Those birds which migrate from one location to feed, breed and raise their offspring are known as migratory birds. Mostly they are coming from unfavorable condition to some favorable place having sufficient food, water, resources and safety. In World and Asia, the largest migratory sanctuary is Chilka lake bird sanctuary in Odisha and second Khichan Bird Sanctuary located in Rajasthan, India. Kulik bird sanctuary is also 2^nd largest sanctuary in Asia. It is located in Raiganj Uttar Dinajpur. It is popularly known as Kulik Bird Sanctuary because the river Kulik flows beside the sanctuary. The shape of the sanctuary is that of the English alphabet "U". During monsoon the river water enters the sanctuary, which supports a wide variety of food for the birds. All know that biodiversity is the indicator of health of the ecosystem. Biodiversity supports ecological services including, purification of the water and air pollution, prevention of erosion, and stabilization of CO₂ levels. Biodiversity boosts ecosystem productivity for each species, no matter how small. All have an important role to play. Ecological life supports biodiversity. Rich biochemical cycle is noticed in the forest which keeps a balanced ecosystem, food chain, and biodiversity. It is important that necessary nutrients are provided by the soil and the roots of phototrophic green plants take the nutrients through the process of root osmosis from the soil. The term biochemical cycle because plants play an active role in the cyclic pathways of nutrients.

Birds generally migrate in flocks. Most migrations are latitudinal- from North to South also, some migrate from East to West.

Winged species from different countries from southeast Asia have started flocking to the Kulik Bird Sanctuary near here a month ahead of their normal arrival with experts suggesting that conducive climatic conditions induced by lockdowns in different Asian countries propelled their flight. Every year, migratory birds, especially cormorant, night heron, egret, and open-billed stork, reach the sanctuary from July onwards. The sanctuary is spread over 370 acres on the banks of the Kulik.

The two major factors which support migration of the birds in these regions are as follows:

Availability of plenty of food supply: Food and nesting space are simple answers. And additional reason is Kulik river and agriculture field. agriculture field is source of apple snail. River water is source of bird’s food like, fingerlings. Birds migrate to improve their chances of survival and reproductive success, and while temperature may influence bird migration, birds can survive in very cold conditions. The main reason for bird migrating is to follow their stomachs (aka food) and find safe spaces to nest.

Many birds migrate in line with vegetation changes. Wind, precipitation, and temperature affect the changes in vegetation and consequently availability of food and space for nesting; which is why bird migration is seasonal. These resources and conditions make areas more live-able at various times of the year. Migrating to areas with more abundant resources improves chances of survival and also impacts breeding success. There has been less pollution in many Asian nations because of lockdowns induced by the novel coronavirus. Hence, it seems, climatic conditions became conducive to the birds’ flight, and they reached Kulik earlier than the normal time. We hope the avian population in Kulik this year will surpass the record. "Around 10,000 birds have already reached the sanctuary about a month ahead of the usual time and more will fly in during the coming weeks,” Raiganj divisional forest officer Somnath Sarkar said. Pramika Lama said strong two reasons for bird migration are – fingerlings and apple snails. Apple snail is favorite food of open bill bird. Mainly four types of birds are migrating- Open bill, Black crowned night Heron, Great egret and Cormorant.

A suitable place for nesting, Deciduous Trees: The study already discussed Kulik forest is mainly mixed deciduous forest. These broad-leafed trees lose their leaves in the winter, but in spring, they are filled with flowers and buds, and many of them produce fruit for birds. In the autumn and winter, the leaf litter from deciduous trees is also a fine source of food for ground-feeding birds, and it also provides nesting material in the spring. The best deciduous trees for birds include larches, mesquites, maples, oaks, and willows. They make their nest from softwood, and thorny plants in the surrounding area and make hard material. Every day they do repair their nest. Since trees are so helpful to birds, adding the appropriate trees to your landscaping will allow you to attract a wider variety of bird species that increase our Kulik sanctuary.

