Hanging in there

3 January 2014 by Malcolm Campbell, posted in Biology, Science

Yet between the lines there’s an implicit demurral; the habit stays with us, albeit it’s puerile: when Darwin saw squirrels, he saw more than Squirrel.” From the poem, Darwin’s Bestiary, by Philip Appleman (1926- )

It’s difficult not to be heartened by persistence in the face of adversity.

For weeks, the grey squirrel nests have been awe-inspiring. Perched high amongst the bare branches, they had been soaked by heavy rains, whipped by strong autumn winds, and laden with early winter snowfall. Against seemingly high odds, the nests continued to cling to their lofty branches. squirrelsnest1

And then came the ice storm.

The ice storm did its damage with great stealth. There was no lashing wind, no torrential downpour. There was merely a persistent frozen drizzle – not a sheet, but the finest of veils – adhering to every surface as it misted the landscape. It was not so much a lambasting as a basting. In its wake, it left a landscape glazed in ice.

That glazing was a burden too great for many.

Stalks of dried grass that had the week before stood steadfast above snow banks were felled by their icy load. Branches, small and large alike, could not withstand the weight and snapped – peppering the ground with woody debris. Even tree boughs and trunks that had seen scores of years come and go could not contend with this newest challenge. With frightening claps they split, cracked, splintered, and were ripped asunder – crashing dangerously to earth with a burst of shards of wood and ice. Tree trunks were torn down the centre, in halves, thirds, or quarters, as the largest boughs each toppled in a different direction as they were forced to earth. brokentree

The ice storm left in its wake a sight of random devastation.

Even still, with ice-laden branches tumbling to earth everywhere, the grey squirrel nests persisted. They remained steadfast, these seemingly hodgepodge clusters of arboreal bric-a-brac, firmly in place, hanging on with the greatest tenacity. The nests seemed to thumb their noses at the conditions, as if to say, “Is this all you’ve got? Bring it on!”

Grey squirrel nests are nothing if not a testimony to incredible animal industry – the capacity of other creatures to create something with a lasting legacy – something that can withstand whatever the passing seasons may throw at it.

What is truly remarkable is that the nests appear flamboyantly, or at least hastily, constructed – creating an apparently disorganised assemblage.

How had these clumps of dried plant matter, hurriedly wedged into the forks of branches, survived?

The short answer is that the grey squirrels that reside here are impressive nest builders. squirrelsnest2

Grey squirrels are the dominant squirrel species here. Originally forest-dwelling rodents, grey squirrels were introduced to urban environments around 150 years ago, and have thrived in that environment since. While urban living has created a semi-domesticated animal, comfortably inhabiting an environment in very close proximity to humans, grey squirrels still retain much of their forest-dwelling habits. Nest building is one of them.

Grey squirrel leaf nests, or dreys, are one of two types of nests that they construct. Through particularly inclement weather, grey squirrels reside in a nest, a den, that they construct in sheltered hollow – generally the cavity of a tree. By contrast, the drey is constructed out in the open, made of leaves and twigs, on the branches of the tree. It is exposed to the elements, and, therefore, not the preferred location for nesting during the foul weather of winter – it is stereotypically a summer and autumn dwelling. This said, squirrels can reside in dreys throughout the winter months. What’s more, dreys can be re-used year-after-year, provided they persist throughout the bad weather.

And this is the marvel of the drey – they do persist. squirrelsnest5

Dreys are constructed by squirrels during the summer and autumn. In fact, in some regions of their range, grey squirrels have two peaks of drey-construction activity ; one mid-summer and one in late autumn. It has been hypothesised that drey construction coincides with the emergence of juvenile squirrels from their maternal nest, as they strike out on their own. This may be the case, but what is certain is that construction occurs when conditions allow the harvesting of growing plant tissue – leaves and twigs are stripped from growing branches, and incorporated into the drey.

