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Squirrel Nests in the Morgan Arboretum

Squirrels are mammals that belong to the Sciuridae family, which is included in order Rodentia (rodent) (Macdonald 2009). They are among the most common and widespread mammals and are found in almost all regions of the world, excluding the arctic, Madagascar and desert areas. These large-eyed, bushy-tailed, diurnal (active during the daytime) animals are relatively unspecialised, instantly recognisable and can often be quite clever (Macdonald 2009). Tree squirrels always descend tree trunks head first, digging their sharp claws into the bark like an anchor (Macdonald 2009).  As they are generally perceived as attractive or “cute” animals, most people do not consider them pests like other rodents. For example, their distinctive way of feeding by squatting on their haunches and holding food in their forepaws can be considered very amusing.

An eastern grey squirrel enjoying some fresh acorns.

An eastern grey squirrel enjoying some fresh acorns.

Popular conceptions have led many to believe that squirrels hibernate. However, most species, including the ones found in the Morgan Arboretum, stay alert during the winter and rely on the nuts they have stored in the autumn (Macdonald 2009). Shockingly, a single squirrel can bury several hundred nuts every year. Individuals can smell nuts buried as deep as thirty centimeters, but many are never found which results in inadvertent tree seed dispersal (Steele & Al. 2006). Instead of hibernating like many other mammals, they must exit their nests to dig up the nuts they have stored in the previous months. Tree squirrels cannot spend more than two days inside their nest without having to exit and forage in the cold (Macdonald 2009).

Eastern grey squirrels (Sciurus carolinensis) and red squirrels (Tamiasciurus hudsonicus) are the two species of rodents found in the Morgan Arboretum of McGill that construct perched tree nests, also known as dreys (McGill Arboretum 2013). Spherical shaped dreys are constructed with small branches, twigs and grass, and are insulated with dry grass, moss and fur (Macdonald 2009). The number of dreys is a good indicator of the abundance of squirrels, but grey and red squirrels have very similar nest building strategies (Don 1985). This makes it nearly impossible to differentiate to which species a drey belongs (Gurnell et al. 2004). Red squirrels tend to prefer coniferous forests since they prefer to feed on pine cones over the fruits of maple, oak and hazel trees that make up the majority of the grey squirrel diet (Riege 1991). These nests allow the squirrels to function at a very low metabolism during most of the winter (Reynolds 2008) and are so well insulated that temperatures inside are usually around twenty degrees celsius higher than the outside temperature (Macdonald 2009).

A squirrel’s drey, or tree nest, built on top of a beech tree in the Morgan Arboretum.

A squirrel’s drey, or tree nest, built on top of a beech tree in the Morgan Arboretum.

Research question:

Is there a relationship between squirrel nest abundance in the Morgan Arboretum and the forest types and tree species they are found in?

Methods:

To test our research question, we sampled within seven 75×75 meter plots in the Morgan Arboretum. We tried to sample a variety of forest types, such as maple-dominated forest, beech-dominated forest, and coniferous forest. To create our sampling area, first a tree in the type of forest we wanted to test was marked with a piece of flagging tape. Two lines forming a right angle from this tree were then made, using two people walking 75 meters from the tree and marking a tree at the end of their lines with flagging tape. They then each turned 90 degrees towards each other and walked another 75 meters, marking the point they meet with flagging tape to make the final corner of the square. We then recorded the GPS location and forest type of the site.

The research group would then walk around the perimeter of the square, stopping every time a squirrel nest was spotted to record the species of the tree and the diameter of the tree 1.5 meters from the ground. A picture was then taken of the nest, and the tree was marked with flagging tape to avoid counting the same trees more than once. This procedure was repeated until the entire perimeter of the plot had been surveyed. Finally, a diagonal cross section was made through the plot, walking from one corner to its opposite to record any nests that resided in the middle. A final walk through was then done to remove all flagging tape from the area.

A map showing the position of the nests we found in the Arboretum, mainly in mixed deciduous forests, dominated by maple and oak.

A map showing the position of the nests we found in the Arboretum, mainly in mixed deciduous forests, dominated by maple and oak.

