Here's my first BiF photo posted here (I think), and one of the first ones I like from the Rf 100-500mm. This killdeer is one of a flock of a dozen or so that spends afternoons in a field near our home. They love to stay far away and usually fly just below tree-lines, but this one flew a little high and I was able to get a shot. I was only at 324mm because I needed a little extra field-of-view to keep it in my EVF.
I thought I'd try the much lighter 100-500 with the 1.4x TC after yesterday's outing with the heavy 400/4 + 2xTC. Here's a Skylark at 700mm f/10. These birds are very small and a decent focal length is needed but 400/500mm is easier to handle.