I use the AF-finetune to calibrate my lenses and cameras. Usually this works out pretty well. Calibration will set the camera to not front-focus or back-focus, the focus will land where you want it - on the spot where focus was attained.
What I have found almost all the time when shooting birds in flight is that focus is harder. Sure, nothing news worthy there.
But what is most frustrating is when a bird is flying at the camera focus is rarely on the eye. And it is because the eye is rarely in the center of the frame where the selected focus point is most likely to be.
I've speculated and toyed with changing my AF-finetune specifically to be setup for this type of bird flight shot, but it is rare that I do it. The setup is to change focus to bias towards front-focus. With the birds flying at the camera, and the head likely the closest part of the bird, you want focus to be closer than what you will normally get.
So, I rarely do it because how many flight shots towards the camera in a row do you get and if it is just a few is it worth blowing all the other shots?
On this day I was shooting the Herons as they were nest building and they kept flying a similar route.
I got some of the best focus in this setup than I have ever gotten before... Normally my lens (600mm f/4) is set to about -14AF I think, and I changed it to -18. I tried a few settings, and the following weekend I tried with my 500mm and experimented more. I got better results with the 600mm at about -4 on top of my normal calibration.
Have you ever tried this technique?
Or are you already happy with your birds in flight towards the camera shots - so no need to tweak this setting?