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An early winter morning. K.P. McFarland

Field Guide to December 2020

By Vermont Center for Ecostudies December 2, 2020

December is off to a gentle start this year; the annual blanket of snow and ice has yet to drape across the land. As we move into this chilly month, you may find yourself wondering how wildlife adapts and survives each winter. Cozy up with our Field Guide to December and a warm cup of tea to learn how species from birds to bats and mice to moose face the coming cold-weather challenges.

Black-capped Chickadee © Kent McFarland
Black-capped Chickadee © Kent McFarland

How Do Songbirds Survive Vermont Winters?

By Julia Pupko

I think everyone is in agreement that Vermont winters can be bitterly cold. While many mammal species have the ability to put on large amounts of fat and/or hibernate, songbirds need to remain light enough for energy-efficient flight and do not hibernate. Some songbirds risk a dangerous annual southern migration, sometimes flying thousands of miles to return to warmer regions. But how do resident songbirds and those that visit Vermont in winter survive?

The primary issue that comes with overwintering in cold, snowy regions such as Vermont is simply not freezing. Maintaining one’s body temperature (which for birds is generally around 105 degrees F) requires high caloric inputs, meaning that many songbirds must forage nearly continuously during winter days. They need to intake enough calories to support their daily activities and store up a fat reserve to get them through the night. This is complicated by the fact that there is a large reduction in available food sources and quantities in winter. As a result of balancing reduced food resources with the need for higher caloric inputs, many songbird species implement multiple strategies to efficiently forage and retain energy.

Black-capped Chickadees provide an excellent example of the diversity of different mechanisms one species may utilize for winter survival. To improve the chances of finding food, they forage in groups, which increases the probability of finding food sources and allows Black-capped Chickadees to learn about new foraging techniques and food sources from each other. To efficiently retain heat, chickadees fluff out their feathers and shiver, using opposing muscle groups. To further conserve energy, they prioritize heating their body’s core by reducing the temperature of their extremities. They regulate their foot temperature at about 30 degrees F, with cold blood from the feet warmed upon reentry to the main body by warm blood that is on its way to the feet, thereby reducing heat loss. At night, chickadees nestle down in sheltered cavities and drop their body temperature (by as much as 22 degrees F!) to reduce heat loss and energy output. The process of lowering the body temperature is called regulated hypothermia.

Another small songbird that overwinters in Vermont is the Golden-crowned Kinglet. Unlike Black-capped Chickadees, Golden-crowned Kinglets are not cavity nesters. By day, they forage in groups, seeking out frozen insects, spider eggs, and pupae under bark and around conifer trees.  To survive the brutal nights, Golden-crowned Kinglets roost in groups on sheltered branches, sharing body heat. Like Black-capped Chickadees, they also utilize regulated hypothermia to save energy.

View December checklists from years past on Vermont eBird and observations of all songbird species that have been uploaded to iNaturalist in December to help you identify the birds of winter in your own back yard.

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Yellow Birch roots encrusted in snow, following germination on a short snag © Julia Pupko
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Soil samples from agricultural fields for a laboratory study on nitrous oxide emissions © Julia Pupko

Changing Snow Patterns and Freeze-thaw Cycles

By Julia Pupko

Climate change is impacting Vermont’s winters already. Based on current trends, Vermont is predicted to have a 4.8 degree F increase in December’s average temperature between 2039 and 2069 from the average temperatures reported between 1968 and 2000. Already, December average temperatures and Vermont’s annual average temperatures are showing an upward trend. With the increase in average temperatures has come more variable temperatures, with temperatures in winter months fluctuating between freezing and above-freezing in greater frequencies than they have historically. Additionally, there has been a reduction in snowpack depth and cover consistency. Snowpack accumulation early in the year before the soil fully freezes keeps soil temperatures at or slightly above freezing throughout the winter. While a thick, consistent snowpack keeps the soil at relatively constant temperature, reductions in snowpack depth and consistency exposes the soil to air temperature fluctuations, causing it to freeze and thaw with air temperature changes.

