Setting for 90D and 100-400mm II for birds in flight

[AdoNetok]

I just learn to use Canon 90D and Canon 100-400mm II for birds in flight.
I set up C2 for this project.
I try to take photos for Forster's tern at the bay where most time is sunny.
Here is my setting.

Camera: AI Servo, ISO: Auto, Drive: Fast shooting, 1/2500, 7.1, 9 points focus,

Lens: FULL, AF, STABILIZER: ON, STABILIZER MODE: 1

C FN II AUTOFOCUS:
1. Tracking Sensitivity... -1
2. Acceleration/deceleration tracking... +1
3. AF Point Auto Switching... +2
4. AI Servo 1st image priority... +1 Focus
5. AI Servo 2nd image priority... 0

There are two things I want to improve:

1) For the first photo, it is hard to catch (focus) the flight bird,
2) Focus is still not perfect.

Please help me.

[Waddizzle]

The first photo is probably hard to capture because of your Image Priority settings.  With 1st Image Priority set to [Focus] the camera will not fire the shutter until it gets a focus lock.  You want to get. The first shoot fired off as quickly as possible.  

Try setting 2nd Image Priority to [Focus].  This may slow down your frame rate, but your images will always have a focus lock, hopefully on your desired subject.  I do not see any point in firing off continuous shots and many of them are OOF.

[jrhoffman75]

Birds in flight is extremely difficult. Small target. Rapid moving subject.

If you don’t have subjects in the near range you could set your lens for the distance range rather than Full. 
See page 563 in the manual to stop lens drive when AF not possible. Then you can manually try to get subject in focus range and then use AF.