Calcutta: A black kite, its wings entangled in a kite string, landed on the balcony of a north Calcutta house on Sunday, renewing birdwatchers' concerns about the sharp thread injuring birds in flight.

Kite strings are made of nylon and often laced with manja (a mixture of glass and glue), aiding flight but also adding to its sharpness. Such strings do not snap and instead get entwined in the wings of birds.

"We often hear of such injuries to birds, especially black kites. Birds cannot see the string while flying and get entangled in it," said Shubhankar Patra, a veteran birdwatcher.

Black kites, common in the city, are large, slow-flying birds. Being scavengers, they keep hovering in the sky in search of food before swooping down.

"They spend a lot of time in the sky and that makes them more susceptible," said Sumit Sen, birdwatcher and honorary wildlife warden, South 24-Parganas.

The black kite that fell on the balcony of a Duff Street house on Sunday afternoon was rescued and taken to the Wild Animal Life Rescue Centre at Salt Lake.

"The bird was trying to drink water kept on the balcony. I noticed that the bird could barely move and was being surrounded by crows who it seemed would pounce on the injured bird as soon as I moved away," said Palak Dutta, a banking professional.

Dutta wrapped the bird in a piece of cloth and put it in a cardboard box. "The black kite was not in a condition to flutter its wings and slipped into the box rather quietly," he said.

The Duff Street resident called an NGO, which asked him to contact the animal rescue centre in Salt Lake. "They were unable to send anyone and so I took the bird to the centre, where they had to cut the strings to free the bird," Dutta said.

A forest department official at the rescue centre said the bird had been kept with other kites and would be released as soon as it recovered.

"There are about 30 black kites at the centre at present. The kites are released depending on how fast they recover. Some kites also get dehydrated in the summer months," said the official.

Birdwatcher Patra said most kites recovered with proper treatment.