An endeavour has been made in this paper to assess the environmental quality of a habitat by analyzing migration trend of Night heron bird population. The study has been conducted based on secondary data in particular and primary data in general. Secondary data has been collected from books, journal and official records of forest department, Govt. of West Bengal, India. Quantitative techniques like growth rate of Night heron bird population along with their rate of change in migration at four year interval have been collected. It has been derived that flood in the river Kulik adjacent to the Kulik Wildlife Sanctuary is the foremost causes of fluctuation in the migration of Night heron bird population. This is because of lowering down of environmental quality and dilapidated situation followed by an overall collapse of the ecosystem caused by the occurrence of flood in the sanctuary.