Rice Fields Used as Feeding Habitats for Waterfowl throughout the Growing Season

Abstract

The role of rice fields as feeding habitats for the two main waterfowl herbivores, Eurasian Coot (Fulica atra) and Mallard (Anas platyrhynchos), in the Ebro Delta, a Mediterranean wetland in northeastern Spain, was investigated. Exclusion cages and tethering experiments were deployed within a rice field at the beginning of the growing season (summer 2010) and before harvest (autumn 2010). In summer, waterfowl abundances were low, but cage experiments detected rice field damage by waterfowl grazing through a significant reduction in plant biomass (although consumption was undetectable using tethers). In autumn, waterfowl abundance increased and tethering experiments detected consumption of rice plants with developed seeds, whereas cage experiments did not show grazing effects. Gut content analyses indicate that Mallards are mainly granivorous, feeding mostly on seeds of spiral ditch grass (Ruppia cirrhosa) and rice (Oryza sativa), while Eurasian Coots are herbivorous, feeding mainly on macrophyte leaves. However, stable isotope analyses and mixing model results showed that in the long term both species seem to acquire most of their dietary needs from rice plants and sago pondweed (Potamogeton pectinatus). Dietary analyses confirm the importance of rice in both species' diets but also suggest that waterfowl may undergo seasonal dietary variations. These are mostly influenced by changes in the availability of food resources in the area rather than by their nutritional quality. This study confirms the ecological importance of rice fields as a complementary feeding habitat for waterfowl during the growing season in Mediterranean areas. It also highlights the importance of including these habitats in wetland management for waterfowl conservation.