A one-month-old cygnet has died in suspicious circumstances at Beijing Zoo, said zoo officials Tuesday, the latest victim of visitors who throw inappropriate food to the animals.

Ye Mingxia, the zoo's media officer, said the baby swan, which was born on July 1, appeared drowsy and weak during an examination on July 24.

"It died after the emergency treatment failed that day. Twelve  corn kernels were found in its stomach which is not what the keeper feeds them," said Ye.

Ye said the corn is probably  responsible for the indigestion and worm infection that caused the young swan's death.

On Tuesday afternoon, several children were feeding ducks and other waterfowl with steamed buns alongside their parents, all ignoring signs saying "Feeding is forbidden."

"This inappropriate feeding killed the animals out of love, but many visitors still don't get it despite the warning signs," said the organizer of the zoo volunteers, surnamed Wang.

Wang said the group started eight years ago, and includes 100 animal lovers who take turns to help introduce the animals, maintain order and "stop visitors from throwing everything they have in their hands."

"But us being at the zoo is still not enough to protect the animals during the tourist season; didn't feeding by visitors kill Quanquan too?" said Wang.

Quanquan was a Sichuan snub-nosed monkey, an endangered and nationally protected animal, who died in April from indigestion as well. Safety glass has been built around the monkey enclosure after his death.

However, a tourist surnamed Jiang, who regularly visits the zoo, said people still try to attract the monkeys by throwing popcorn in the gap between the glass and the ground.

"People think feeding them is perfectly fine as long as the fruit's carefully washed, but it's not," said Wang.

"There's a regular feeding routine for the animals we need to stick to," she noted.