Swan Sanctuary said if a swan's mate is killed or disappears, the surviving bird goes through a similar grieving process to humans, the Mirror reported.
Then the swan either finds a new stretch of water to live in, flies off to join another flock or stays where it is.
A spokesman for the RSPCA said: 'This is a really sad development and it is very upsetting to hear about the death of this poor swan.
'We are investigating the previous distressing incident and we would urge anyone with information to contact our appeals line on 0300 123 8018.
'Swans, their nests and their eggs are protected under the Wildlife and Countryside Act 1981.'
This comes soon after the Met Police launched a hunt for a jogger who allegedly booted a cygnet in Richmond Park in south-west London, on June 8.
Police suspected the jogger kicked the newly-hatched cygnet, leaving the tiny bird with brain damage. It died two days later at a sanctuary in Shepperton, Surrey.
The suspect is a white male around 60 years old, is 5ft 6in and is balding. He was wearing black running shorts, a vest and a running top with a logo on it at the time.