This video from the USA says about itself:
5 August 2016
Mother Gray Squirrel spends much of the day and night in the owl nest box nursing her newly born “kittens” as baby squirrels are called. She has added some nesting material and when she leaves she covers the kits up so any bird poking its head in the nest box might not see them as they would be easy little meals. They are totally helpless and their eyes will not open for at least another month.