As I was walking through the park yesterday, I saw a squirrel building a drey. It was selecting leaf fronds, chewing them off and carefully tucking them into a mass of other branches to create the drey. I guess winter must be coming!
In his 1974 opus, Mammals of Louisiana and its Adjacent Waters, the late eminent ornithologist George H. Lowery Jr. wrote that Grey squirrels — as well as Fox squirrels (S. niger) — create two types of dens.
The first type is simply a den within a tree cavity, while the second type consists of a ball or dome-shaped mass of leaves and twigs (left). According to Lowery, these dens can be located anywhere from the ground up to 12m (39ft) in the trees and are referred to as dreys. Dreys are typically about the size of a football (i.e. 30cm / 1ft diameter), with a combination of twigs, moss and leaves woven into a thick-walled shelter – the inner cavity is generally 12cm to 16cm (4.7 to 6.3in.) in diameter.
The dreys are lined with mosses, lichen and bark and it is not uncommon for a single squirrel to have several dreys. Indeed, a nursing female will have several ‘alternative’ dreys located near to the natal drey – if the natal drey is disturbed, the female will move her young to a new drey. In broadleaved woodlands, the cavity of a tree may be used as a drey – in the case of Grey squirrels, such nests will usually be in oak or beech trunks, with the centre hollowed out and an entrance gnawed to a diameter of about 10cm (4in.).
Grey squirrels tend to have two ‘types’ of drey, built according to the seasons; winter dreys are those described above, while summer dreys are, according to Corbet and Harris’ Handbook of British Mammals, “shallow platforms of twigs”. Indeed, in his fascinating 2003 book, Winter World, Bernd Heinrich discusses how some Grey squirrel dreys to be appear haphazard piles of twigs, while others are very well-constructed. During his studies in North America, Heinrich found many that were simply “piles of junk” — Heinrich suggests that these may have been ‘fake’ nests used to distract predators — but that some were very robustly engineered. Describing one drey in an oak tree on his driveway, that he examined after it was blown down in a heavy rainstorm in mid-January, Heinrich wrote:
“The outside layer of the 30-centimeter [12 in.] diameter globular nest was of red oak twigs with leaves still attached. The twigs had therefore been chewed off the tree during the summer. Inside this rough exterior I found layer upon layer (twenty-six in one spot where I counted) of single flattened dried green oak leaves. The multiple sheets of leaves served as watertight interlocking shingles, because the nest was dry inside. The leaf layers sheltered a 4-centimeter-thick [1.5 in.] layer of finely shredded inner bark from dead poplar and ash trees. This soft upholstering enclosed a round, cozy 9-centimeter-wide [3.5 in.] central cavity.”