Cameras generally have two focus options: Single-shot (or AF-S) and Continuous Servo (or AF-C). With AF-S, the camera fixes the autofocus. If you move the camera or subject after this, the focus point will not change. With the mode set to AF-C, the camera will continuously adjust autofocus as your subject moves. There are now also hybrid systems where the camera itself makes the right choice based on the scene analysis.

Newbie Tip:
See how you can change the autofocus points on your camera yourself and set it to autofocus-continuous mode. Next, practice changing your autofocus point to place your subject off-center in your composition. If your camera can't adjust the autofocus point, set the camera to single-shot, focus on your subject and then adjust your composition. Make sure you always focus on the face on the eye.

Pro Tip:
Some cameras have an extra autofocus button on the back of the camera. We use it often and activate the autofocus with our thumb. Especially when tracking moving animals. The moment we want to stop because the subject has sat/stands still, we can stop. The advantage is that the camera retains its focus point if, for example, an animal passes in front of the scene or there is an unexpected movement in the horizontal plane.

How do you make this image?
Lens: Long zoom lens
Settings: Low ISO, Aperture Open
And then: Set your camera to autofocus continuous with a single autofocus point. Move this autofocus point with the bird's eye and shoot when you've got the right composition.