The increased frequency of soil freezes and thaws (or freeze-thaw cycles) has multiple impacts on Vermont’s ecosystems, including increased greenhouse gas emissions from the soil and stress on trees. As the soil alternates between frozen and thawed, water from melting snow and ice fills the soil. Water percolating into the soil flushes nutrients to microbes that have been reactivated by the warmer temperatures. Temperature change and nutrient influx sparks microbial growth, which quickly depletes the limited oxygen supplies in the soil, causing the conditions to become anaerobic (lacking oxygen). In soils with high nitrogen content, such as farm fields, these are the perfect conditions for denitrification, a process that occurs in microbes where nitrogen compounds (like nitrate) are assimilated into the organism and broken down into nitrogen gas (which makes up over 70% of Earth’s atmosphere). However, the process is often incomplete, leading to the release of nitrous oxide (N2O), which is created as an intermediate step during denitrification. Nitrous oxide is a greenhouse gas 298 times more potent than carbon dioxide (CO2). The increased number of freeze-thaw cycles that come with climate change may be causing an increase in N2O emissions from soils, increasing greenhouse gas concentrations in the atmosphere, and further fueling climate change.

In addition to increasing greenhouse gas emissions from soils (particularly from agricultural soils), snowpack changes and increased exposure to freeze-thaw cycles impacts tree root health. While tree growth is mostly concentrated in the summer months in Vermont, tree roots can remain active in soils that remain at or slightly above freezing. As previously stated, thick, consistent snowpack provides the necessary insulation for soils to potentially meet this criteria in winter. Under these conditions, tree roots can continue growth, repair damaged roots, and may help the tree prepare for the coming growth season. On the flip side, a deep freeze can damage fine tree roots in the top layers of the soil and inhibits any growth or repair of roots before spring. This means that trees will have to allocate more resources towards root growth and repair during the growing season to compensate for fine-root loss during the winter, which may add detrimental levels of stress on some trees. One study showed that increased freeze-thaw cycles caused trees to over-compensate their fine root production, reduced their nitrogen uptake, and changed nitrogen cycling in the ecosystem. Multiple freeze-thaw cycles can also increase the risk of trunk crack formation and may cause the tree to begin to bud out early if there is an extended warm period, further injuring the tree when new growth is killed upon the return of below-freezing temperatures.

This is not a full list of all of the implications climate change, reduced snowpack, and increased freeze-thaw cycles may have on Vermont’s terrestrial ecosystems. Much remains unknown about the full depth of impacts from these factors, or how interactions between them may change our ecosystems.

Little Brown Bats hibernate together in a cave. © jrosenth (iNaturalist)
Little Brown Bats hibernate together in a cave. © jrosenth (iNaturalist)

A Not-so-Peaceful Winter Nap

By Emily Anderson

Bats are not everyone’s favorite critter. However, they humbly remove up to 1,200 insects per hour in the summer months. Without bats, sitting in your backyard on a sweltering July evening would become unbearable. When the weather turns cold, six out of Vermont’s nine bat species head indoors, settling deep in caves referred to as “hibernacula.” During the winter months, these bats slow their heart rate and metabolism, and remain in a state of torpor to minimize energy loss, rarely waking except when disturbed or in response to warmer temperatures.

This type of hibernation is not unique to bats – other small mammals that remain in Vermont all winter long fall into a similar state. However, for Vermont’s hibernating bat species, it carries an extra risk. Over the past decade, white-nose syndrome (WNS) has plagued bat populations. Pseudogymnoascus destructans, the fungus responsible for WNS, loves cold, damp places, making hibernacula the ideal locations for it to grow and spread. And hibernating bats, whose immune systems get suppressed during torpor, make it easy for the fungus to proliferate. Once introduced to its host, P. destructans invades skin tissue, causing the characteristic white fuzz on both snout and wings. Infected bats also experience increased water loss, frequent arousal from hibernation, and greater depletion of fat reserves. Ultimately, these conditions weaken infected bats, making many cases fatal.

As a result, several bat species have declined in Vermont. Little Brown and Northern Long-eared Bat species are particularly affected, having experienced a 75-90% population loss between 2008 and 2011. Currently, both species are listed on Vermont’s Endangered Species List, along with the Indiana Bat, Tricolored Bat, and Eastern Small-footed Bat. Despite these grave statistics, some individuals do survive. In some cases, epizootics like this give evolution a strong push in a different direction, encouraging the proliferation of traits promoting greater resistance or tolerance to infection.

