An investigation is under way after the mysterious deaths of 30 swans in Cardiff.
Officials say the animals died within a three-week period and the cause is unknown.
The Welsh capital has a population of around 200 of the birds, which are often seen in Roath Park and Cardiff Bay.
Cardiff Council has confirmed these two locations, both popular with families, were predominantly affected.
Although bird flu has not been diagnosed, as a precautionary measure, people finding dead swans are advised not to touch them.
The Animal Health and Veterinary Laboratories Agency (AHVLA) said the Welsh Government was looking into the matter.
A statement by the AHVLA said: "Reports of increased levels of mortality amongst wild swans in the Cardiff area are being investigated.
"Notifiable disease, including avian influenza, has not been diagnosed, but the underlying cause of these deaths is unknown at present. Investigative work is continuing."
Councillor Ashley Govier, Cardiff Council's cabinet member for environment, said: "I am deeply concerned about this situation, and am personally monitoring the work done by council officers and our partners to resolve this as quickly as possible.
"I understand how distressing it will be for members of the public to come across dead birds in this number, and I have asked our parks and river officers to pay careful attention to ensure any further instances are dealt with quickly.
"I will be speaking to our partners to ensure our city has an effective response."