These are the leftovers of a snail kite’s dinner, shells of the Apple Snail.
But wait…. Apple snails aren’t this big.
Turns out this is a Brazilian apple snail brought over by the aquarium trade, which has colonized the Everglades in great numbers and created the usual problems of vegetation destruction and excessive poop.
Note the quarter for scale.
The snail kites are happy, though, one of the odd cases of an invasive species helping out a threatened native.