I get two kinds, the first seem acclimatized to people somewhat and you can get within ten feet or so before they scoot, then the second kind are really skittish and won't let you get close at all. With the skittish ones I use a technique I learned from my cat. Get as close as you can slowly, at some point you'll see they are onto you. Stop at this point and let them get focused back on their hunting. When they're all wrapped up in the hunt, move ever so slowly toward them until they become aware of you again. With each bit of ground you gain, get some shots! They have a kind of body language that with enough stalking them will tell you when they are just feeling threatened and when they are ready to bolt. When they're close to departing, get you're shutter speed up and pull the zoom out a little, put the focus on spot and you should get some cool flight shots if all goes well. Oh and turn off your camera beep for focus lock because that can spook the more timid ones. Of course this is pretty generalized info but it should help you a tad I hope