Birds at the Alipore zoo water body. A Telegraph picture

Calcutta has lost a winter signature — migratory birds at Alipore zoo — and climate change could be the reason. The zoo used to play host to about 20,000 feathered visitors every winter from various parts of the world, including the Himalayas and even Siberia, in the 1980s. The figure dwindled to 4,000 in the last decade. This year, migratory birds were absent on the vast water body at the centre of the zoo.

“We don’t know the reason but it’s a fact that migratory birds totally skipped the zoo this year. Even last year, a few birds had come,” says zoo director Subir Choudhuri. According to him, not only has the number of birds gone down consistently over the past two decades, the duration of their stay has gradually changed. “Initially, the birds used to spend the period between December and February in Calcutta. The period has gradually shrunk and shifted towards March over the past few years.”

What may be the reasons behind the disappearance of migratory birds from Alipore zoo? No study has yet been carried out in the city to understand the link, if at all, between climate change and behaviour of migratory birds. However, a 2005 report — Climate Change and Migratory Species — published in a journal of British Trust for Ornithology suggested that the two are related.

Eighty-four per cent of the migratory bird species on the scheduled list face some form of threat from climate change, the United Nations Environment Programme has warned. Climate change could hamper migration, concluded another study, which further pointed out that birds that migrate large distances are greatly affected due to changes in climate on their flight path.

In Calcutta, senior forest department officials believe that increase in air and noise pollution along with disappearance of wetlands and agricultural fields around the city — these areas serve as the main food basin for the birds — are the primary reasons behind migratory birds shunning the city.

“If pollution was the only cause, the number of migratory birds that flock to spots other than the zoo — many of them away from urban centres and not polluted — would not have dwindled,” stated an expert.

“It is difficult to pinpoint the reason behind migratory birds disappearing from the zoo. Most probably, a combination of several factors, including micro-climatic changes in and around Calcutta, are to blame,” said an ornithologist.