The osprey had an unwanted visitor to their nest. A third bird attempted to scare our pair out of the nest. You can see the bird in the nest get agitated and then quickly move to a defensive position.
She (we believe it was the female in the nest) almost does a back flip trying to ward off the intruding bird with her talons. The intruder comes back a couple more times. The male (we believe was the bird outside of the nest) comes to her aid and helps chase the intruder away. It happened so quickly, its almost hard to see.
However, check out the still photos below (click them for a larger image) and watch the short video clip provided.