What are Those Clumps in Trees?

Caitlin Dempsey

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As the weather shifts towards winter, deciduous tress start to shed their leaves. With leaves first turning from green to a shades of yellow, orange, and red, eventually the tree will become completely bare of foliage.

Deciduous trees drop their leaves in winter to save energy and survive the cold. Without leaves, trees don’t need to use as much water, which is harder to get from the frozen ground in colder climates. Also, without leaves, the wind can blow through the branches more easily, which helps the tree stand strong during winter storms.

With no foliage left on the branches, these deciduous trees reveal the secret hideaways of squirrels. If you have ever seen a clump of leaves and twigs tucked up high into a tree, you are probably observing what is known as a drey.

A clump of dried and browned leaves in a bright green leaved deciduous tree.
A dense clump of dried leaves and twigs in a tree is mostly likely a squirrel’s nest, known as a drey. Photo: Caitlin Dempsey.

Bare branches reveal squirrel hideaways

With the shedding of leaves starting in the fall, squirrel dreys, which are usually hidden in the canopy, become visible. Dreys are clumps of leaves, twigs, moss, and sometimes fur that are nestled between branches. The outside of the drey is woven with these materials to form a shelter with a hallow inside. These dreys are one type of squirrel nest, used by squirrels for staying safe, sleeping, keeping warm, and raising young.

Most squirrels build dreys as a solitary activity. Squirrels have also been known to take over older dreys that have been abandoned.

Squirrels also create nests from dens

Dreys are not the only nests created by squirrels. Squirrels will also seek refuge in dens, typically a tree cavity to which needles, moss, and fur have been added for comfort.

Types of dreys

University of Edinburgh biologist Andy Tittensor noted that there are two types of dreys. One is a temporary drey, used primarily during the summer months for resting and for shade as one way for squirrels to keep cool in hot weather. These temporary dreys are a shallow saucer-like shape that often has no roof. Squirrels use these dreys temporarily during the warmer months. The second is a permanent drey which is fully enclosed with one or more small entrances that lead to a hallow interior.

Dreys are easiest to see on deciduous trees after the leaves have dropped but you can also spots these nests in evergreen trees as the dried needles will fade to a reddish-brown color and stand out against the green pine needles.

A squirrel's drey with reddish, dried needles in a pine tree.
A squirrels drey, with reddish, dried needles, stands out agains the green pine needles of a tree. Photo: Caitlin Dempsey.

What species of squirrels in the United States build dreys?

In the United States, several species of squirrels build dreys, and these include:

  1. Eastern Gray Squirrel (Sciurus carolinensis): Commonly found in the eastern U.S. and introduced to the western part of the United States, these squirrels are known for their gray fur and bushy tails. They are adept at building dreys in urban and suburban areas as well as forests.
A gray squirrel on a branch in a tree.
The Eastern gray squirrel is common in many parts of the United States. Photo: Caitlin Dempsey.

  1. Western Gray Squirrel (Sciurus griseus): Native to the western U.S., particularly along the Pacific Coast, these squirrels have a distinct, more silvery-gray coat compared to their eastern counterparts.
  2. Fox Squirrel (Sciurus niger): The largest species of tree squirrel in North America, the fox squirrel is known for its varied fur color, ranging from gray to reddish-brown and tan fur on the underbelly. They are widespread across the eastern, midwestern, and some western states.
A brown and tan squirrel sits on the base of a palm tree.
A fox squirrel. Photo: Caitlin Dempsey.
  1. Red Squirrel (Tamiasciurus hudsonicus): Smaller than the gray squirrel, red squirrels have reddish fur and are found in both coniferous and deciduous forests, primarily in the northern and western parts of the U.S.
  2. Douglas Squirrel (Tamiasciurus douglasii): Native to the Pacific Northwest, the Douglas squirrel is a small, energetic species with a preference for coniferous forests.

What do squirrels use dreys for?

Dreys have a number of uses for tree squirrels.

For warmth and weather protection

Dreys that are enclosed provide protection for the squirrels against weather conditions. Dreys provide shade from the sun, keep the squirrels dry during storms, and are warmer than the ambient air during the winter. A study of dreys that was done in 1935 measured the temperature outside and inside 65 empty dreys during the winter in Russia. Isakov & Lubimov found that while temperatures outside were well below freezing (ranging between -1° and -10 °C) the temperatures inside the dreys were much warmer with temperatures more than 10° to 18° C higher inside the next (ranging between 1° to 10°C).

A similar study by University of Helsinki researcher Erkki Pulliainen found similar temperature differentials. For his study, Pulliainen measured the external and internal temperature of 74 dreys inhabited Red squirrels in Lapland repeatedly over four winters. What Pulliainen found was that the body heat of the squirrel raised the internal temperature of the drey to 25° C (77° F). Even when the squirrel had exited the drey, the temperature stayed elevated and never went below 3° or 4° C and quickly reached 25° C within 10 to 30 minutes of the squirrel returning.

To hide from predators

As prey animals, squirrels are at risk of predation from hawks, falcons, and other aerial predators. Dreys provide a quick and safe place to retreat to when a bird of prey is circling overhead.

A safe place to raise a brood

These nests provide a safe and warm environment for the baby squirrels, known as kits or kittens. The mother squirrel builds the drey to be sturdy and comfortable, using materials like twigs, leaves, and grass. It’s in these nests that she cares for her kits until they are old enough to venture out on their own. Isakov & Lubimov also noted that female squirrels will often evict their male mates from the drey in preparation for raising their young.

References

Isakov, J. A., & Lubimov, P. M. (1935). Biology of the hares and squirrels and their diseases. Mantejfel, P. A., (Ed.) Moscow.

Pulliainen, E. (1973, January). Winter ecology of the red squirrel (Sciurus vulgaris L.) in northeastern Lapland. In Annales Zoologici Fennici (pp. 487-494). Societas Biologica Fennica Vanamo. https://www.jstor.org/stable/23731714

Tittensor, A. M. (1970). Red squirrel dreys. Journal of Zoology162(4), 528-533.

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.

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