One of the most thorough analyses of grey squirrel drey construction was undertaken by William Fitzwater and William Frank in 1941. They noted a paucity of information on grey squirrel dreys – the most detailed published description up to that point was a handwritten report from 1940.  To fill this gap in knowledge, Fitzwater and Frank investigated 146 dreys in a wildlife sanctuary in the state of Connecticut. squirrelsnest6

The dreys showed great industriousness. The dreys comprised a foundation of decaying leaves built on a supporting platform of twigs, on top of which was constructed an outer shell of leaves and twigs enveloping a tightly woven inner core. The squirrels used a remarkable diversity of plant species to construct the nests – thirty-nine species of plants were represented in the 146 dreys. Broad-leafed tree species dominated the leaf material used to construct the dreys – woven into the layers of the nest. Dreys are impressive in their size – occupying a volume of almost 50 litres – but equally impressive in their mass – weighing in, on average, at around 1.5kg. Their volume provides great insulation against the elements, while their weight means that they are not taxing on the tree – and are easily supported in the crux of the branches where they are located.

What was perhaps most remarkable was the selectivity the squirrels showed in locating their dreys.

Of the thirteen species of trees in which Fitzwater and Frank found dreys, just five species accounted for more than 80% of the drey locations. Strikingly, more than 50% of the dreys were found in white pine trees alone. Clearly, there was some tree selectivity, but this may have been due to a number of secondary factors, not the least of which was tree size. squirrelsnest7

In fact, tree size turns out to be an important determinant of where grey squirrels locate their dreys. Fitzwater and Frank found that the grey squirrels prefer good-sized trees, with an average of a 35cm trunk diameter, and a height of 17m on average.

Fitzwater and Frank’s findings are broadly consistent with those of others who have investigated preferred drey locations since. While more recent studies are scarce, a trio of investigations from 1993, 2005 and 2008 explored drey location across a number of sites.

In all instances, the number of dreys investigated was relatively low, but in all cases, the squirrels showed some degree of preference for a tree type to site their nest, even given a variety  of species to choose from. Interestingly, the preference wasn’t always for pines, or even other conifers, as might be inferred from Fitzwater & Frank’s study. Indeed, often it was broad-leafed tree species, like oaks and maples, which were preferred. What seemed to be a constant in all studies was tree size. Tall trees were most frequently chosen as nest building sites. What’s more, the nests tended to be located at higher locations within those trees. Why might this be? squirrelsnest9

In a wonderful study conducted as a university undergraduate project, Natalie Robiou examined why the squirrels might choose certain trees to build their nests. She examined where grey squirrels situated their nests on her university campus. Again, sample size was low, but the trend was for squirrels to situate their nests in taller than average trees, and in trees that were more likely to have canopy connections with other trees.

Robiou hypothesised that the major driver for greys squirrels to locate their dreys in particular trees was avoidance of predation. By placing their nests in high trees, they were less likely to be preyed on by ground predators. By situating them in trees that had good connections with neighbouring trees, the squirrels both avoided the need to travel from one location to another on the ground, and also had routes to escape predators in the canopy. Of course, there could be other explanations, not the least of which could be availability of foodstuffs in the trees in question, and in the neighbouring trees.

Another determinant might be resilience – durability – resistance against inclement weather.

Why might this be a reasonable determinant?

Well, first off, because the dreys do seem to be durable. In observing 24 grey squirrel dreys over the past couple of months, it was clear that they were able to survive whatever weather came their way. This was underscored by the ice storm. Despite the number of downed branches, none harboured dreys. Not one. It was rather astonishing. In fact, all 24 of the dreys observed over the past months remained where they were. squirrelsnest10

Now, it’s possible that many other dreys were constructed during the year, and only the most robust survived into the autumn. This implies that the squirrels built a good may more dreys, and that those that remained are merely the survivors of some sort of selection process. This is a plausible possibility.

On the other hand, it seems equally plausible that there would be value to building dreys that persisted. Given the time and energy invested in drey construction, and the value of having a drey situated in a good location, why not construct dreys that are built to last?

The value of a persistent drey is reinforced by the lifespan of grey squirrels, which may be decades in duration. Constructing a drey that can be used year after year would be a useful investment of time and energy for such relatively long-lived creatures.

Might it therefore be possible that the squirrels situate their dreys in the best locations for their nests to resist the elements?

