2023 Volume 72 Issue 1 Pages 47-56
Bean Goose Anser fabalis middendorffii is one of many waterfowl species wintering in Japan. Flocks of Bean Goose frequently forage on submerged water chestnuts (Trapa species) at their wintering sites which consist of ponds and lakes. While foraging process, the birds did not always eat all of the fruit they picked and discarded many before swallowing them. We hypothesized that the geese select fruits based on their size to achieve high foraging efficiency. To test the prediction, we conducted field research at Katano-kamoike (pond), which is one of the main wintering sites of the species in Honshu. In 2009–2010, 2015–2016, 2016–2017 and 2017–2018, exclosure experiments were conducted to investigate the size of fruits that were consumed. In the exclosures, the density of small water chestnuts was higher and the average dry weight tended to be lighter, suggesting that the geese were selecting small-sized fruits. Observations of foraging behavior showed that small fruits were more frequently eaten than estimated. Additionally, the time required to consume large fruit was three times longer than that for small fruit. These results suggest that Bean Goose selects small water chestnuts for greater foraging efficiency.
