Magpies steal stuff in most forms of fiction. There are no Magpies where I live so I'm not sure if that's true in real life, but at least in most works of fantasy they like to steal things.
When raiding birds nests, if it is a magpie nest there should be a chance to find some stolen trinket in the nest. Maybe it can be drawn from the global ditch pool just like fishing and digging up trash.
Idk I just think it would be neat to raid a magpie nest and be like "oh hey someone's poor man's belt is in here, those sneaky magpies!"
Also when they attack you I think it would be annoying but also neat if they had a chance to yoink some 1x1 item out of your inventory, and you only get it back if you kill them. They'd prioritize low-value things like bones, offal, seeds, forageables, would attempt to ignore high value things like pearls, gold, localized resources - unless that's all you're carrying. They would not steal equipped items.