I thought this would be a good place to start, since it is a passion of mine to constantly try and capture birds in flight. If you are up for a challenge, they will certainly provide one. Even if you forego the identification of several thousand species native to your chosen location, certain wide spanning characteristics are common among all flying animals. The first of these is that the larger the bird the slower its wings move to maintain flight, with the inverse being true for the smallest of species. As far as I can tell this is likely due to the sheer volume of air they are able to displace with a wider surface area on their wings versus a common hummingbird or bumble bee.
It is vitally important to realize what size of bird you want to capture in order to set your shutter speed accordingly. There is no reason to have to set your shutter speed to anything higher than 1/2000sec for any bird to stop their wings completely. A super fast insect wing might demand 1/2500sec. So why not leave the shutter speed at 1/2000 for all of your bird shots to guarantee frozen wings you might ask. Simply, if you are mega rich shooting a $10,000+ f2.8 lens on a D4s at 11fps then you are likely not reading this and have your own techniques. Light is your friend, when you have to start pushing the limits to get that great sunset glow or its merely a darker overcast day any additional light you can capture will help dramatically. Most of us using a super telephoto lens 400mm or longer are between an aperture of f5.6-6.3. So in our exposure triangle we have two choices, up the ISO adding noise or reduce the shutter speed and risk motion blur. My personal taste is to go to ISO as a very last resort, after all many of the shots you take will need to be cropped to 100% to see the detail in the bird.
A gliding hawk, eagle, raptor, heron, pelican, swan, goose or the like rarely need a shutter speed faster than 1/250sec to stop these large birds in flight. That extra 4x light factor from 1/2000sec gives you a couple additional options. One you could increase that aperture to around f8-11 on a sunny day guaranteeing focus as well as getting that ISO to your cameras native ISO which is generally 160-200. Ultimately you gain the best possible image of your birds in flight. Learning a couple functions of your camera will also greatly increase your “keepers”. I personally shoot all of my images in full Manual mode, even though I could get nearly every shot equally as crisp using Aperture priority Nikon/Aperture value Canon, I find it varies more than I like mainly because of the need to use your cameras built in light meter to make that last second decision for you. Once you move out of rapid fire and into trying to compose the shot of the bird in flight it just plain no longer works as you would like it to and is metering off of whatever the center of the frame is aimed at. Two main things to adjust when you are going to shoot birds in flight are your focus type and focus button. Even point and shoot cameras have a continuous focus, or servo that you can set to constantly focus as long as you hold the focus button down. Many add the option of choosing how many focus points to use in combination with continuous focusing, I would recommend starting with 9 or the closest option your camera has to it and slowly working your way to one as your tracking ability increases with practice. The issue arises when your shutter button and focus button are the same button, the half way pressed button is extremely unstable and difficult to track a moving subject with. Most newer DSLR’s even consumer grade cameras have the option of “back button focusing” or at the very least the ability to assign a focus button other than the shutter button. If you want to get better at tracking and capturing birds in flight these are the fundamentals of success.
Obviously all the other “rules” of photography apply, for wildlife they are a bit different and sometimes more difficult. But Try and get the sun on your back with the action in front of you, the more light is generally the better, the quality of light is more important than the quantity as always. Fill the frame, the closer you get to your birds in flight the more amazing and sharp the birds will be, however composing the image with some background has a more artistic appeal. Most importantly get your camera out and have fun. I am no expert on the subject but have a fairly extensive portfolio of birds in flight and I love the constant challenge they provide.