Here is another Forster's tern frame. This one was taken in even dimmer conditions, hence ISO 10,000. I was impressed the camera could focus with the Extender 2X III on fast flying tern under such low light.
1DX MKII 400 DO II+ 2X III. ISO 10,000. f/8 at 1/1600sec hand held. Processed with DPP 4.4 Hope you like it. PS. there was no sun in the sky, I added the catchlight in post.