Feeding Ecology of the Black-necked Swan Cygnus melancoryphus in Two Wetlands of Southern Chile

Abstract

The diet of Black-necked swans, Cygnus melancoryphus (Anatidae), was studied in two southern Chilean wetlands in 1993–1995; the Cruces River Sanctuary and the Chihuao-Pichoy River, both near Valdivia. The diet of swans was analyzed by using microhistologic techniques of cell remains in feces collected in the field. The analysis showed that swans were eating predominantly the most abundant submerged aquatic plant, Brazilian waterweed, Egeria densa (Hydrocharitaceae), at both Cruces River (91.9% of diet) and Chihuao-Pichoy River (71.7%). The foraging action of this species was estimated using four forage exclosures in the Chihuao-Pichoy River wetland free from swan grazing, which were compared with foraged controls. Aquatic plant biomass taken in the exclosures was significantly higher (p < 0.05) than biomass sampled outside the exclosures. Cygnus melancoryphus might play an important role as a regulator of aquatic plant biomass, thus contributing to cause a delay in ecological succession. The potential impact of this slowing down is discussed in terms of a natural ecological process in wetland environments.