I have been using the Canon Rebel T8i and my kit zoom of 55-250mm on birds in flight and experimenting with as many settings as I can. To really get great shots I tend to go to SS speeds 2000 to 4000 and try and keep a small f stop in good light. My first choice was Av mode Shutter priority for our cameras. This way I could choose my aperture quickly and watch the SS vary to a high number in the viewfinder. I put my focus element on single shot if I could or the next setting for small center focus, rarely all45 points unless the bird was flying in an empty sky. I soon learned that many of my shots were using iso 1600-800 and I got SS’s 2000sec to 4000 and my fstop was generally around f8 + in good light. As the light changed I had to adjust and take what I could get at full zoom of 250 mm I can only open up to f stop of 5.6. Also experiment by setting your ISO to Auto! You might like it very much...many do.

Now I am using Tv setting, shutter speed priority and just set what I want and think will be good for conditions to start. I am trying the Auto ISO setting here because it is so fast changing for different lighting I am getting more keepers. I get more good green lights in the viewfinder saying it is good to go...and I can concentrate on smoothly following the bird in flight. I have gotten 5-7 shots on continuous shooting and all were good keepers.

You NEED to be at least using AIFocus it works like the One Shot Focus does until the bird starts moving then it switches to continuous focus just like AIServo does! After a short while you will like it! Don’t let the youngsters have ALL the good things. Lol... then for birds in flight give a serious try to Continuous Focusing. (AI Servo) that will work best most times for the moving subject. I try and use the small 9 point focus on moving subjects but if there tons of clutter around your subject go back to single point if it doesn’t work for you. I save the 15 and 45 point focus points for clean backgrounds and empty skies, but hey I’m still learning just like you!

I am saving all Manual for last for a serious trial. I’ll know my camera better by then, it will be easier to get around. Good luck and enjoy....my favorite BIF shot so far was on my granddaughter’s t6 so it works wonderful, go for it! Walt

Continuous focus. Start off with higher shutter speeds (at least 800 but 1000, 1250 or higher would be better.)until you get good at it. Using a tripod/ no tripod? If so, rest your left hand on top of the lens above the tripod to help dampen vibrations. It helps me a lot on a tripod. Some say VR should be turned off on a tripod & should be turned off at high shutter speeds. Experiment & see what happens. Practice , practice, practice.

Steve Perry has some vids on you tube that might help as there are others.

Rich
In any fluid flow (air over water in this case), the friction at the boundary between the two causes the air to slow down and the water to get dragged along by the air (relatively slowly, but it can keep building to a storm surge). The further away from the water surface (up) the less the air is slowed, so the wind just tens of feet up is faster than the wind right at the water. "Boundary " in fluid mechanics refers to any interface between a fluid or gas, fluid and solid or gas and solid (the ground, pipe or canal walls, wings or foils, etc), or even between two streams of gas or fluid (look up Kelvin-Helmholz clouds). You are correct that the smaller area of the wing is also a factor