To my mind there is nothing more spectacular and captivating than capturing beautiful images of birds in flight.
Birds are wonderful creatures, some of them are so beautiful. They range from tiny Fairy Wrens and finches to the very large Eagles, together with the giant Cassowaries and Emus. There is also such a large range in the colours of birds.
In order to capture birds in flight on your camera I have found that you need very long lens generally in the order of 300mm or more, I really like the 300mm focal length and have used that for the majority of my bird in flight photography. Shorter lenses can be used if you select a spot where birds frequent on a regular basis and use a remote shutter release, I have had some success with this method but it is rather hit or miss.