use a fast memory card to allow fast burst capture
shoot when the sun is low in the sky (your shadow should be at least as long as you are tall) or just below the horizon, or in cloudy light although this may not suit many birds aesthetically
have the wind and sun behind you or if you want a side on shot, have the wind coming from your side but not blowing towards you (birds like to take off and land into the wind)
try to have a nice background - preferably not busy and out of focus - this is helped by ensuring the background is some distance away from the bird
preferably have the bird in same plane as the sensor to ensure adequate DOF covering the bird and have the bird at similar level to the camera to keep the viewer engaged
ensure the bird is not looking away from you when you take the shot
usually the bird's closest eye is the most important part - so wait until it shows a catchlight reflection of the sun or bright sky and focus on the eye preferentially
birds in flight are usually easiest when birds are flying predictable and repetitive courses such as when building nests or coming and going from a source of food, or leaving or coming back to roosting spots.
usually having the bird fill a third to a half of the frame is ideal, this leaves options for minor cropping and reduces the chances of accidentally cropping out a wing or the legs
keep the camera steady
aim for as long a focal length as you can achieve eg. 500-800mm in 35mm terms
aim for aperture f/8 (in FF equivalent) or less so you can have good blurring of the background, adequate DOF and a fast enough shutter - usually f/5.6-f/8 works best for most subjects but f/4 gives a nicer background blur for larger, more distant subjects but this requires a super-expensive super heavy 600mm f/4 telephoto
if you are not panning:
aim for shutter speed of 1/3200th sec for most birds in flight (1/1600th may be OK for larger birds or birds in the distance, while 1/4000th may be needed for fast moving smaller birds, those that are closer, those that are flying towards the camera or flying with the wind behind them)
turn off IS as it will not be needed at these shutter speeds
if you are panning, you might get great shots even at speeds around 1/20th sec but ensure IS is ON and set to panning mode (unless this is automatically detected)
lowest ISO that will allow the above - if shooting in low sunlight at f/5.6-8, then ISO around 800 or so
use MANUAL exposure (as camera auto modes will be fooled by changing backgrounds such as dark trees, then bright sky, etc) and expose for the whites to ensure you do not get any blown highlights on the birds although some use Manual with AutoISO for many situations such as consistent scene tonality but with changing light such as a partly cloudy day
if light levels fall, adjust shutter speed (or increase ISO but this is slower to do) to keep the meter in the same position
set a focus range limiter if you have one as this will speed up AF and in some cases avoid AF locking on foreground or, if you are lucky enough to have an Olympus OM-D E-M1 with programmable focus range limiter, avoid locking onto a background
prefocus on a distance where you predict the bird to be to make it easier for your camera to lock AF
consider C-AF tracking (AI-Servo on Canons) and start tracking early, or if your camera is not good enough, lock AF if the bird is not moving closer or further from you
consider burst mode unless you have a noisy dSLR and not a mirrorless camera and you don't want to scare the bird
consider using the smallest AF region that you think you can keep on the bird and have it centred on where you would like the eye to be in your composition so you are not locking focus on the wing
practice tracking
take off shots
set shutter speed to half-1 stop faster as you are likely to suddenly jerk the camera
set aperture to half-1 stop smaller to allow for more DOF to counteract a sluggish AF lock on in C-AF
look for signs a take off is imminent and then keep your eye in viewfinder and ready to shoot:
consider using a Pro-Capture mode if using Olympus cameras when you are waiting for the bird to take off as this will allow you to avoid missing images from your delayed reaction time