Josh,
First, these are very nice pics. I especially like the comp in #3.
As for the cameras, I have a 1 Ds Mk2 and a 40D and a 500 and my experience is that I get better birdie shots with the 40D. I think mainly because it is a lot easier to use so its probably more me than the camera. But for example to change the ISO you need two fingers of your left hand to press two buttons down while you turn the wheel with the other hand. Try handholding a 500 and doing that. I now use my 1DsMk2 for more structured shots and my 40D for field work. So I would second Tim's suggestion to try and get a 500 F4 and keep the 40D. His point on the relative depreciation rates is also very important as far as maintaining the value of your investments.
Martin
Hi Josh, wow, lots of decisions here on gear and some interesting comparisons and suggestions!
Bird shots look great, I like the creative comp on the wire
To use your 500 on your 40D you can tape the pins on your TC
Not having had the use of a 500 4.5 I wonder its autofocus speed compared to the F4/ IS version The IS is a wonderous help on the still shots IMO, allows the lower ISO's which hold up even better to crops.
The IS is a wonderous help on the still shots IMO, allows the lower ISO's which hold up even better to crops.