Hi Hutber. Thanks for your nice comment 😊
Focusing on flying birds with long zoom-lenses is quite a challenge with no auto-focus and a very limited dept of field. To overcome this, I have built an extended focus wheel with a suitable gear ratio. Combined with a lager screen and a lot of practice, (not to forget pure luck) it is possible to get the flying birds.
For the wildlife video I have used a variety of Canon lenses from 16-35mm to the zoom lens 200-400mm with extender giving a zoom range from 200-560mm. Hope this answered your questions.