Now set the Nikon models aside for the moment (we will keep things simple by just considering one brand at a time) and let's look at what cameras bird photographers actually use most often. Overwhelmingly, the answer is one of the higher density 1.6 crop models: a 40D, a 400D, or something similar. There are two reasons for this. Obviously, not everyone can afford more than a 50D or a 450D. But we can quite easily control for that confounding variable by only considering photographers that also own one of the big whites - we can pretty safely assume that most people keen enough to spend $10,000-odd on a lens are also keen enough to spend a good deal more than $2000 on a camera if that's what they want.

And what do we find amongst bird photograhers who own big whites? They fall into three main groups. I wouldn't like to sound too certain of the exact break-up between the three, but all three are well-represented, and at a guess I'd rank them in the order below:

Very high-density 1.6 crop bodies (40D, 50D, 400D, and 450D, though still a few 30D, 20D and 350D users around as well. 40D is the most common, 50D is catching on fast.)
Medium-low-density 1.3 crop bodies. (1D III, still a few 1D IIs around as well.)
High-density 1.0 crop bodies, (1Ds III, 1Ds II).

What happened to the 5D? Where is it listed? It isn't listed - because practically no-one uses a 5D for bird work.

Why would you want to use a 5D, given that a typical half-decent shot opportunity is only going to give you 2MP worth of bird picture? A D3 or a D700 would give you a pro-build body, faster frame rate, and better high ISO performance (compared to a 5D) but even worse finished-product resolution, at just 1.9MP. All three 12MP full frame cameras, none of them cheap, give you fewer pixels on the bird than an ancient 10D would. All three are very fine cameras, but none are suitable for bird work, except under exceptional circumstances - such as if your name is "Andrew" and your target species is the Laughing Kookaburra.

So why, if the crop cameras like the 40D and 50D give you so much more finished-product resolution, do many dedicated bird photographers not use them? Why would you spend all that extra for a 1Ds or a 1D knowing that you are going to have lower finished-product resolution?

Many, perhaps most, of the really, really good bird photographers use a 1Ds because they judge that they are good enough at it to get really, really close and fill that much wider frame up. It's significantly harder work, but if you succeed at it, the results speak for themselves. Also, notice that the latest generation of full frame cameras now have pixel density just as high as the old 20D and 30D had - so that's getting pretty respectable. And, of course, these are cameras with top-class autofocus systems, superb viewfinders, and just about every other possible useful feature - once you use a 1 Series body, everything else seems pretty clunky.

The 1D III is another popular choice amongst this same group of photographers (ones who own big white glass and can thus be presumed able to use whatever camera they want). (There are still some 1D IIs around as well.) On the face of things, this one seems harder to explain - after all, you take a big hit in resolution compared with something in the 40D class. You don't get as much resolution so you arguably need to be even closer than you do with a 1Ds, but you still get all the pro body features like viewfinder and top-class AF system, plus two other things: a very fast shutter repeat rate, and very good high-ISO performance. It's a trade-off. Some people think it's worth it, some don't. (In theory you could take this approach even further and have essentially the same sort of camera but with even better high ISO at the cost of even worse pixel density - that's what the D3 amounts to. Most bird photographers apparently regard this as one bridge too far, and I agree: of all the currently available cameras from Canon and Nikon, I'd put the D3 second last on my wish list, superior for bird work only to the 5D, which is almost equally low resolution but much slower and lacking things like the pro AF system too.)

And, of course, a good many top-class bird photographers with an effectively unlimited choice of gear nevertheless use 1.6 crop cameras, mostly 40Ds and now 50Ds. They would rather work within the limitations of a non-pro body, with non-pro auto-focus and less than stellar feature set, but not have to crop so hard. Yes, the larger cameras deliver better quality pixels, but there is still much more detail available from a real-world crop of a focal-length-limited (e.g.) 50D image than there is from one taken by a lower-density body.

Personally, I think there is merit in all three approaches. I really like the wonderful picture quality delivered by the 1D III (there is a lot to be said for fat pixels), the clarity and rich, subtle colours, the focus speed and accuracy, the ability to push the ISO any time I need to, the big, clear viewfinder, and the all-round handling qualities. In exchange for that, I sacrifice a lot of reach - with the cumbersome 3.9kg 500mm f/4 lens, the 1D III delivers less detail than a 50D does with a little 1.4kg 100-400/5.6. Or, to put it another way, to get an equally detailed picture I have to get just as close to the bird with the 1D III and 500/4 as I do with the 40D and the 100-400, and that can be quite a lot harder. But, most of the time, I think it's worth it. Apart from the focus system, the frame rate, and the richer resulting image, I've got an extra stop of aperture up my sleeve if needed. On balance, this is my preffered strategy.

But there is a lot to be said for using something like a 50D instead, and quite often I do. (Especially since I replaced the rather lack-lustre 40D, which to my mind never really improved much over the old 20D, and certainly didn't deliver any better image quality, with a 50D.) In general, I think it's usually better to use 50D and 1.4 converter than 1D III and 2X converter, and quite often better to use 50D bare lens than 1D III and 1.4 converter. The reach is about the same either way, but the focus is faster (perhaps not quite so accurate) and you have an extra stop of aperture available.

As for the full-frame, high-resolution method, I can't say from practical experience. One day I'll buy a 1Ds III and see for myself ....... except that by the time I can afford one they will be up to the 1Ds LXVII.