We describe interspecific and intraspecific kleptoparasitic behaviour in Mallards Anas platyrhynchos, attempting to steal Zebra Mussels Dreissena polymorpha from other Mallards and Eurasian Coots Fulica atra. Both Coots and Mallards were most often attacked by Mallards perpetrating kleptoparasitic attacks when they handled large or intermediate-sized prey items. The probability of attack was nearly halved when the foraging birds had small versus large prey items in their bills. The overall probability of success of a kleptoparasitic attack was lowest when the attacked birds had small prey items, but higher if they had intermediate or large prey items. Kleptoparasitic attacks carried out on Coots were more often successful than those on Mallards. The results suggest that the optimal foraging theory, where animals tend to maximize net benefits, may be applicable to kleptoparasitic behaviour.