you have a geriatric border collie - of course you are concerned for her. I doubt very much a sparrow hawk will harrass you dog -she will not be perceived as natural prey to the bird. If it makes you feel any better, just compare the size, shape and movement of the pigeon to that of your dog - completely different - if you normally preyed upon pigeons, would you go for a collie dog? I think not! If you can, why not go out in the garden with your dog for a while, I'm sure it's not necessary but it will at least, make you feel better. Don't worry.