I recently learned that part of birds' winter survival strategy is to just straight up burn fat by shivering to generate heat. And that they eat and burn a huge number of calories (in bird terms) in a single cold day to be able to do that. It's why I'm extra vigilant about my bird feeders when it gets extra cold up here in New England.
https://swibirds.org/blog/birds-in-cold
I can imagine having that extra chonk padding in winter allows the owls some margin of safety, in case they struggle to find enough calories in the moment when the weather gets really nasty.
"the small animals can cool their internal temperature by 10°C to 30°C. This slows their metabolism by as much as 95% and protects them from starvation"
Holy crap, that's insane!