I’ve been at photographing birds for almost as long as I have been shooting with an SLR/DSLR. Birds, notably ducks and gees are some of the first subjects almost everyone takes a photo of.
Over that time I have learned a lot, and the biggest issue is that for birds in flight, luck and technique iare the champs.
Equipment can help somewhat, longer lenses, more AF points, but that only helps not makes the shot.
One of my favourite BIF shots was shot using a Pentax PZ-1 with a K300/4 MF lens and the Pentax 1.7x AF adaptor.
BIF is largely in my opinion luck, or in some cases technique with a fixed location (bird feeder) and flash. Check out this post, and others by this forum member, they are excellent examples of birds in flight, at a feeder.
Blue Jay in Flight (Vintage Glass) - PentaxForums.com
My experience is the keeper rate at BIF is very low, maybe 1%
If you have a specific bird you want, I recommend learning it’s habitat and behaviour, because that may help you get shots. Some birds, (osprey, kestrels, etc) can hover before diving at their prey. If you recognize they are hunting, you can track and wait for them to hover, then luck and timing can get a great pose
But nothing equipment will give you, except AF will ultimately help, the rest you need to learn on your own