Pigeons.

Where did they come from? How did they end up in our city? Setting up home on the ledges of our lecture buildings?

Sherman provided the answer:

Once there was a beautiful dove who lived in the country. She decided to seek fortune and fame in the big city. So, she flew to Toronto to make a home ... and married a rat, and had a litter of rats-with-wings (aka. pigeons). And the rest, as they say, is history.

Now, these pigeons terrorize me wherever I step. They never seem to move out of my way, those obstinate creatures. They bob their heads in a very creepy manner. And their "coo"-ing is absolutely repulsive. Although I have many pigeon stories, one of the most disgusting things I have seen them do is devour street meat. Made me think twice about what was in my hot dog.
And really, when you're eating a hot dog, you shouldn't think about what is in it, if you want to enjoy it.