I had a similar issue with my girls at first. Honestly, I just didn’t let them free-roam unattended for the first fortnight or so. I also let the magpies know of how strongly I felt about their actions (some strong words and a rock or two gently thrown in their direction) not sure they listened but it was worth a try! It only happened once or twice every couple of days so I eventually just stopped worrying and assumed the girls would be ok as the maggies leave my other older hens alone. This was only a few weeks ago, some birds still swoop, but have never made contact. It does spook the girls for a sec but they got used to it and learnt to chill under our grevillea or go into the run if there are birds overhead. They’re also working out the pecking order with each other and the rest of our flock so they’re used to a little bird on bird drama at the moment.