I'd best 'fess up to what these are. A few replies have mentioned pigeons, and that is what they are for; in Scots parlance they are #doocots (dove cots), these ones fortified. However, they have nothing to do with pigeon racing, rather they are a cultural & social phenomenon where the sport is to lure birds from neighbouring doocots. It's complicated but here is a fine essay by Douglas Stuart on their cultural significance. https://lithub.com/douglas-stuart-on-the-defiant-spirit-of-glasgows-doocots-private-pigeon-lofts-on-public-land/