Might it also be that these sites happen to be within tall trees – on strong branches – locations that have already withstood the test of time? What’s more, tall trees within close proximity of others might be buffered from the adverse effects of wind, rain, ice and snow. squirrelsnest11

All told, the squirrels have chosen locations that ensure that their dreys will last.

Of course, it may be that many factors come into play – escape from predation, connectivity to resources, shelter from the weather, durability.  Clearly, determining which factors figure most prominently in a grey squirrels choice of a nest site is a matter requires further observation – and proper replication and statistical analyses!

Regardless of the factors that determine a squirrel’s choice of a drey site, the fact remains that these remarkable structures persist from year to year.

We tend to be impressed by our own monuments, of our impressive cleverness, of the ability of our constructions to withstand the elements. But we are not alone in this regard. Many fellow creatures, like the grey squirrel, share the capacity to create a lasting legacy. Their dreys tough it out, hanging in there when the going gets really rough. Dreys are a wonderful reminder that many efforts, when properly constructed, and located, can persist against the odds.  They can provide us with shelter, a safe place, when it is difficult to imagine things getting worse.

Images: All photographs by Malcolm M. Campbell.

References: squirrelsnest13

Benson E (2013) The Urbanization of the Eastern Gray Squirrel in the United States. Journal of American History 100: 691-710

Cox K, Glaid J, Ondrasik P & Scheler S (2005) Nest-site preference of gray squirrels. Journal of Ecological Research 7: 36-42

Fitzgibbon, CD (1993) The distribution of grey squirrel dreys in farm woodland: the influence of wood area, isolation and management. Journal of Applied Ecology 30: 736-742

Fitzwater WD & Frank WJ (1944) Leaf nests of gray squirrel in Connecticut. Journal of Mammalogy 25: 160-170

Robiou N (2008) Urban Wildlife and Leopold’s Land Ethic: “The squirrels on a college campus convey the same lesson as the redwoods....”. Environmental Studies Project Report, Fordham University

Thompson DC (1977) Reproductive behavior of the grey squirrel. Canadian Journal of Zoology 55: 1176-1184

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4 Responses to “Hanging in there”

  1. Chris Buddle Reply | Permalink

    Great post, Malcolm - I learned a new word! (dreys). I've also been keen to study the arthropod fauna that resides in the dreys -it'll likely be a combination of parasites, but also critters that take advantage of the protection, warmth, etc. In fact, every time I see a drey, I imagine all the little creatures making homes inside these structures. To an entomologist, the squirrel is merely a 'creator of habitat' :)

    • Malcolm Campbell Reply | Permalink

      Super comment, Chris! For what it's worth, Fitzwater & Frank report on arthropod species found in grey squirrel dreys in their 1944 paper ("insect" survey found on page 168). While 60% of the dreys contained fleas (flea infestation is one of the few things that will drive squirrels from their dreys), a multitude of other arthropods were found. Fitzwater and Frank focused on insects, particularly fleas, but they do comment on the presence of spiders and nematodes as well. The Fitzwater & Frank paper is a wonderful read, if only because they emphasise that their work points to the need for more research. They are particularly well-aligned with your keenness to examine the ecosystem created within the dreys, noting that "commensal relationships between the insects and the squirrels would make an interesting study". Getting started on this would make a wonderful undergrad project!

  2. Laura Timms Reply | Permalink

    Thanks, Malcolm - very informative! I've often thought that I would like to include urban squirrel ecology in my research somehow. Anyways, thought I'd add my anecdotal observation that I did see a few downed dreys in my neighbourhood, post ice storm. Unfortunately the squirrels don't have a lot of choice in our area - almost all the big trees are manitoba maples, and they lost a lot of branches. Happily, the squirrels in the big spruce in my backyard are still safe a cozy.

    • Malcolm Campbell Reply | Permalink

      Thanks for the kind feedback, Laura! Tree choice for siting dreys by squirrels is such an interesting research question. I look forward to seeing someone do a comprehensive quantitative analysis. Seems like a project that might have potential for citizen scientists, or an interested group of undergrads!

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