Kestrels are in the small group of raptors where males and females can be distinguished by their coloration. Like most other raptors the female kestrel is larger then her male counterpart but her coloration is much more muted and earthy. This is a defense mechanism for when the female is on the nest incubating her eggs. since it is smaller then many other raptors the kestrel is often on the other side of the predator/prey relationship. Her brown striped pattern is camouflage that helps her blend in with the sticks around her nest. The male does not help with incubating the eggs and so it is not necessarily for him to be as inconspicuous. As you can see from the top photo the males are much more brightly colored.