One of my favorite photos to capture are birds in flight (BIF). It is a challenge and rewarding at the same time. Sometimes you can track them coming in towards you so you can get ready for them and compose your shots. Other times they just come upon you and you have little time to react and get a few shots, like this series. My favorite combination for this is the Canon MkIV and the Canon 400mm DO IS f/4 lens, sometimes with a Canon Series III 1.4X teleconverter. The 400mm DO f/4 is a lightweight , shorter in length the a normal 400mm, easy to handle and track on a subject lens, perfect for fast moving subjects such as flying birds. Some do not like the DO series of lenses, but if you learn how to use them and learn how to adjust your files it is a real winner. It is also only 4.3 lbs, so it is relatively lightweight to carry for long periods of time. This Snowy Egret just came out over the trees and was close in so there was little time to react and compose my shots.