Let's look at your sentence: 'Kestrel hang in the air to spot their prey. They do it by flapping their wings fast.' There are a couple of small changes we can make so it sounds better and is grammatically correct:
- Change 'Kestrel' to 'Kestrels' because we are talking about more than one kestrel.
- Change 'hang' to 'hangs' because when we talk about what one type of animal does, we add 's' to the verb.
- Change 'fast' to 'quickly' because 'quickly' is the right way to describe how they flap their wings.
So, the corrected sentence would be: 'Kestrels hang in the air to spot their prey. They do this by flapping their wings quickly.'