Searching for Squirrels, Finding the Night

Written by Rachel Garwin

A week ago, I joined my friend Teage (a Field Naturalist alum) and a group of his UVM students on an “owl prowl,” Teage’s own euphonic term for a night hike.  We gathered at the edge of Centennial Woods, where gauzy tufts of white pines and bare hardwood twigs strained the clear moonlight.  A wall of darkness met our eye-level gaze, while the raspy sounds of drying beech leaves in the understory added to a sense of disquiet.  The primary goal for the evening was to listen for flying squirrels and call them in.  Before we entered the darkness, however, we observed a requisite pre-night-hike ritual.

Since leaving a well-lit environment too soon for the darkened woods might lead to undesirable confrontations with tree trunks and branches, Teage informed us about night vision while our eyes adjusted.  Human eyes require 20-30 minutes to become accustomed to low light conditions.  Coincidentally, it finally gets dark about 30 minutes after the sun first sets.  The students oohed appreciatively at the revealed secret of the universe.

I considered Teage’s implication, reflecting on whether twilight length was consistent enough to provide a uniform selective pressure.  The period between sunset and full dark (termed “Civil Twilight”) varies with latitude and season, as it reflects the time the sun takes to drop 6° below the horizon.  On the same night, civil twilight in Burlington, VT, lasted 9 minutes longer than in Bogota, Colombia, a city near the equator.  Atmospheric conditions and local weather can also affect our perception of available light from the setting sun, which increases variability.  Pole-to-pole variation aside, the length of civil twilight appears consistent enough at low and middle latitudes to suggest the plausibility of the relationship (though it does not prove it).  What mechanisms would select for correctly timed physiological processes?  Perhaps some hairy, fanged predator was involved?

Suddenly, I heard Teage ask, “Rachel, do you have anything to add before we head into the woods?”

Bits and pieces of the night hikes I used to lead rushed to mind.  Out of the torrent, what would be most relevant to this group of students?  Our pupils dilate to accept more light, just like a camera aperture changes size.  Our eyes comprise not only lenses (e.g., the cornea) that focus light, but also a receptor structure (the retina), which translates received light into neural signals our brains can understand.  The retina, in turn, is made of two types of cells: rods and cones.  Rods are far more abundant (about 17 for every cone cell); however, the cones are concentrated in the center of the retina.  Responsible for receiving color and fine resolution, cone cells are better suited for working in high-light environments.  Rod cells cannot understand color, but they register exceptionally more light than cone cells.  The operational ranges of the two cells overlap to some degree; in true twilight, both cells help parse the dim picture before us.

Instead of a long-winded physiology lesson, I settled on a practical and safety-oriented piece of advice.  “If you’re having trouble finding the trail in the dark, try using your peripheral vision.  Your rod cells—the receptor cells that are really good at picking up light—are arranged on the periphery of your retina, so that part of your eye sees better in low light conditions.”  With that, we were off.

As the trail contoured the side of a hill, I followed it with my feet as much as with my eyes.  Hard packed dirt spotted with only a few fallen leaves firmly resisted my feet, whereas my wandering steps sunk into noisy leaf litter.  After a few quiet minutes, we angled from the trail and picked our way down the gentle slope.  Looking out of the bottom of my eyes—as if I wore tiny slivers of half-moon spectacles—proved the best technique for avoiding the tangle of hardy ferns and woody shrubs.  Teage motioned for us to stop; this would be our first attempt to call the flying squirrels.

I listened intently.  Wind gusted through the upper boughs of white pines and red maples.  Still-hanging, papery beech leaves rubbed together, sending bursts of unwarranted excitement running through my mind.  Sirens howled close-by.  One of Teage’s students fired up the iPod, and high-pitched “chip chip chip” alarm calls radiated into the night to serve as bait.  I cupped my hands around my ears to exclude the droning car engines circling the outskirts of the woods.  Still nothing.  Perhaps the squirrels’ huge eyes, dominated by rod cells, picked us out as we muddled through their habitat.

