The white feathers that adorn dhaks are mostly of killed egrets and using them is illegal, the state forest department has warned Durga puja organisers in the city.

The dhakis, most of whom hail from rural areas, kill egrets for their feathers before they head to the city for the autumn festival. The feathers are supposed to catch the eye of the organisers of big-budget pujas that pay well.

The Wildlife Protection Act, 1972, bars the killing or hunting of egrets but the timing of Durga Puja has made the birds easy prey to poachers. Egrets breed between May and October, when they do not fly as much as they do at other times, explained a forest department official.

"In all the meetings we have had with puja organisers, we have told them that they should desist from hiring dhakis who use egret feathers," said Subhankar Sengupta, conservator of forests (wildlife). Forest department officials are invited to meetings between police and puja organisers before the festival.

In 2014, two dhakis who had adorned their dhaks with egret feathers were arrested at Sealdah. The news of the arrests spread among the dhakis and the feathers were not used as widely last year. "We need to reiterate the point every year so that the use of the feathers can be stopped gradually," said Sengupta.

Forest officials feel that if puja organisers stop hiring such dhakis, the use of egret feathers will automatically reduce. Many puja organisers insist on decorated dhaks, pressuring dhakis into falling back on the traditional option.

Puja organisers who do not want blood on their hands sometimes find it difficult to identify egret feather. Not all white feathers used on dhaks are of egrets: some are poultry feather.

Veteran birder Shubhankar Patra said there were simple ways to distinguish feathers of egrets from those of cocks and hens. Egret feathers are about 1.5ft long whereas poultry feathers are less than a foot long. "Being longer, the egret feathers are more flexible. When a dhaki dances, the egret feathers swing more. Also the milk-white egret feathers are brighter than the feathers of hens and cocks," explained Patra.

He said use of egret feathers has reduced over the years but the practice still persists.

The most common egrets in south Bengal are Cattle Egrets, Little Egrets, Intermediate Egrets and Great Egrets. Among them, Intermediate and Great Egrets are killed in large numbers.