It’s very strange and sickening.
This happened on my local canal where I often go running.
http://www.burytimes.co.uk/news/radcliffenews/4064426.Mystery_over_swan_deaths/

The canals not in the best nick (its disused now as a waterway) with beer cans and assorted rubbish in parts of it but there is still loads of wildlife. This year I saw a pair of swans with perhaps 10 signets, and there was a big flock of geese with loads of goslings. Then there were assorted coots and moorhens with young, along with a number of different duck species (including the amazing looking Goldeneye duck). Sad times. Apparently killing animals can be a precursor to serial killing.