I keep three nocturnal cameras [ bird box cameras] trained on feeding dishes in a rural garden, and have a very good idea about the comings and goings of the local hoard of hogs. Usually between 6- 10 regularly in summer. I had to rescue two [250g in December] , and the rest dissapeared around the 12th December, presumably to hibernate.
No sightings at all for one month, then the last two nights, and yesterday afternoon at 15.00 we saw several. The sun had been out, but air temperature remained about 0 degrees, with frost on the ground. Good appetites.
I have seen hogs out in -5 degrees in the past, [>500 g] and they do get through winter OK. But when there is a big drop in temperature they do know that they need to stock up, so will awake , eat for much longer than usual, and then go back to hibernation [not proved]. However, how would they know that they should come out of hibernation to stock up ? My hypothesis is Air Pressure. Two nights ago the pressure climbed quickly, the air cooled and that would be enough to give them a signal. So whilst we have this high pressure on the UK, be especially vigilant to feed them if they arrive.