Since the leaves have fallen, it's easy to see the dreys, squirrel's nests. I realize Eastern Grey squirrels don't hibernate -- they're active throughout the winter -- but on cold winter days, I have often looked up and wondered...

Is there really a little
squirrel body up there?

In the past, when my curiosity got the better of me, I would search for how squirrels survive the winter and never quite had this question answered, instead reading of them sheltering in old tree cavities. And although that may be true, there is a shortage of old trees in relatively young neighborhoods.

This is an Eastern Grey Squirrel ~ Sciurus carolinensis, and, yes, this one is black, the melanistic form. While we have both the grey and black colouring here, studies have shown that the black squirrels have a higher cold tolerance. From Wikipedia...

when exposed to −10 °C, black squirrels showed an 18% reduction in heat loss, a 20% reduction in basal metabolic rate, and an 11% increase to non-shivering thermogenesis capacity when compared to the common gray morph

In the nearby bush, there were some large dreys. They're typically built like hollow globes of 30 cm (12") in diameter with outer layers of loosely woven leaves, bark and twigs, and inner layers of softer materials such as grass, leaves and fur. A squirrel-sized opening, or two, are oriented towards the tree to prevent rain from getting in and hopefully to keep raccoons out since they will prey on young squirrels.

In the bush, the dreys are well above ground level, 9-13 m (30-40') up in the crotch of a tree. In my neighbourhood, they are often lower out of necessity because the trees are younger.

A female squirrel may build 4 or 5 of these within range of her primary nest. Then, if she feels one is dangerous, she may move her young to another. A drey might also be abandoned due to an insect infestation.

Squirrels don't necessarily build their own nests. They may use nests built by other squirrels and even abandoned by other squirrel species. And although they typically have one of their own, males and females may share during mating season, and when temperatures fall below freezing and the squirrels stay in their nest, they may share to stay warm.

Her coat thickens for the winter and although she has many food stashes, she can live off her body fat when necessary, and yes, she really might be up there.

In warmer months, squirrels may sleep on a platform of twigs and leaves rather than within their drey. It becomes difficult to tell whether something is old and abandoned or never was a nest.