Oystercatcher, Haematopus ostralegus, single bird in flight, Slimbridge Gloucestershire, January 2018.
Taken with the Olympus Em1 Mk2, 300mm f4 lens plus 1.4 extender. 1/8000th at f5.6. 1600 iso. No crop.
I was using the Olympus Dot Sight Viewer. This is a small device that fits to the hot shoe of the camera. When powered on there is a red dot which you line up onto your subject. It needs calibrating in the same way as the sights on a rifle, but is simple to do.
Instead of looking through the camera viewfinder you use the viewer, which is clear, bright and flicker free to follow the bird in flight. It is much easier to follow fast flying birds. The downside is you can’t judge the image size and have to hope the AF is working well.