References:

Don, B. A.C. (1985). The use of drey counts to estimate Grey squirrel populations. Journal of Zoology, Vol. 206, No. 2, pp.282-286. DOI: 10.1111/j.1469-7998.1985.tb05656.x. Retrieved from: http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7998.1985.tb05656.x/abstract

Gurnell, J., Lurz, P.W.W., Shirley, M.D.F., Cartmel, S., Garson, P.J.,Magris, L. and Steele, J. (2004). Monitoring red squirrels Sciurus vulgaris and grey squirrels Sciurus carolinensis in Britain. Mammal Review, Vol. 34, No. 1-2, pp.51-74. DOI: 10.1046/j.0305-1838.2003.00028.x. Retrieved from: http://onlinelibrary.wiley.com/doi/10.1046/j.0305-1838.2003.00028.x/full

Macdonald, D. (2009). Squirrels. In The encyclopedia of mammals (New ed., p. 151). Oxford: Oxford University Press.

Morgan Arboretum. (2013). Mammals. Nature. Retrieved from: http://www.morganarboretum.org/nature-en/mammals.html

Reynolds, J. C. (1985). Autumn-winter energetics of Holarctic tree squirrels: a review. Mammal Review, 15: 137–150. doi: 10.1111/j.1365-2907.1985.tb00395.x. Retrieved from: http://onlinelibrary.wiley.com/store/10.1111/j.1365-2907.1985.tb00395.x/asset/j.1365-2907.1985.tb00395.x.pdf?v=1&t=i26o09oh&s=a5370763d1325426f65775ddb46a96f87443b03c

Riege, D.A. (1991). Habitat Specialization and Social Factors in Distribution of Red and Gray Squirrels. Journal of Mammalogy, Vol. 72, No. 1, pp.152-162. Article DOI: 10.2307/1381990. Retrieved from: http://www.jstor.org/stable/1381990

Steele, M. A., Manierre, S., Genna, T., Contreras, T. A., Smallwood, T. D. and Pereira M.E. (2006). The innate basis of food-hoarding decisions in grey squirrels: evidence for behavioural adaptations to the oaks. Animal Behavious, Vol. 71, No 1, pp.155-160. DOI: 10.1016/j.anbehav.2005.05.005. Retrieved from http://www.sciencedirect.com/science/article/pii/S0003347205003374

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About Crystal Ernst

Hakai Institute postdoctoral scholar at Simon Fraser University (B.C.)

9 comments on “Squirrel Nests in the Morgan Arboretum

  1. I like your project – fascinating. I often stare up at these nests when I’m walking about in a forest. How many years do the squirrels use the same nests? (or, stated another way, how often do they rebuild nests?).

    • Dreys will usually last for less than a year. However, many squirrels will build more than one as temporary shelters. These temporary nests are not built as strong, won’t last very long and aren’t used during the winter. Also, younger squirrels typically construct poor nests at first, but as they gain experience, the dreys last longer.

  2. I was just wondering if you encountered any problem during the course of your experiment? Very nice picture by the way!

    • Thanks Kim!

      There were a couple problems we encountered throughout our data collection- our method involved walking through large sections of the forest and sometimes it was tricky to make sure our lines were straight when making plots. Also, sometimes there were sections of forest we wanted to sample in but couldn’t because the forest was too thick to walk through. As well, as time progressed more and more leaves would fall from the trees. This made spotting the nests easier, but made it much more difficult to identify the species of tree they were in!

  3. Wow! On top of being “cute”, squirrels are brave enough to forage in the dead of winter! I was wondering if you knew for how long they can stay out of their nests. Also, even though they don’t hibernate, do they build up a protective layer of fat in fall? How can they withstand such cold temperatures?

    • Thanks for your question!

      One method squirrels use to survive the harsh winters is to alter their behaviour so they only leave their nests in the middle of the day when temperatures are warmest, and stay inside otherwise. And they do indeed build up fat in the fall, but this is more to use as a reserve when food becomes more scarce.

  4. Thanks for your question!

    One method squirrels use to survive the harsh winters is to alter their behaviour so they only leave their nests in the middle of the day when temperatures are warmest, and stay inside otherwise. And they do indeed build up fat in the fall, but this is more to use as a reserve when food becomes more scarce.

    • I would really like to get in touch with someone with much experience about squirrels. I would like to become a shelter for squirrels and would love to learn as much as possible. Who could I contact ? Thank you

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