you wont.
It depends what they are eating as well.
When i was around my inlaws in April two magpies flew down into the garden and started to eat the frog spawn.
This is quite common evidently.

You donot actually say what they are eating. What are they feeding on in the garden?
If you put a bird scarer up you will be scaring of all the other birds as well

Try a Magpie trap, a friend of mine caught 46!!!!

Someone told me to hang old CD's or DVD's up around the garden as they don't like them flashing as they move, I haven't tried it as yet but will be soon.

Yes, one of my neighbours does this and caused speculation that he was losing the plot, poor bloke!

We have them hanging in our veg garden