Perhaps the most challenging aspect for bird photographers is capturing pleasing images of birds in flight (BIF). However, It can also be the most satisfying when you do get it right!
Photographing Birds in Flight (Part 1 - Gear)
Your gear will help greatly in this regard, faster prime (fixed focal) lenses being ideal as well as a camera body that has good auto-focus abilities and higher frames per second. In general, zoom lenses do focus slower than fixed lenses. The speed at which a lens attains focus also slows down when teleconverters are used, even when using a fast prime lens.
Camera bodies like the Canon 1DMKIV or the newer Canon 7DII are excellent choices for bird photography owing their powerful auto-focus (AF) capabilities. On the lens front, the inexpensive Canon 400 5.6 lens has long been used by bird photographers to get their bird-in-flight images. The Canon 300 2.8 IS II or the new Canon 400 f4 DO IS II lenses are also excellent choices for improved BIF success.
Read Part 2 - Focus and ISO here.