Your lens will be sharpest at f8. To hand hold at 250mm, the rule of thumb is 1/250th of a second. I know you have IS but if the bird is moving even a little you still want a fast shutter speed to freeze the motion.

For the sharpest image, if you don't have bright enough light for 1/250th at f8 you will need to boost your ISO rather than using f5.6 or a slower shutter speed.

Try ISO 400 or 800 if your lighting is overcast.

Your lens will be sharpest at f8 Dxomark lens report. To hand hold at 250mm, the rule of thumb is 1/250th of a second (and preferably faster). I know you have IS but if the bird is moving even a little you still want a fast shutter speed to freeze the motion.

For the sharpest image, if you don't have bright enough light for 1/250th at f8 you will need to boost your ISO rather than using f5.6 or a slower shutter speed.

Try ISO 400 or 800 if your lighting is overcast.