Unusual nest boxes

Whilst on  a very wet walk in Wales today I became aware of baby birds chirping.  I knew it was close but couldn’t think where it was coming from.  After standing and observing for a while I was surprised to see a great tit entering what looked like an owl box? - much larger than a robin box.  I patiently waited and sure enough it popped up with a fecal sac in its mouth.  This huge box seemed a strange place for great tits to nest to me.  I have seen blue tits nesting between tree roots and I have also seen them in metal ash tray type boxes with holes, but these both seemed quite safe from predators.  This box seems very vulnerable and open enough for squirrels, magpies, jays, woodpeckers etc etc.  It made me wonder if anyone else has seen unusual choices of places to raise a birdie family???


J