This happened outside my office window. We suspect the squirrel was trying to protect her babies which are somewhere near by perhaps even inside the wood beam where the hawk is looking closely several times during the video.

Video captured by the Sony HDRPJ710V video camera in full auto mode at 1920x1080x60p with final output rendered at 1280x720x30p shot through dirty office window glass. I applied autocontrast Adobe Premiere CS6 to fix the contrast problem caused by shooting through the office window, and a slight gamma increase to compensate for the darkening from the autocontrast.