Shifts in site use by spring-staging swans Cygnus sp., from traditional to artificially reclaimed habitat in the Gulf of Finland

Abstract

Variation in the abundance and habitat use recorded for three swan species – the Bewick’s Swan Cygnus columbianus bewickii, Whooper Swan Cygnus cygnus and Mute Swan Cygnus olor – at their main spring staging areas in the coastal waters of Neva Bay, Gulf of Finland, were studied from 2018–2022. The count sites were grouped into two distinct habitat types: “traditional” (defined as natural shallow coastal water areas with low levels of anthropogenic disturbance, supporting a range of emergent vegetation), and “reclaimed” (defined as the shallow waters created behind artificial sand embankments during and following their creation). The numbers of swans using traditional sites varied widely, but never exceeded 121 Bewick’s Swans and 292 Whooper Swans at any one site. At the reclaimed sites, numbers of the two yellow-billed swans increased progressively following completion of the reclamation work, with peak counts ranging from 0–442 Bewick’s Swan and up to 160 Whooper Swans. In contrast, Mute Swans in any one area never exceeded two individuals. Surveys of potential food plants used by swans in summers 2021 and 2022 demonstrated the prevalence of Perfoliate Pondweed Potamogeton perfoliatus at the main stopover sites close to reclaimed areas, together with Hairlike Pondweed Potamogeton trichoides and the green algae Cladophora glomerata. These results suggest that, with the continued gradual colonisation by aquatic vegetation of shallow waters formed near reclaimed areas, there is potential for the number of swans feeding in these areas to increase, provided that the sites are protected (e.g. from disturbance by human activity) and continue to provide suitable habitats for the swans.