It's The Tower of London. Not Big Ben. There should be six. According to some dates back to Celts. Great for the tourists.
When I was a kid (60s,70s) it was believed that nailing a black bird to someone's door was a form of a curse or sign of evil. Couldn't find sauce for that but I still believe it! After Q who knows. So many old traditions seem to have alternative meanings to those in the know.