We don’t typically use entrance reducers on our hives in winter. Here’s what bees from one hive did to the entrance. This colony was wintering in a 5-frame nuc box. Bees sealed the entrances with propolis, leaving only a small entrance hole, the size they wanted. We like to encourage bees to use lots of propolis, as it increases bees resistance to disease.
In the photo below, both entrances are propolized: the standard entrance just above the bottom board, and the round entrance that is partially blocked by the plastic circle.
This is a close-up of the bottom entrance.
Here is a close-up of the round entrance. They left a small opening to come in and out here too.
This propolis below is on the lid ventilation hole. Bees left just enough room to climb in and out through three smaller holes. You can see a bee poking its little head out in the top photo.