Like Rolfe said ,it was the warning note that squirrels use .They hang on to a note for a certain time ,which to other squirrels denotes the type of prey in view .For a bird of prey the note is high pitched,a sort of 'cheeeeeeeeeeeeeeeeeeechak -chak',very much like the mating call but higher pitched .Usually done motionless .Ground predators are exposed with short 'chak-chak -chak 'but continuous until the threat is gone .Usually done with the tail flicking over the head .