Hi Gilly
Been away for a couple of nights, with no Internet access, on Exmoor.
From others who have kept chippies outside I know that they need time to acclimatize to being outside if they come from indoors. They grow thicker fur and start to build up food stores. But they are quite hardy, and outside chipmunks are less prone to illness as they're tougher all-round. Now would be a good time of year to get some, I see someone on this board has 40 chipmunks to re-home, too many even for your aviary though. Mine are all kept indoors, and even though they are supposed to withstand the cold mine often head for the radiators, they really like the warmth if they can get it.
I read of people with a shed joining on to the aviary, that they have lined the shed part with insulation slabs, then covered the insulation with cladding (lots more work). I've been up to Cleathorpes and experienced the 'lazy wind' up that part of the country, too cold for a southerner like me. So I would be kiting out the shed part with insulation. But I think they could still go in to their winter sleep even if they had a shed. Perhaps others visiting here would like to chip in with how their outside chipmunks behave during the winter.