That is the problem with wildlife. It takes so much light to capture the image at some point in the afternoon you do not have enough light.
That is why I suggested setting the maximum for aperture and ISO. When you can no longer get enough shutter speed you have reached the limits of your equipment.
I wasn’t sure the 7D allowed you to set the limits on the Auto ISO. If it does then just set your maximum. I would also suggest good noise software, I say this because I was comfortable using the 7D to an ISO of 1600 back when I was using it and you had mentioned 800 as your limit.