The following sequence was taken with the Canon EOS 40D in high speed continuous burst and also illustrates my technique for tracking birds in flight. All shots were taken in Manual Exposure Mode (M) at 1/800s, f/8.0, ISO 400 and are full frame. I only use the center autofocus point for flight shots. The subject is a Tricolored Heron. It's not a very fast flying bird, but it makes for good practice. I recommend practicing on herons, egrets, and ibis when learning how to take photographs of birds in flight.

	I don't start the burst mode until the subject is completely in sharp focus within the viewfinder. If you start bursting before the subject is in complete focus, it will be difficult to achieve sharp focus as the burst is occurring. It is also important to note that on the 20D, 30D, 5D, 40D, and other cameras that use the 9 point Canon AF system; the area of sensitivity is larger than the small indicator squares. I use the center weighted metering circle to keep the subject in the general vicinity of the center sensor. The metering circle is a larger target, and although the sensor sensitivity does not extend throughout the entire circle, the subject usually remains in focus.
	While the target is flying toward you, it is easier to keep it within the central sensor and maintain good focus.
	Most birds move up and down slightly when they flap their wings, so their flight path is not always a straight line. For many birds the up and down movement is not enough to displace them from the focus point. However there are birds like the Belted Kingfisher and the Pileated Woodpecker that have erratic flight patterns due to how they flap their wings.
	To conserve energy many birds will keep their wings in this position and glide for an extended period.
	As the bird starts to move past the photographer's position it becomes more difficult to keep the sensor on the front of the bird.
	In this position the autofocus will actually move slightly and lock on the bird's wing. If you don't have an aperture small enough to get a sufficient depth of field, the bird's head will appear soft and slightly out of focus. I find that f/8 usually gives me enough depth of field for most flight shots.
	As you can see the bird is now well off the central autofocus point but still remains in focus because it is still within the metering circle and the background behind the central sensor is not busy, even though it is relatively busy at the bottom of the frame.
	The bird's up and down motion will return it to within the range of the central sensor. That's a juvenile Cooper's Hawk in the background.
	Again the bird is still within the metering circle, so focus is maintained. The bird is now way past the photographer's standing position so it's getting harder and harder to keep the bird targeted and the central autofocus point is now drifting to the rear of the target.
	By turning your body along with the passing bird you can catch up to it and get the center autofocus point back towards the front or center of the target. This takes some practice to achieve amd the shots from this point on will have the bird flying away from the photographer, yielding not very good results.