Why aren't witchers hunting during winter too?

When the witchers are wintering at their respective schools aren't there monsters that are winter based and only last until spring? Couldn't they hunt all year round and amass higher rewards for higher priority targets?

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Answer

My answer is speculative, based on my admittedly limited knowledge of medieval era and the Witcher world.

There are 3.5 reasons I can think of:

1. As AcePL stated, northern realms tend to have harsh winters. Presumably this also affects the life cycle of animals and beasts, including those witchers are supposed to hunt. Presumably, it affects them to a point when they go to hibernation in order not to consume too much energy, since finding food is hard at winter.

And this brings us to second clause:

2. Medieval life is year-cycle based. People lived from harvest to harvest. In between, they fed on what they harvested. They tried to limit their activities in such a way that they wouldn't have to go out much. This also applies to wildlife - they are year-cycle based too. Why do we care? Because humans and the wildlife actually feed monsters, and in absence of both monster have to go by saving energy, which would make them hard to find and hunt down.

Lastly,

3. Witchers are humans too. I always assumed they liked to gather at their stronghold in Kaer Morhen to exchange stories, experience, and be among friends. Note that since witchers do not stay at one place when they hunt, it would be hard for them to find refuge in winter. Big cities where there is plenty of shelter do not (often) have pest problem; and smaller villages can't (often) provide shelter for one more mouth.

As a matter of fact, we are somewhat limited in our knowledge of the witcher world, since we can only witness one school, and a declining one at that. There is (to my knowledge) no direct evidence that witchers don't hunt in winter. It's just that the remains of the School of wolf don't.