Duck example strategy pattern - Head first design pattern

Question

I want to ask something about duck example on this book that made me confused and I feel contradictions.

1. Problem

Answer 1

The strategy pattern works when you favor composition over inheritance http://en.wikipedia.org/wiki/Composition_over_inheritance

This is a good practice because you can change the behavior of a class without having to change any code. And you don't need a huge tree of classes either. You also can change the behavior of a class dynamically.

What it does in the example is that defines "behaviors" in the parent class. In the parent class you define that a Duck can have a flying behavior and a quacking behavior. But it doesn't mean the children classes have to have quack or fly.

You can have a nonflying duck and when you call "fly" it will do nothing because we'll have a "non-flying" behavior.

Instead of hardcoding what a duck does in the class, you can change the behavior of this duck whenever you want.