Culling began today in bird-flu hit Nadilag and its surrounding areas of West Tripura district as the state government said neighbouring Bangladesh could be the possible source of infection. As the H5N1 virus was confirmed in blood samples of dead birds from Nadilag village by the High Security Animal Diseases Laboratory in Bhopal, the authorities started the operation, setting a target of culling 20,000 chicken in eight gram panchayat areas surrounding Nadilag in four days.