I have a camera and lens combination that is arguably the worst possible for BIF. A Sony A77 with a Sony 500mm f8 reflex lens. A slow lens which is slow to focus and can only use the central focus sensor which is specially adatpted to it. After spending half an hour trying to get one shot from a gang of seagulls wheeling over a public park and failing to get one I was inclined to agree it was totally useless for BIF. Even aeroplanes flying overhead I often failed to find in the viewfinder before they'd gone over the horizon.
The problem was aiming in the thing. It had such a narrow view that if I pointed it at something and couldn't see it in the viewfinder then absent other visual clues in the viewfinder, which there aren't in a blue sky, there was no way of knowing which way to move.
So I fitted a 1:1 red dot gun sight, 1:1 being the kind you use with both eyes open, one looking through the scope. It superimposes a red dot in the middle of what you see. I calibrated it so that whatever I put the red dot on was smack under the central focus sensor. Using that it was instant to aim the lens, and not hard to track it under the central focus sensor while it locked focus. From not being able to find a big slow flying bird at all I had to be selective to stop filling up the shot buffer with dozens of shots of every passing bird of any size. Most not quite in focus, but a fast burst of around five would usually have one in sharp focus. If the flying bird was close enough I could with a bit of luck lock focus on the eye.
Not suggesting you should get a 500mm prime! Just pointing out that a red dot gun sight can make BIF photography much easier. I mounted on lens barrel to minimise parallax. I glued it to a stout cardboard tube slightly too small to fit over the lens. When slit open that provided a firm and precisely aligned grip on the lens.
Red Dot gun sight on 500mm reflex lens by
Chris Malcolm, on Flickr