For birds in flight, I usually use a shutter speed of 1/3200. However, with large-winged birds that typically flap their wings at a relatively slow rate, I will go down to 1/2000th only if the light is low. If I have enough light, I prefer a faster speed to guarantee tack sharp motion shots without pushing the ISO too high. In this picture of king vultures in Costa Rica that I took from a blind, I was lucky. Even the tips of the wings are sharp with 1/2000th of a second. To keep the flying bird in focus as it landed, I used continuous focusing (AI servo) and my focus point arrangement was a center cluster of 15 points. I felt this was a large enough area to lock onto the bird, yet it was small enough so the AF mechanism wasn't influenced by the background. My other settings were f/5, and 3200 ISO. I used a 100-400mm lens.