Birds

I remember one fascinating phenomenon I observed last (my first) Autumn in Rome - millions of birds dancing in the sky every evening at Lungotevere. As you approach the river, you first hear the disturbing noise and then you see the amazing show. Everybody stands stunned and look up the sky. I have no clue how those birds follow one another and change direction in less than a second.
But this show has also its price: streets are covered by a new carpet and I definitely wouldn't like to be an owner of the cars parked at the river those evenings...

An umbrella is also needed if you pass by the neighbourhood!

Some more reading here: http://essentiallyrome.com/tag/lungotevere/