Abstract
Mallard (Anas platyrhynchos), a surface-feeding dabbling duck species, has often wintered in the northern climate that freezes over most of their preferred habitats of shallow lakes, rivers and wetlands. A small wintering population was observed in mid winter at the entrance of Lake Mokoto, a brackish lagoon along the Sea of Okhotsk. They dived repeatedly to the bottom (1 m deep) of the lagoon and took benthic organisms. Their foraging success was high, averaging 78%. Most of their prey consisted of bivalves similar to or larger than their bill width (20 mm). They had a mean diving duration of 5.8±2.4 s SD (n=53), with a maximum of 12 seconds; and had a mean handling time to swallow their prey of 9.7±4.5 s SD (n=53), with a maximum of 21 seconds. The mean length of diving and handling bouts combined was 15.6±5.8 s SD, with a maximum of 31 seconds. Although theoretically they could have shortened these bouts by taking smaller bivalves, they tended to take larger bivalves that resulted in prolonged bouts, probably because more large bivalves remain given that the dominant Aythya diving ducks tend to take smaller bivalves during their stay at the same locality in autumn. Mallards were estimated to consume from 1.1 times to 3.5 times their body mass in bivalves in order to meet their daily energy requirements, when they selectively foraged on benthic bivalves. This range in amounts arises because of the varying quality of the bivalves. The northern brackish lagoon of Lake Mokoto, with its abundant benthic resources has played an important roll in supporting Mallards that have changed their foraging behavior in mid winter from surface dabbling for vegetation to diving for benthic bivalves.
