Fast shutter speed. But of course that lowers the total light hitting the sensor, meaning that the aperture must be wide, and/or the iso must be high. But modern sensors can handle really high iso, well into the 1000's. So I pick shutter speed and aperture depending on the situation and let the camera handle iso. Bright conditions, F7.1 to 9 for depth of field, and 1/2000 to 1/5000 depending on the speed of the subject. For lower light, F5.6 to 7.1, and 1/800 to 1/2000. The lower the light the lower the shutter speed. So, low light and fast subjects = hard. Luckily modern cameras have controls that make changing these variables very quick, and you just make your own guess for each situation. The most important factor is AF. You must use continuous AF for BIF. But all brands and models have different forms of AF, so finding the best for each situation is really trial and error, and I  highly recommend back button focusing. Happy hunting! Oh, and swifts are imo the hardest BIFs to capture. I'm still working on them.