What do you call a duck that steals?
A robber ducky.
Interesting Fact: Mallards nest on the ground on dry land that is close to water; nests are generally concealed under overhanging grass or other vegetation. Occasionally, Mallards nest in agricultural fields, especially alfalfa but also winter wheat, barley, flax, and oats. Both urban and wild populations readily nest in artificial nesting structures. Pairs search for nest sites together, typically on evening flights circling low over the habitat. Occasionally nests are placed on floating mats of vegetation or woven into plant stems that rise out of the water. ( https://www.allaboutbirds.org/guide/Mallard/lifehistory )