Anyone who thought that love and affection are strictly human characteristics has clearly never heard of the swans of Side Park.

There, on a pond in Chaoyang district, live six big birds - three black swans, two white swans and a goose.

On Sunday, the white female swan surprised Liu Hangjian who looks after the birds by walking away from the water across the frozen surface and over to Liu's room, apparently to raise the alarm and let him know that her mate was missing.

"She used her beak to knock on the glass and kept making a noise," he said. "She looked very anxious, which is something I had not seen before."

He said he went to the pond to see what was the matter and noticed that the other white swan, the male, was missing.

Liu immediately called the Beijing Wildlife Rescue and Rehabilitation Center and was told that the center had just received a call from a resident who had spotted the missing white swan in a river near Xibahe, Chaoyang district.

Parks staff and other would-be rescuers went to the river to try to round up the bird but it flew off again.

Liu said that his crafty swan eluded those trying to catch it and next turned up close to a beauty salon near the Chongqing Hotel.

"Barbers from the salon caught him and called the police," he said.

The swan was back in the park early Monday morning and Liu clipped its wing feathers so it will not be able to fly off again.

He said he might not have noticed the bird was missing for some time if not for the love of the swan's mate.

The whole time the male swan was missing its female mate looked like a worried wife waiting for her husband, he added.

"During those hours when her husband was missing, the swan wife looked so sad and even circled in the sky hoping to see her husband," Liu said.

He speculated that the warmer weather of the previous few days may have impacted the male swan.

"Swans are migratory birds that are very sensitive to climate change," he said.

"The rising temperatures may have tricked him into believing spring was coming."

The five swans were gifts from the Baoding Animal Park in Hebei province and are aged between two years and three years, which is young for a swan.

Swans are well known as being birds that mate for life.