Seven out of eight newly-hatched cygnets have mysteriously disappeared from Pittville Lake in Cheltenham.
The cygnets have been disappearing over the past week
The baby birds, born to a breeding pair of swans at the park on 25 April, have been vanishing over the past week.
Ranger Wayne Sedgwick said: "The last time we lost a cygnet of this age was in 1991, so to lose seven in such a short space of time is exceptional.
"Although the circumstances are unclear, it seems unlikely that the cygnets have died from disease."
It is thought the birds could have fallen victim to natural predators such as foxes or pike, although foul play is not being ruled out.
"Our priority now is to protect the remaining cygnet and members of the public have a vital role to play," said Mr Sedgwick.
"We would urge all dog walkers to keep their dogs on a lead when close to the swans and their remaining cygnet, particularly as the protective instinct of the male swan will now be at its strongest."
Anyone who has seen or heard anything regarding the young birds is urged to contact the Cheltenham Borough Council.