Birds in flight is a challenging task. It takes a good fast focussing camera, a fast focussing lens and lots of practice,. I am fortunate to have a number of falconries near where I live at which regular flying displays take place. My local camera club organised some tuition at the Barn Owl Centre in Gloucester. Slowly we all improved our sharp shot hit rate. I use a Canon 5D III which has very good focussing performance and my favourite lens is a prime Canon f5.6 400mm. There are better ones for this task but the lens I chose focusses very quickly, is pin sharp and affordable. Not too big and heavy either.