I relaxed my eyes and marveled at the increased input from my peripheral vision.  Intermediate wood ferns stood distinct from the ground; before, they had dissolved into the dimness.  It seemed we had been away from bright light long enough for rhodopsin, a photopigment in the rod cells, to build up.  Rhodopsin and other photopigments in the cone cells help increase light sensitivity within the receptor cells, though at different rates and degrees. Cone cells, which never develop the light sensitivity attained by rod cells, take only 5-7 minutes.  Rod cells, however, may need over 30-45 minutes to achieve full light sensitivity.  In the presence of too much light, these photosensitive compounds break down; enough time in dark conditions is thus needed for photopigments to build up to a functional level.

The flying squirrels proved reticent, so we walked through the underbrush back to the trail.  The undergrads hesitated less between steps, and they seemed to run into fewer obstacles.  After walking a circuit across Centennial Brook and back along the lower slope on the other side, we paused to make their eyes’ night adaption more explicit.  Crouched beneath a closed hemlock canopy, we covered our left eyes and stared at the flame of Teage’s lighter with our right ones.  I smiled, startled by the pervasiveness of the “Pirate Patch” myth.  As lore would have it, pirates did not wear eye patches to cover gaping eye sockets.  Instead, they kept one eye in darkness to allow them to see below and above deck without needing time for rhodopsin to develop.  While no historical evidence supports this story, the folks at MythBusters put their weight behind its plausibility and likely had a role in its propagation.

Teage flicked off the lighter, and we switched our “eye patch” to the light-blinded eye.  Most students commented they could see more precisely with their night-adapted eye than with the light-blinded one.  Removing my hand, I winked back and forth at the hemlock boughs above.  Sure enough, my left eye discerned individual twigs against the dark sky; my right eye saw only fuzzy dimness.  While not proof that Blackbeard covered one eye so he could rush up to a darkened deck from a lamp-lit cabin, our experience supported the possibility.

We emerged from beneath the dense hemlock canopy onto a grassy hillside, where moon-cast shadows danced at our sides.  No longer relying on peripheral vision, the students carelessly walked down the trail towards home.  I smiled.  An hour ago, they had hung together timidly in similar light levels, still uncomfortable with moderate darkness.  Now they practically ran.  While the flying squirrels had remained elusive, the students found something more powerful: the ability to stride confidently through the night.

Will You Need a Warmer Hat This Winter?

by Carly Brown


Wolly bear (Pyrrharctia isabella) caterpillar

Wolly bear caterpillar (courtesy IronChris,


Isabella tiger moth (Pyrrhartia isabella)




Young Scientists Plot for Smart Urban Forestry

by Liz Brownlee

“Wait until you see the accuracy of our plot,” calls the lab team.

The four undergraduates burst with pride, oblivious to the prickling raspberries and thick brush that edge the Intervale forest.

They stop me midstride.  As their lab teacher, I’m fully equipped with aerial maps, GPS, first aid kit, phone, and extra rain gear.  I’m planning to cruise through this pasture towards the soybeans and into the swampy forest.  My goal is to check in with teams at twelve more study plots, spread over the 350 acres of forest and fields.

This lab project is a real, on-the-ground application of their natural resource training.  Most weeks these first-year students learn the basics of field work in big, broad fields (fisheries, stream ecology, forestry, etc.). The field data they record in their weekly labs is important: they use it to learn how to create papers, reports, and essays.

This week, though, the data is headed to city hall.  We’re studying the Burlington urban forest to help the city make informed decisions about valuing and managing the forest.

I need to check in with the next group. It’s incredibly important that the teams describe each plot with accuracy and efficiency.  I know that all the teams are anxious to prove their worth, to share their budding knowledge and field skills.  But this team’s stories cannot wait.

Elliot, a bright-eyed skier from Utah starts first.

The maple we used to mark our plot was so big, our measuring tape couldn’t reach around it, he says.

Aptly named and observant, Hunter chimes in.

There was a deer trail, but that’s it – otherwise it’s untouched.

Eric, a tattooed, intelligent former Marine, knows this isn’t entirely true.

How should we record the land use on this plot, Liz? This isn’t a park, and it’s on the farm but not cultivated land.

We brainstorm the forest’s values: recreation and flood control top the list.

