I have a 1D3, and had a 7D, which I sold when I got it. I also have a 5D3, and although I don't "bird", I thought i would pass on that:
"If you aren't focal length limited, my vote is 1D3 all the way. And a great bargain at the going prices IMO."
True ... the other weakness of the 1D3 is the LCD screen ... it's the first body I have had where the images on the computer are always better than they looked on the back of the camera!
The 7D and 1D3 are going to be very close at higher ISOs. You are not going to gain too much. You might be better learning how to set up the 7D for higher ISO shots, and how to post process them. You should be able to go up to 3200 at least."
In my experience, there is much less processing required on 1D3 images as compared to 7D images to get to the same result, especially at ISOs above 800 or so.
"I shoot equine sports. I have a 1D3 and a 7D and the 7D is all I use. I actually much prefer the ergonomics of the 1D3 but for me the additional crop and the higher resolution make the 7D the winner. The noise is usually only an issue for me above ISO3200 but, I use noise reduction software for all my images from the 7D."
Exactly right in my experience, the 1D3 has better ergonomics and more easily handled noise levels at high ISOs. If you don't have the long lenses, the 7D may well be the better choice, however.
"I shoot ISO3200 on my 1D3 all day long Not preferred but it's great for me. To be fair though, I've learned more about photography using the 1D3 than I ever did with my 7D so I often wonder If I would use a 7D better now."
In my experience ISO 1600 and 3200 are very usable from the 1D3 with less post processing than similar images from the 7D.
I would sum up the 1D3 as having near instant response and that it is a delight to use.
5D3,1D4,S90,6S&Moment Lenses,Hero4Silver,GPS-E2,2x580EX,430EX,90EX,EF16-35L f/4 IS,Samy SYTS24-C 24TS,EF24-105L IS,EF50 f/1.4,EF70-200 f/4L IS,EF300 f/4L IS,EF100-400L I IS,Kenko DGX 1.4X,Canon 2X TC Mk II, RRS&Pro-Media L-Brackets,Manfrotto MHXPRO-3WG & Roller 50,Sirui 306&K-20,Giottos MT-7371&MH-3300,Velbon ElCarmagne 530,CamRanger,Phottix&Canon Remotes,Lowepro Backpack,ThinkTank Retro 20&Modular System,OpTech straps,Lexar/San Disk Cards