The following question is asked with canon equipment as examples, because that's what I know. Nikon users should feel free to translate to their brand. The question is generic.
Most folks seem to agree that amongst current (February 2012) APS-C (1.6 crop factor) Canon cameras the 7D is the clear choice for bird photography. It's faster, has more pixels, more autofocus points, and generally outshoots any other Canon APS-C. It also costs the most. I think there's effective consensus on that.
I also notice that amongst the pros, given the choice of all Canon cameras, not just APS-C, the most common choice for a primary body is the 1D Mark IV; and I'd like to understand why. The Mark IV has three big disadvantages:
#1: It costs four times as much. Nuff said.
#2: It weighs more than a 7D.
#3: It has a much larger pixel pitch than a 7D. This means you need longer, heavier, and more expensive glass to get the same number of pixels on the bird. This vastly multiplies points #1 and #2. (Caveat: I'm assuming you're not shooting pelicans at point blank range here. If your primary location is the St. Augustine Alligator Farm or equivalent, then you may actually want a wider angle.)
Given those disadvantages, what factors lead people to choose the 1D Mark IV, and how relatively important are they? Possible advantages I can see to a 1D Mark IV are:
#1. Faster autofocus
#2. More accurate autofocus
#3. More autofocus points
#4. Ring of fire
#5. f/8 autofocus
#6. Better sensor; higher image quality*
#7. Better high ISO performance*
#8. Am I missing anything?
* Image quality has to be compared after the images are cropped to the same field of view, and one image is reduced in size to the same number of pixels as another.
What makes the 1D Mark IV worth more than $10000 more than a 7D? (after accounting for the more expensive lenses you'll need to put the same number of pixels on the bird). Why carry this camera instead of a 7D?