There is no point in shooting squirrels unless you happen to own a commercial forest where large numbers do serious damage to young trees.They have no effect on ordinary farming.
Magpies are not a pest at all. People in towns think they are and complain about the magpies taking eggs and the poor little chicks of garden birds. That's an innocent's view. Fact is, if there are more magpies about then there must be more garden birds about. So it's a good sign, not a bad sign, if magpies are on the increase.Their numbers reflect the health and abundance of their food supply.
Now, shooting rabbits can be quite another matter.They breed like rabbits, obviously, and can clear quite an area of crops.However, in practice, hardly anyone bothers because their depredations are small given the large scale of modern fields and the scale of arable farming.In this village, all arable, a couple of men go out shooting them but the interest is much more in getting them for the pot , local restaurants etc, than the pest control