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The first observations of infanticide by a precocial avian species, Spot-billed Duck (Anas poecilorhyncha). were recorded on a small pond at Yatsu tidal flat, central Japan, from May to June 2000. where six families of conspecific and one Gadwall A. strepera family, respectively, occupied territories. A female Spot-billed Duck with 11 ducklings attacked ducklings of all the other families killing eight conspecifics and three of the Gadwall. High breeding density (15.0 families/ha) may have increased the aggression of the female.

The pair relationship of ducks lasts until the male deserts his mate during the incubating or hatching periods (Sowls 1955). Spot-billed Ducks (Anas poecilorhyncha), which are widely distributed in eastern Asia and breed throughout Japan (del Hoyo et al. 1992), also have a similar breeding pattern. Spot-billed Ducks lay from 8 to 13 eggs from April to early July, and their incubation period is from 26 to 28 days (e.g. Kobayashi 1985). After hatching, only the female is followed by the ducklings which find food for themselves. Here, we observed infanticide in Spot-billed Ducks and report the first case in a precocial avian species.