We used to have a bird box on the wall in our garden. One year a pair of blue tits nested in it. We watched them from the window to-ing & fro-ing with food for the chicks & all the darling little heads poking out, preparatory to fledging. Then a magpie (or maybe a pair) pulled the roof off and ate the lot.

All part of the garden food web, of course, but annoying just the same.   They are thought to be responsible for some of the decline in garden songbirds, which is serious in some places.

Beautiful plumage.