Abstract
To identify the feeding preference of waterfowl wintering in a freshwater wetland, we conducted experiments both in the laboratory and in the field. In the laboratory experiment, we fed the stems, leaves and tubers of the waterweed Hydrilla verticillata to a Spot-billed Duck Anas zonorhyncha that was wild-caught using a mist net at Hayasaki near Lake Biwa. The captive duck was fasted for 12 hours, then the feeding test presented the two parts of the plant at the same time, but on separate trays, and the duck clearly preferred tubers.
In the field experiment, we selected sites with two water depths. The shallow site had a water depth of 30 cm, and the deep site’s water depth was 60 cm. Both sites were vegetated with the H. verticillata community, and were located near the estuary of the Echi River (Echigawa) between the Echigawa Bridge and the Kurimi Bridge (Fig. 1).
For examination of tuber density, we established eight survey quadrats. Four quadrats were placed in shallow water and four in deeper water. Each water depth had two bird-exclusion quadrats (covered with nets) and two controls (uncovered with nets). We measured tuber density before and after the arrival of wintering waterfowl, and found that tuber density was significantly higher in the deeper water site than in the shallower water site prior to the arrival of waterfowl, and that foraging by wintering waterfowl significantly reduced the density of tubers at the deeper site (Fig. 3). Thus, we concluded that the waterfowl wintering at the study sites preferred H. verticillata tubers over the leaves and stems, and selected foraging sites based on tuber density.