I change my route, unable to leave the pull of their enthusiasm.  Elliot fires questions as we walk the farm’s puddle-filled gravel road.

We didn’t see any sign of invasive species like emerald ash borer – that’s good, right?  And the ground was covered in mud from the hurricane and the flood – do you think the forest will rebound before teams come next year?

The team answers most of their questions before I can chime in, and I know that I’m no longer needed.  I head for the next site when Carly, another lab teacher with twenty more students, calls from the Old North End of Burlington.  We check in, cars honking in the background on her end.

Over two hundred UVM students are taking the pulse of the forest, from tree height and canopy dieback to ground cover and plant-able space. They’re studying over one hundred plots throughout the city, and all in just two weeks.  We’ll crunch the numbers with a program called iTree that’s being used worldwide.  The city will use the results to decide where and what to plant next, and how to properly value a forest that cools homes, mitigates pollution, and absorbs storm water runoff.

They’ve tracked changes in the forest’s health.  They’ve built confidence as budding natural resource scientists.  And next year, two hundred new students will do it all again.

A Closer Look at Cones: Norway Spruce

by Doug Morin



What was that, I wonder?  Never mind, I have to focus.


Bang? Was that a bang?


I couldn’t help myself.  I opened the window and look down to the garage and driveway.  Nothing moved.  The neighbors weren’t even home.  Back to work.


I raced over to the window, catching a flash of rust-colored fur bolting along a spruce branch to the inner tree.  I looked down; the driveway was covered with spruce cones.  I stayed put, waiting to catch the culprit red-handed.  A minute later, the squirrel ran boldly out one of the long spruce limbs, 40 feet above the ground.  It ran to the end of the branch, hung down off it’s back feet, grabbed a cone with its front feet, chewed the cone’s base for a few second, then let it fall.  thwackclunkbang……… The cone tumbled to the ground, hitting the neighbor’s roof, the side of our house, then my housemate’s car.

Norway spruce. Note the swooping branches and drooping branchlets. Source:

Over the course of the last week, the squirrel dropped about 200 cones into our yard and driveway, by my estimate.  The cones were coming off a Norway spruce (Picea abies) tree in our backyard.

Native to Europe, Norway spruce is one of the main trees in the forests of Germany, Switzerland, Austria, and Russia.  In the U.S., it is commonly grown as an ornamental and in plantations, but rarely establishes on its own.   It is widespread throughout the cities and suburbs of the Northeast, so keep an eye out and you will start seeing it everywhere.

Norway spruce may be the tree most easily identified from a distance.  Once you get the search-image, you will be able to recognize it while driving 60 miles an hour on the highway.  An evergreen, Norway spruce has short, dark needles.   The trees usually grow 50-80 feet tall and two feet in diameter, and often have branches almost all the way to the ground.  And, most importantly –here’s your 60mph field mark— branches off the main stem arc upward (“swooping”) while branchlets growing from the main branches are long and hang down (“drooping”).  Swoop and droop – it’s that easy.

Now, back to the cones.  When you imagine a cone, I bet you think of a dry, brown one, light as a feather.  But, cones are not always so. The dry brown ones most of us imagine have passed maturity and already released their seeds.  In contrast, the cones pelting our house were still developing – leathery, green (or pink early in the season!), and dense.  Plenty dense to dent a car, as we discovered.

But these cones are only one of the two kinds of cones conifers produce.  The big cones we tend to think of (and the kind now all over my driveway) are female cones.  They are usually between 1 inch and 6 inches long depending on the species and produce seeds under their scales.  Squirrels eat the seeds, explaining why our squirrel was amassing a collection of female cones.  Lesser known are male cones.

Separate structures from female cones, male cones tend to be small (1/2 inch or less in length) and not as long lasting (they often disappear in days or weeks).  They produce pollen for a short time in the spring then, having fertilized female seeds, their job is done, and

they die back.  Interestingly, the difference between male and female cones explains why the squirrel was dropping cones from high enough to bombard our roof.

