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Cameras
My first DSLR

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My 10D with battery grip |
My first DSLR was the Canon EOS 10D. I only really considered Canon and Nikon equipment, and I chose Canon after reading The Art of Bird Photography by Arthur Morris, who has used Canon equipment for decades. I also did some online research, and the popular opinion was that Canon had the edge in auto-focus and image stabilisation. Is this still the case? The fact is that if you choose one of these major brands you'll have the equipment necessary to produce good quality images.
I bought the 10D as it seemed like a good entry-level model and the price was about right. Considered a 'Pro-sumer' model, it was aimed at the serious enthusiast rather than the professional. It has since been replaced by the 20D, 30D, etc, etc. I had no major complaints about the camera, although certain aspects of its performance began to frustrate me. These are issues you should consider when investing in a new camera for bird photography.
In continuous drive it would shoot a maximum of 3 frames per second (fps) and for a continuous burst of nine RAW images. For birds on the move (e.g. flight), 3 fps is not really enough. Also, when the nine images had been taken, it seemed to take forever for the camera to release sufficient memory to take a further picture.
Where possible, I will check the histogram after taking the first shot before taking further shots, but it took the 10D too long to save the image to disk and make the histogram available for viewing.
The fps, continuous burst for RAW images, and recovery speed are all important factors in bird photography.
My Current Camera

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The 1D Mark II N |
Although always intending to upgrade my camera, I was 'encouraged' to do so early when I bent one of the connecting pins on my 10D when reinserting the CF card. I later had the camera repaired by Lehmans in Stoke-on-Trent for £150, and now use it as a spare.
I was unsure of which way to go, as it seemed at the time that there was a significant compromise between speed of shooting and image file size. I went for the Canon EOS 1D Mark II N. The 1DS is simply too expensive and arguably the file sizes unnecessarily large for me.
The 1D has a shooting rate of 8.5fps and a 22 RAW image continuous burst. The speed of response of this camera is amazing. 22 images is plenty for action shots and the memory is released and images (including histograms) made available for reviewing extremely quickly.
Of course, this camera was replaced by the 1D Mark III and then the 1D Mark IV (it will never end). If starting again, I would choose the 1D Mark IV. My current camera is superb for bird photography, but the 1D Mark IV shoots as 10fps with a continuous burst of 28 RAW and includes many additional features.