"There's a crow in my owlbox!"

This cri de coeur has become more frequent in recent years. In fact, the "crow" in question is a Jackdaw, which is a cavity nester, unlike the Rook or the Carrion Crow. And the reason the complaint about "crows" taking over nestboxes meant for Barn Owls or Kestrels has increased is because the number of Jackdaws has increased, and quite markedly so over the last few years. You may or may not love this small corvid, beady of eye and grey of nape, often seen tumbling around in groups calling its name, a sharp "jack", but you can't deny that it's a successful creature.
According to the BTO, the reason it has increased is because it adapts very quickly to changes in the environment, and in particular can exploit new food sources, which means it has an improved chance of raising more young. It also exploits nesting opportunities, and will usurp a Barn Owl or Kestrel nestbox given half a chance. In other words, it's a generalist; the more specialist a creature is, the more vulnerable it is to change, because it simply can't adapt. Think of the Giant Panda, one of Nature's dafter inventions.

One can't really blame the Jackdaw for nicking other birds' accommodation, but he is a horribly untidy squatter. You don't have to climb up to a Barn Owl or Kestrel box to know if Jackdaws are in occupation: you will see an ungainly tangle of twigs sticking out of it. They fill up the box with twigs, making it unusable by other species. Normally, these nestboxes don't get cleaned out, but after the Jackdaws have been in there, it's up the ladder, facemask on, and a hand rake to get rid of the crud. Birds of the crow family (which includes Magpie and Jay) have a bad name, but there's no denying they are great survivors. And the Jackdaw, seen in a certain light, is quite rakishly handsome.