I am a wildlife photographer. I have the 5D MKII which I've used since it came out (also the EF 400 DO II). I am ready to upgrade the camera but not sure which upgrade is best. I like the idea of the 5DS because of the large file size and ability to severely crop. This would enable me to get the fine detail of feathers on small birds at distances of 30-50 feet. However, most of my shooting is hand held. So I am concerned about the 5DS sensitivity to shake. I am also shooting in woods often where light is not great. So I am thinking that maybe the 5D MK III might be the better choice.
There are a few other things to consider and, if I were you, I'd be considering the 7D II.
When you shoot wildlife and particularly "birds in flight", the camera is typically in "continuous" burst mode (so you press the shutter and the camera goes click-click-click rapidly capturing many frames. As you review the frames, you'll find many are rejects but, with enough frames captured, hopefully one is not just usable... but a great shot.
You cannot control the actions of the subject -- rapid bursting of shots improves your odds.
The 5Ds images are so large that the buffer fills quickly and you'll only get a few shots before it's full. The 5D III images are smaller and it takes longer to fill the buffer. With a fast CF card, you may get 15 RAW images before the buffer is full.
The 5D III has a pixel size of 6.25µm squaree. This larger pixel size gives the camera more sensitivity to low light and the 5D III can shoot at higher ISO and have lower noise than the 5Ds or 7D II
The 5Ds has a pixel size of 4.14µm square. This gives it the ability to resolve finder detail (assuming the lenses you use are good enough to deliver that much detail AND that you are shooting at a low enough focal ratio to avoid diffraction issues.) But what it gains in resolution, it looses in ISO performance.
The 7D II has a pixel size of 4.1µm square. This is roughly the same as the 5Ds but the 7D II is optimized for action photography, has a larger buffer, and since it's a crop-frame camera with smaller image sizes, it can hold significantly more images in the buffer while doing continuous shooting... before the buffer fills up and it has to wait for an image to finish writing to the memory card before it can shoot another image.
"... , if I were you, I'd be considering the 7D II."
This is a good answer. But a second choice and the most sought after 'birder' camera there is, the 1D Mk IV. You will have to find one used as it is no longer in production (2012). Birders are keeping used prices up for this best of all APS-H Canon professional cameras.
The 1D Mk IV will make that 400mm DO really come to life. BTW, I consider 400mm minimum for birding but the 1d4 will make it appear as a 520mm f4.