Previously, a cygnet had a "brick dropped on it" a year and a half ago and a female swan with eight cygnets became seriously injured, and later had to be put to sleep, after having a stone thrown at her, a local resident said.

Source: https://www.walesonline.co.uk/news/wales-news/pigeons-found-shot-dead-welsh-27554412

They fit the cygnets with a ring bearing identification numbers, and then release them back into the water.

Source: https://www.nytimes.com/2023/07/27/world/europe/uk-royal-swans.html