Super capture, Lillian. With that lens, from messing around with one, for me I stick to nothing lower than ISO 400, and at that strength of telephoto use either a bean bag, leaning firmly against something, or a tripod if handy. Hand-held is precarious...The slightest movement at 500mm will reduce the sharpness unless the shutter speed is as high as you are able to go and the conditions are bright outside.
But hey.......it's a great Robin shot, amigo! That's what counts.
Dave.