Birds are everywhere. You may even have a bird in your neighborhood. The most critical technical issue is to get close. If you’re patient, you’re in luck. No matter how close you get, it’s rarely close enough. That doesn’t really qualify as “wild.”
Use Nikon’s Continuous AF (AF-C) mode while shooting Birds in Flight. These modes let the cameras track moving subjects. Set your camera to use all the focus sensors. In Nikon this is the Dynamic AF Area Mode, whose icon is a box in the middle with little dots all around it. These modes let the camera use different AF sensors as the bird flies around in your frame.