This newly fledged female Cooper's Hawk is mantling to show her sibling it is time to back off. The two of them have been chasing each other through the trees and play time is over. Raptors usually mantle over prey that they want to hide from others. It makes them look bigger, more threatening, and covers up their meal. So it is a bit unusual to see this behaviour from below. I'm amazed at the feather tracts on the back of the head and neck. What a moment.
Here she is transformed into a different creature. She is still keeping an eye on her sister though. There are 4 fledglings, 3 females and a male. The females are busy chasing each other and constantly begging. The male is further along - out on his own, hunting, flying with purpose as he patrols a different patch of the arboretum. I think the males develop more quickly because they are smaller. It is probably advantageous to leave the nest before your three gigantic, voracious and, fierce sisters get their act together.
Another sister gets in a little quiet flight practice near a reservoir. Photographing Coops is challenging. They are so quick and unpredictable and once they get a little sense they become nearly invisible. I never once saw the parents who keep a very low profile. I hope the kids stick around awhile.