Male cones on left, Female cones on right. Sources:, Wikimedia Commons

Most trees concentrate male cones on their lower branches and female cones on their higher branches.  This serves an evolutionary role: it prevents self-fertilization. With male cones down low and female cones up high, pollen from male cones must get blown by the wind to get high enough to reach a female cone. This wind will usually carry the pollen to another tree.  If, however, the cones were intermixed or the males were on top, the pollen would fall directly into its own female cones.

So, if the tree wants to mate with another tree, rather than itself, it puts its female cones up high… giving them plenty of time to accelerate as they fall before pelting roofs, cars, and the occasional unsuspecting bystander.





Time Slips

By Danielle Owczarski

Crickets sound their high-pitched hum, blaring sirens swell and shrink, sweat percolates in overlapping areas, distant music floats through the moonlit breeze, insomnia returns, a kingfisher chatters along the lake shore, a main sail flaps in the breeze – all signifying the shift to summer in Burlington. Spring has its moments with its ephemeral blooms, but with summer, we shed the last layers of clothing. White skin peeks out no longer shy, soon many shades darker and prominently displayed. We relish the summer months without the taunting of spring’s undulating meteorological moods. Like our seasonal feathered friends, we break out into bold colors and partake in the ritualistic dance of the Burlington Jazz Festival and music on the waterfront. The neighborhood druids welcome the summer solstice by gathering around the Earth Clock in Oakledge Park, drumming in the sunlight’s energy.

Midsummer festivals, boldly celebrated in pre-Christian times and still today in many cultures under an altered guise, take place around the solstice. The Oxford English Dictionary states that summer has its origins in Old English as sumor, sumere, and somera among other variations first recognized in text c. 825, referring to its role as the warmest season of the year. And while summer more accurately applies to the astronomical solstice then to the actual beginning of the seasonal and climatic change, it is understood as a time of fertility, growth, and warmth. Bonfires and heavy drinking are the most common signifiers of midsummer revelries throughout the world, the celebration linked to the birth of John the Baptist, born six months prior to Jesus. However, stripped of its religious shroud, in the silence and beauty of the early morning, summer is the celebration of energy transformed into life.

Despite the simplicity of its title, summer is dynamic, generating an ever-changing understory and flow of biotic change. Those of us who work outside during this time of year follow the influx of hatching insects good and bad. Appearing first are the black flies, followed by the deer flies and mosquitoes. While the former two usually abide by shorter seasons, the mosquitoes continue hatching throughout the late summer months. Overshadowed by the annoying, small flying insects are those who act as predators keeping them at bay.

Dragonflies, unbeknownst to some, live most of their lives (up to four years) as armored nymphs in calm aquatic environments. The warm sun of early summer heats the waters, enticing the mature nymphs to the surface where they crawl out and clasp themselves to tall grasses, wood, and stone. Here they begin their hatch. Attached to a cattail leaf above the water, the nymph’s exoskeleton begins to dry and crack along the back where the dragonfly emerges, compressed and translucent, expanding into a world of bug feasts, avian maneuvers, sex, and egg laying. Their terrestrial phase occurs no longer than two months causing their numbers to dwindle by late summer.

Those of us working inside this time of year suffer through the mocking rays of light filtering through paned glass windows. The vigor of those coming in from outside tickles the nerves and draws conscious thought away from tasks and into daydreams of lake breezes on bare skin, lush winding hiking trails, and the transforming hues of clouds low on the horizon. For most of us trying to take advantage of the summer day, a strange phenomenon occurs, while the length of the day in relation to sunlight increases, time seems to move more quickly. Weekends fill with social obligations, and the list of things to do and places to visit become too numerous to accomplish. Summer spurs the endless chase of the elusive present moment.

In nature, as the season progresses, the once electrifying greens of spring begin to dull. Before we know it, tomatoes are ripening into deep reds and purples, and the goldenrods and asters are blooming. In town, the college students are moving into their dorms and young children board the yellow bus while we wait patiently in traffic. Some welcome the change with anticipation of harvest festivals, cider donuts, and corn mazes, while others feel depressed by the notion of winter’s encroaching darkness. Despite our resistance, the mayfly, stonefly, and caddisfly, each year during this time, hatch, lay their eggs, and die. The brook trout rise to eat, and a small red maple leaf flutters, dropping slowly to the water’s surface.

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