Fill the frame
With birds in flight, the key is to fill the frame, and more often than not this means using a long lens (see page50formoreonphotographing birds in flight). Having said that, with fast-moving birds it’s best to start tracking them when they’re small in the frame, and to start shooting only when they start to fill it. If you don’t, with a long telephoto lens you may not even be able to find the bird in the viewfinder, let alone frame it!
EXPOSURE 1/6000 sec, f/11, ISO800 LENS Nikon AF-S 600mm f/4G ED VR