It's called panning. Use a low aputure, the lower the aputure the less light will get in when you open the shutter which means you can use a slow shutter speed, say 1/15th of a second. Maually focus on a point where w/e you want to shoot will pass by so you wont need to focus it while taking the shot. Watch the subject thru the camera but try to keep it in one spot on the view finder else it will turn out all blury then when it gets to the point you focused on take the pic but dont stop following the bird till your sure the camra has taken the pic cause the slower the shutter speeds, the slower the camera takes the pic.
I'm still a noob at using that technique but it can make a bird in flight look really amazing especially when it's gliding. The duck in that pic was flapping it's wings so they turned out a bit blury. I hope that made sense lol.