My ducks lay during the winter too; although their egg production does decrease a little bit. At first I thought it was weird, but I don't think it's harming them at all, so I just go with it. Laying is usually induced by amount of daylight and temperature. Mine live in a garage, so they have more light than a wild duck would. The garage is also heated, so they must think it is still ok to lay.
Where are your ducks sleeping? Maybe the light or temperature in their bed is what is causing it. If you want to decrease their egg production, limit the amount of daylight they're exposed to.
Edited by JJpuck - 1/21/16 at 3:28pm