bird photographers crop their images a lot during post-processing in order to make the bird bigger, if you don’t have enough pixels you wont be able to do this without the image starting to look pixelated.
6: Crop sensor is good enough.
I say this knowing that there is a huge debate about this, but crop sensor cameras are cheaper and wildlife photographers like them because they appear to make the bird bigger on the produced image. Full frame cameras do produce slightly better-quality images, but the difference is minimal and difficult to see in most examples. I would estimate the around 75% of the photographers I know and talk to, some of them professional, use crop sensor cameras. Don’t feel you need to pay out big bucks for a full-frame, if you can afford it you might want to but otherwise crop-sensor is fine.