Magpie thirsty work?

Hello Crewe55, I've seen other corvids (Magpies, Crows, Jackdaws, etc,) soak bread (from discarded sandwiches and such like) in the nearest puddle or water source so would suggest it is something they naturally do.

For other people who may find themselves reading this post:
Bread is not suitable food for any birds (including swans and ducks) and thus not advised to be given to them although I understand from what you wrote that you did not provide the bread which the Magpie brought in to your garden; for anyone reading this it is far better to put a tin of dog food out for Magpies and other corvids if you want to feed them or cheap raw mince chicken or beef but not ham, bacon as it contains too much salt content. In the winter months I used to make raw pastry for them which is a cheaper way of providing fat based food for colder months with plain flour (supermarket own brand), lard but no salt must be added. A small amount of grated mild cheddar cheese can also be added (not to much as that also contains salt). To the flour and lard (half fat to flour ratio) you can add a little seed/suet pellets, etc, and bind together with just enough water so it is not sticky.

In spring/summer when they are raising chicks I would avoid the fat based foods like pastry, fat balls and instead give them meat based tit bits (but never use discarded fat from a roast as it can be dangerous; stick to plumage, etc, ) In essence, do not feed bread to garden birds or waterfowl; (once again I am including this for other people who might be reading your post) much better to give them a little seed mix (no wheat) and if feeding ducks, swans, there are special blends available that float on the water for a while giving the birds time to eat it before I sinks and contaminates the water.