Mallard ducks do not migrate from Ireland in the winter. Native Irish Mallards overwinter and their population is increased by migrating ducks from Iceland. Sean and his family would be quite comfortable spending the winter in Ireland.
When Sean and Voley are preparing for the winter Voley insults the squirrels to get provisions for the winter. Sean tells Voley that "ducks don't eat nuts and berries". The RSPB lists the food of wild mallard ducks and includes acorns and berries at the top of the list.
Voley tells Sean that he cannot stay with him over the winter because he needs to hibernate. Water voles do not hibernate. They spend more time in their burrows over the winter months but Sean could easily stay with Voley as he would be awake and active.