The Mute Swan (Cygnus olor), a species of southern origin, has shown a large-scale increase in numbers and expansion of its breeding range against the background of climate warming and the ban on hunting since the end of the 19th century. This paper presents the results of studies of the biology of this species in the eastern part of the Gulf of Finland, where the species first appeared on breeding grounds in the late 1980s. Its dispersal from south-west to north-east following earlier ice-free dates has occurred, and the Mute Swan has now established suitable nesting habitat along the entire coast of the Gulf of Finland, with the exception of the inner parts of the Bay of Vyborg, where ice-free dates are very later. The main breeding areas of the species are island areas of the Gulf of Finland, with up to 90% of birds nesting in southern areas with moraine boulder-sand island shallows. The rocky skerries on the northern coast of the Gulf of Finland are less frequented by the Mute Swan, and most nests have occurred in the last decade. As a species of southern origin, Mute Swans appear in breeding areas and begin egg-laying very early, as soon as ice conditions permit. A change in habitat preferences of the species compared to inland waters is clearly visible on islands - most birds breeding on low grass meadows. Nests on unvegetated beaches and flat rocks are common, and even in forest habitats on islands. Swans are highly tolerant of other large birds (gulls, geese and ducks) with which they often form joint settlements on islands. There is a significant change in territorial behaviour among Mute Swans on islands of the moraine zone with extensive shallow waters. Here, at different sites from 26.18% to 69.39% of pairs bred in dense colonies without individual protected sites. Individual sites of many other swans were often greatly reduced in size compared to typical individual sites of swans on lakes. These pairs of swans seemed to form dense associations of individual sites. Apparently, unlike other swans of the northern hemisphere, the adaptations of the Mute Swan to marine nesting have an ancient origin and are related to the history of its formation in the miocene in the coastal zone of the Paratethys Ocean and its derivatives.