Yes, it is a very good use for that lens.
I used it a lot more often for soccer where I needed to cover a lot of area without moving, it is a very sharp lens with very fast focusing and it is not very heavy compared to wide aperture telephoto primes.
Photos below were shot with my EF 400 f5.6 with 1DX Mark II body several years ago at a spring afternoon soccer match.
Rodger
I really like the 400 f5.6L but with wildlife, they are usually further away than you would like for your lens but sometimes too close. There is no perfect focal length but I think the 400mm is a good compromise between the shorter zoom lenses like Canon's excellent EF 70-200 f2.8 and the very heavy and very expensive EF 800 f5.6. Often you will wish for more "reach", other times your reach is too long for the situation, and when you are lucky it is just right.
The first photo was on a very cold day on the Illinois side of the Mississippi river north of the Quad cities. I was using an EF 800 f5.6 plus a 1.4X extender because the eagles were staying on the far side of the river and even that 1,120mm combo was marginal in terms of reach. Earlier in the year, I was in the middle of Iowa photographing eagles with the bare 800mm lens when a goose decided to land in front of me and the 800mm was too much telephoto for that surprise landing. Later in the day, a duck decided to put on a walking on water show at a great distance for my 400 f2.8 lens.
Another possibility is one of the third party 150-600mm "super zoom" lenses, I bought a Sigma for a hiking lens a couple of years ago and I am very impressed with its image quality. It isn't in the same league as Canon's prime glass but it is surprisingly good. The last photo is one shot with it today. So a "super tele zoom" is another possibility with a little more versatility and it does add image stabilization allowing you to drop shutter speed for non-moving wildlife when light conditions demand.
Rodger
Ernie,
I definitely wouldn't run out and buy a telecon just for the 400 f5.6 BUT if you have one for use with other better suited lenses or plan to acquire such in the future, it might be a better solution than cropping depending upon the camera. The combination either requires very careful manual focusing or a camera that will autofocus at f8 and preferably allows the user to dial in perfect AF calibration for the combo because the DoF is quite shallow at moderate working distance.
I love the design of the 400 f5.6, it is light and well balanced with a very convenient built in metal lens hood. It was my wildlife hiking lens for years until I bought the Sigma "super tele zoom" and it is a lens I will likely never sell because it does its job so well. I guess when I am too old to run up and down the field with a 400 f2.8, I will switch to daytime only games with the very light 400 f5.6.
The birds weren't cooperating this morning but Friendly Tom, the barn cat, was willing to pose for the 400 f5.6 plus 1.4X combo shot with a 1DX II. Definitely loses sharpness and contrast compared to the bare lens but it does take the 1.4X better than a lot of glass that Canon rates as 1.4X compatible.
Rodger
