Every year, people line up along the Tiber to watch this
Starlings, millions of them, winter in the plane trees that line the Lungotevere, the two roads along the banks of the river. It really is an arresting sight.
The trouble is that the birds are a little, as Wanted in Rome put it, "antisocial" in their habits. It makes walking along the Tiber an unpleasant prospect, and there are several other roosting sites around the City where it becomes unpleasant to walk.
Every year, the City administration tries to "do something" about it. No one wants to just get rid of the starlings, but, well, nature's kind of gross sometimes.