magpies are very smart creatures, and if they are well fed and healthy they can get bored. they do things to annoy other creatures, the "cause and effect" mentality.
some of the magpies here are very annoying. they imitate you walking anf gargle as if they were having a conversation, in the winter they knock snow off of the eves of your house onto your head and go "YARK YARK YARK!" as if laughing. i've even had some drop pebbles and sticks on my head. VERY mischief little birds
they are likely just trying to entertain themselves by annoying your chickens. they will eventually get bored of it, and if you think you can out smart them, try scaring them off. scarecrows won't work, they will just play with it. throwing something at them might help, but if your not careful you may end up entertaining them a bit.
unless you have very very small bantams, magpies are nothing to worry about. they are just being mischief.
i have recently seen the magpies swooping the feral cats at the back of my property. We don't have nests here, but the magpies have young babies with them, teaching them to fly and eat. You would not be able to tell the babies from the adults except that they have shorter tails. They are super noisy as the kids chase mom and dad, begging for food.
i would suspect they are swooping your chickens as they see them as possible predators to their babies.
True that DDawn The maggies in australia only seem to be a nuisance when its Spring when they have eggs and chicks. I don't know about any other species. Kids in Australia make icecream container hats and paint big eyes on the back of them, this is ment to stop the maggies from swooping them because if a magpie can see you looking at it ,it won,t swoop you.
I put a pic in of what Australian magpies look like.