Food items consumed by the bean goose (Anser fabalis subsp. middendorffii) and tundra swan (Cygnus columbianus), two large waterfowl overwintering in Fukushima Lagoon, Niigata Prefecture, Japan

Migratory bird populations have been declining globally for several decades. Migratory species are vulnerable to decline due to the variety of habitats they require over their lifetimes, including breeding, overwintering, and stopover sites. Degradation and loss in any of these habitats may threaten population survival. Preventing further declines in migratory bird species requires understanding their feeding habits across habitat types and utilising appropriate habitat management to prevent qualitative and/or quantitative declines in food resources. We assessed the feeding habits of two large waterfowl, the bean goose (Anser fabalis subsp. middendorffii) and tundra swan (Cygnus columbianus) using DNA barcoding and stable isotope analyses to inform habitat management and conservation efforts within their wintering grounds in Fukushima Lagoon, Niigata Prefecture, Japan. The bean goose overwintering diet consistently comprised Oryza sativa and Trapa natans. Tundra swans also consumed O. sativa as a primary food item, but shifted their consumption to other rice paddy plants, including Alopecurus aequalis and Poa annua, in November and December. Thus, these two species overlap in primary food items, but their utilisation differs during winter. In both species, food item use was dependent on plant species distribution within paddy fields and lagoons. Maintaining these migratory species in Fukushima Lagoon requires conservation efforts to integrate the lagoon and surrounding paddy fields as a foraging area.