What Is A Drey?
This is the time of year when things that have been going on in treetops during the summer and early autumn become visible as leaves fall. Among the most interesting of those things are the big bundles of leaves snagged snugly in branch crotches high up in the bigger trees. These, I learned some years ago, are called “dreys.” They are the homes of squirrels who don’t have holes to nest in. Which in these parts, where the tree population is dominated by hardwoods like oaks and pecans, is most of the squirrels.
Dreys are usually built of twigs, dry leaves, and grass, although they may include a variety of other materials. They are sometimes referred to as "drey nests" to distinguish them from squirrel "cavity nests" that are also termed "dens" and may be in cavities ranging from hollow trees to the attic of your house. (If you get a squirrel nest in your attic, that is grounds for immediate eviction before the squirrels in question have damaged wiring or insulation or even your ceiling.)
A favored height for a tree drey is at least 30 to 45 feet above ground level. Evidently the height and location are determined by the need to compromise between defense against tree-climbing predators like cats and raccoons and the need for the drey to be supported by branches strong enough not to break off in wind or ice storms. The smallest tree around my house that has a drey is a mulberry across the street that’s only 25 feet tall. That drey is on the small side and may be the product of a first-year, amateur builder. The tallest tree with a drey is a red oak at the side of my front yard that supports two dreys, one of them quite small and likely abandoned before completion, the other quite large and sturdy and about 40 feet up – likely higher than any housecat and possibly most raccoons might be willing to climb.
In North America, dreys begin as a collection of small, gnawed-off branches bearing green leaves. Harvesting these branches well before autumn (when the leaves would naturally fall) allows the leaves, though they turn brown, to hang on tightly through the winter, which is important. A finished drey is a hollow sphere, about a foot or more in diameter, with the branches and other rough materials loosely woven on the outside and an inner surface lined with a variety of finer materials, such as grass, moss, leaves, shredded bark or pine needles.
One drey I personally examined, after it fell to the ground during a storm, was lined with fur from my long-haired dog, gathered by some industrious squirrel from areas where my dog napped outdoors. That must have been an especially cozy nest!
There may be one or occasionally two entrance/exit holes in a drey, usually close to the bottom and angled toward the trunk, which keeps rain out. A second hole can also be used for an escape route if the squirrel has a predator coming through the front door. Incomplete or flat dreys that are sometimes seen may be hot-weather sleeping platforms or, as mentioned above, abandoned efforts built by very young, inexperienced squirrels.
Drey construction materials and sizes differ according to squirrel species and region. Eastern gray squirrels, for example, tend to use the leaves, bark and twigs of deciduous trees such as beech, elm, and oak. Southern flying squirrels will often employ fungal growths, deciduous leaves, bark and twigs in their nests, while northern flying squirrels often use shredded cedar bark (among other types of bark), lichens, mosses, leaves and twigs in their dreys. In the Pacific Northwest, the northern flying squirrel employs a common local lichen as the primary material.
Squirrels sometimes occupy a vacant drey that was previously constructed by another squirrel, often of a different species. Dreys must protect against the environment, and require constant upkeep to remain water and predator-resistant. Squirrels often build more than one in a season, with the second as a reserve nest in case the primary drey is disturbed by predators or overrun by fleas or lice. Some particularly well-built dreys have been observed in use for more than a decade by multiple generations of squirrels, although the average drey may be used only a year or two before being abandoned. Remnants of abandoned but well-built dreys may be visible for years.
Male and female squirrels may share the same drey for short times during a breeding season, and during cold winter spells squirrels may share a drey to stay warm. However, females nest alone when pregnant. In North America, squirrels produce broods of about three "pups" twice a year. After leaving the drey, a young squirrel is termed a "juvenile" for its first year of life. The June broods are sometimes born in dreys, but January broods are usually born and raised in tree cavities, if available, which are much safer. Drey broods are about 40% less likely to survive than tree cavity broods, so long as the cavity entrance hole is no wider than about four inches, which can keep out hungry raccoons. I have no statistics on survival rates for my own yard, which has cats, raccoons, and possums, plus occasional large hawks and owls, as potential predators. But despite these hazards, my local squirrel population consistently runs about five to ten adults and juveniles each summer, assisted no doubt by the ample supply of sunflower seeds I keep available in and beneath six tube feeders, plus acorns, pecans, elm seeds, hackberry seeds, and cherry laurel berries in large quantity.
It’s difficult to predict which of numerous potential trees my local squirrels will choose for building dreys. For years they used the big elm trees in the yard just east of mine but one of those is now gone and the survivor has gotten sparse with age and bad health. It hasn’t sported a drey in about 10 years. Also for a long time dreys were regularly built in one or both of the slash pines in a yard behind mine. That always seemed a strange choice since it required hauling in deciduous leaves and twigs from trees some distance away, but the pine needles may have provided a good supply of “hooks” to attach the dreys to, plus more shelter from winter weather.
For the past three or so years, the tree of preference for dreys here has been the fine young oak (previously mentioned) at the side of my front yard. There the attraction may be the profusion of leaves plus loads of acorns for convenient food; and that tree has achieved the necessary height after growing rapidly for 40 years without much competition. What surprises me is that the very tall oak in my back yard (planted by me 45+ years ago), which is upwards of 90 feet tall, never seems to have dreys in it. I’m still trying to figure this out.
Dreys have been a part of our northern hemisphere environment for millions of years and of our human environment for tens of thousands of years. Most of us, I think, never even notice them, much less get curious about them. We should!