When this OP found some unusual spinning objects on a freestanding dock in West Central Florida, they had to turn to the internet for answers. It goes without saying that the wise folks of the world wide web did not disappoint. As it turns out, these devices are basically bird scares used to keep seagulls and pelicans at bay.
You’ve seen all kinds of scarecrows over the years but these are more compact than others and they do their job brilliantly. Besides, they’re not as “scary” looking as traditional scarecrows. More modern solutions also include bird repelling balloons, lasers, and ultrasonic devices.