(But the swans know the truth!)
That's right. Goose POOP. Geese have biodegradable cameras (with live recording) in their droppings to allow their government to spy on people's whereabouts. So now you know why they tend to poop on the path.
Why haven't we noticed these cameras? Well, the cameras disintigrate much faster than the droppings do so the geese much keep returning to drop off some more.
They also try to use a gross "fear" tactic to keep us from observing. You know when the geese fly over people, everyone starts trying to duck for cover and avoid getting pooped on? That's the tactic.
The higher ranks of the goose flock are the lookout/guard geese you see in the flock. The one that stands up straight and watches your every move if you stand ten feet away. That particular goose has a camera installed in the center of its forehead.
Advice for you: Don't be focus of that goose. That means that their government has an eye on you and you might disappear without a trace.