Originally posted by bkpix All good advice here. Keep in mind, though, that in the end, even if you could afford a costs-as-much-as-a-car Canon wonder lens, you still have to get really, really close to the birds to do bird photography. Extra millimeters are not a miracle cure. And shooting with long lenses can be frustratingly difficult.
I mention all this just to keep things in perspective. You can do a fair amount of bird photography with a 300 on a crop body so long as you're willing to stalk, use a blind, or search out big, cooperative birds that don't mind your hanging around with a camera. And even if you manage to buy that wonder 600/4, you're still going to have to stalk, use blinds and find big, easy-going birds!
I'll second this. A 600 f4 takes a shot that is ok at 300 f4 and makes it great. If it isn't ok at 300mm, it won't be much better at 600mm. 400mm will give you great shots. I shot with a 400 f4 manual lens for a year and really enjoyed the length. You have to be very close to get a great shot at 300mm. If you learn to get great shots at 300mm, a longer lens in your hands, with some skill, will give you stellar results.
Length is nice, very nice, but sharpness wide open is even better. Light gives you shutter speed to play with, and keeps iso down, and the faster the lens, usable faster, the more light you have to work with. I find my 150-500, which is nice at f8 is only useful if the sun is shining and the subjects aren't in the shade. Cheaper lenses also don't handle bright light very well without stopping them down substantially. It is extremely frustrating to get a bright day and your shots around water are full of hot spots and flare, and you need to stop down to f11 or f14 to control them, losing the fast shutter speeds you were looking forward to using to freeze birds in flight.
There is a reason why folks spend far too much money on lenses when they are chasing wildlife.