THIS STUDY evaluated the effect of water level reduction in the Caspian Sea in 2000-2022 years on the breeding behaviour of sedentary bird populations in Gizilaghaj Bay. Water level fluctuation in water bodies affects the productivity of many waterfowl and shorebird populations. The diversity of biotopes in Gizilaghaj Bay creates fertile conditions for the reproduction of birds belonging to different ecological groups. Migratory-nesting bird populations from Africa and South-East Asian countries create intra-species and inter-species competition for sedentary bird populations in the Gizilaghaj Bay, located on the southwestern coast of the Caspian Sea. A decrease in water level causes the interaction and communication of bird populations belonging to different ecological groups during the breeding period and leads to the change of nesting places on biotopes. Since the water level fluctuation directly affects the trophic relationships of the birds, it is possible to evaluate the nesting conditions by changing their habitat. 41 species of sedentary bird populations belonging to 11 orders, 21 families, and 29 genera were recorded in the Gizilaghaj Bay during the breeding period. Of these, 3 species are "floating" in the water area of the bay, 15 species breed in reed swamp, 8 in tamarisk swamp, 2 in Juncus swamp, 11 in trees (shrubs), 10 in soil, 7 on shores, and 4 species in old buildings. The decrease in water level in Gizilaghaj Bay has a different effect on the nesting places of sedentary bird populations in biotopes such as reed, tamarisk, Juncus swamp, and the water area of the bay. Studying the effect of water level fluctuation on the breeding behaviour of sedentary birds is important for solving the problem of preserving natural biological diversity.