Bsmooth wrote in post #14757752
I have a place where I can go and practice shooting images of these amazing little flyers as much as I want on weekends.
Thats where i have been with my 1DMk2 and 100-400 for the past 4 weekends.
I have tried all sorts of different settings, and so far i think I have maybe 1 - 4 images that are fairly sharp, and in focus.
All of the above are shot at 1/800 or above.
I also wonder about how fast the lens focuses, as the 100-400 doesn't seem very swift in that department. I also have a 70-200 without IS, and wondered If it would focus faster. These birds actually come within 4 to 10 feet sometime, so I don't always need the range , just the speed.
I've also found I get better focusing against the grass as oopoosed to open sky. It may be because the grass is fairly brown now, and provides a good contrast to the darker backed birds.
I have tried to take shots of them myself using a T2i and a 100-400mm, it's not easy, at all... 
What I did find to help a lot was to zoom out as much as needed to be able to see where the bird is, and follow it until you see it coming at you, or close, then zoom in to follow it until you feel the need to shoot. Trying to focus on them, or, FIND THEM is the hardest part for me.
That helped a LOT, I have a 1DIV but haven't tried to shoot those birds again lately, soon, I will, I'm sure. It sure is FUN though! I love to try, I literally suck at it, but it doesn't matter, I have a good time, and people stare at me like I'm some sort of nut! heh
Try turning off the IS on the lens, and see if that speeds it up for you. Like you said, try the 70-200, you don't need the 100-400mm for those birds, and you will probably get more shots using it too.
Back in the early Spring of this year was my first and ONLY time shooting them, but I learned a lot trying, like that my T2i wasn't fast enough in the FPS department, and that since I now have the f/2.8 400mm lens, the 100-400mm is a TINY lens in comparison, I still have it, so I will try again on the wily little-bitty birds!
Good luck!
Randy