Probably at that stage. Is it a blackbird nest? Whatever it is, best to not open it up for photographing, and even if not disturbed, the extra attention could attract a magpie or similar.
Young birds are unable to thermoregulate - that is they are unable to maintain their own body temperature for at least the 1st week after hatching & generally upto a few days before they are fully fledged. So, at night, & to some extent during the day, depending on the ambient temperature, the young are reliant on being roosted to stay warm & in some degree the insulation of the nest retains some heat. As they (speaking of blackbirds in this instsnce) approach 2wks of age & fledging, they become able to generate their own body heat, requiring less time being kept warm by the mother. Having said this, the hen may leave them for short spells without them becoming too cold, & this will be affected by the temperature.