A bird won’t build a nest if there is no safe, suitable place for that nest. There are many different locations that different types of birds prefer for nesting, and savvy birders will try to provide multiple nesting sites in their yard. Trees are critical for birds in many ways, and they meet all birds' basic needs for survival.

This study was about Raiganj located on the bank of Kulik river. It is situated at 25° 35' to 25° 57' N latitude and 88° 07' to 88° 17' E longitude. Raiganj town in Uttar Dinajpur District in West Bengal. The mean temperature of this site is 39°C maximum. Rainfall is more or less 1500 mm whereas the relative humidity is 89% and winter temperature of maximum 20°C.

Both primary and secondary data were used for Collecting data. Secondary data is collected from some journals, books, official websites, DFO office Raiganj. Premika Lama (Range Officer, Wildlife Raiganj) provided data on names of birds, animals, fishs, and reptiles. With systematic sampling study and area visit (visited 5 times in a month), the nesting process and breeding in the different habitat are observed. Also AQI index, noise and water quality index surrounding forest area are verified. Mainly May and June data are studied to clarify the condition of weather, because of birds’ arrival in these months.

Local birds are kites, owls, woodpeckers, Drongoes, Flycatchers, kingfishers Bulbul, Dove. Migrant birds are Asian openbill, Night Geron, Egrets, and Cormorants. Every year 80000 to 90000 birds are migrating during monsoon.

Table 1 depicts the population counts of different species of birds over the 5 years interval from 1984 to 2022. Asian open bill, Black crowned night heron, Great Egret and Cormorant count has been increasing over the years. Usually, birds move in search of wetlands and change in their population could reflect in environmental health.

In 1984, population of bird’s species were recorded to be 29023 in total of which openbill numbered 22,364, night heron stood at 3,432, the great egret at 2,016 and cormorant at 1,216. Openbill account for the highest number. The data between 1984 to 1988 reflects the real increase in population of all four species. The Asian Openbill population has nearly doubled, it has reached 37,876 while the Black Crowned Night Heron, Great Egret and Cormorant showed a significant growth. In between 1993 to1998 bird population of the Black Crowned night heron and Great Egret declined while the Asian Openbill and Cormorant populations remained stable. Between 1998 to 2003 there was significant increase in population count of all species. Asian Openbill was exceeded 50,000 and other three species of birds experienced notable growth. This period indicates successful conservation, or it is showing healthy habitat quality.

Other year also saw fluctuations in population counts between 2008 and 2013. There was slight decrease in the populations of the Asian Openbill and Great Egret, while the population of Black crowned Night heron and Comororant remained stable. Year of 2018 saw increase of population counts for all species expect Great Egret. These fluctuations could be influenced by climate conditions and human activities. Overall, the population of bird’s species suggest multiple role play of environmental factors besides habitat quality and availability of food resources and human impacts.

Overall result and discussion of the study suggest a strong relation between inflow of birds and temperature variation. The average temperature in the Kulik region was found to be 15 to 20 degrees lesser than in Southeast Asian countries. Sufficient food, and fresh air along with silent, clam environment of Kulik makes it the home for numerous migratory birds. Huge numbers of birds came to Kulik bird sanctuary in the year 2019-2020 which was the time of pandemic- Covid-19. Less degree of noise pollution, air pollution, water pollution and soil pollution provided these birds a quiet place for nesting and sheltering from hot weather at their original countries. Another point to be noted is that bird foods are available because of sufficient rainfall. Fish laying eggs bacause of lot of rain and a lot of fish are appearing in the Kulik River. Because of sufficient rainfall, trees have grown nicely. As a result birds are able to build their beautiful nests in these trees. It is a bitter truth that coronavirus has taken the lives of so many people. On the other hand, Covid-19 saved the environment and created pollution-free atmosphere. Nature is in favor of wildlife. Birds are swaying in the pollution-free air that soothes our eyes and enhances the beauty of the sky.

We are thankful to DFO office Raiganj and Premika Lama (Range Officer, Wildlife Raiganj) for providing data of migrant birds from 2010 to 2020 and for providing reptile names, mammal names, and tree names.