Currently, there is evidence that Little Brown Bat populations might be stabilizing, however none of the affected species are out of the woods yet. Scientists still have many unanswered questions. If you’re wondering what you can do to help support Vermont’s bats, the VT Fish & Wildlife Department provides guidelines and answers. To learn more about Little Brown Bats, check out this episode of Outdoor Radio.

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Isabella Tiger Moth (aka Woolly Bear) © Kent McFarland
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Cecropia Moth © Kent McFarland

December Moths

By Julia Pupko

When I think about moths, I think of a mix of fragility and strength: paper-thin wings that somehow last through all sorts of summer storms to carry the insect in question to wherever it needs to go. What I do not think about is a winter-resistant insect, capable of even making it part-way through December in Vermont. In my mind, they must either migrate or turn into tiny insect ice cubes if they remain. And yet, many moths overwinter in Vermont, emerging in the spring to resume activity as normal. So how do they do it?

Moths have several different strategies for making it through the winter. These strategies involve each of their life stages: egg, larvae (caterpillar), pupae (chrysalis), and adult (moth). Some species bury themselves deep in the leaf litter or soil, while others remain attached to a twig or tucked away under some loose bark. No matter what life stage these moths are in, the vast majority are in diapause. During diapause, growth and metabolic function nearly cease to occur, comparable to the true hibernation strategy in mammals. Diapause is very important, particularly for eggs, larvae, and pupae. If these young stages of moths continued growing at the rate they grow during the summer months, they would emerge in the dead of winter to a frozen, flowerless landscape.

Many moth species go through multiple generations over the course of the year. As summer moves into fall, daylight hours (photoperiod) get shorter and the temperature begins to drop. This seasonal change tells moths, no matter what life stage they are in, that they need to find a spot to hunker down and prepare to enter diapause. Part of the change that many moth species undergo allows them to create an antifreeze solution from the concentrated proteins and sugars in their body, inhibiting the formation of ice crystals in their bodies as temperatures drop well below freezing.

Vermont has many different species of moths, and has species of moths that overwinter in each phase of a moth’s life cycle. For example, moths such as the introduced Spongy Moth (Lymantria dispar), overwinter as eggs. Gypsy moth eggs actually contain larvae that are ready to hatch when winter comes, but instead of hatching, they enter diapause and await spring. Other moth species, such as the Isabella Tiger Moth (Pyrrharctia isabella) and the Fall Webworm (Hyphantria cunea), overwinter as caterpillars. Isabella Tiger Moth caterpillars, or Woolly Bears, overwinter in leaf litter or in sheltered spots on the ground, sometimes becoming entirely encased in ice, relying on the combination of their antifreeze solution and diapause to get them through the winter. Fall Webworms, on the other hand, do not rely on their antifreeze fluids alone, burying themselves in the soil under leaf litter for additional protection from the cold and predators alike.

Moths such as the Cecropia Moth (Hyalophora cecropia) spin a cocoon, in which they pupate. This cocoon is attached to a twig, where they hang until spring comes. Other moths that exist as pupae overwinter may attach their cocoons to logs, in cracks in tree bark, in other sheltered areas, or may pupate in the leaf litter. Other moths, including the Montana Six-plume Moth (Alucita montana), overwinter as adults, tucked away in cracks and crevices, only to emerge and fly once temperatures have warmed up enough for a long enough period of time. Moths that overwinter as adults have a jump on other moth species come spring, as they do not have to emerge from their pupae, hatch from their egg, or grow through multiple life stages before becoming adults and laying eggs.

Keep your eyes open and check out the tree branches and trunks, logs and leaf litter as you stroll through Vermont this December. You may just find a moth, in one of its many stages. Be sure not to handle it, as the warmth from your hand may prematurely awaken it. However, you can snap a photo and upload it to iNaturalist, adding your observation to the ever-growing records of Vermont moth species.

Deer Mouse/White-footed Mouse (genus <em>Peromyscus</em>) © Sarah Carline
Deer Mouse/White-footed Mouse (genus Peromyscus) © Sarah Carline

Mice on Ice

By Julia Pupko

In Vermont, we have two native mouse species and two native jumping mouse species: White-footed Mouse (Peromyscus leucopus), Deer Mouse (Peromyscus maniculatus), Meadow Jumping Mouse (Zapus hudsonius), and Woodland Jumping Mouse (Napaeozapus insignis). Of these species, only the Meadow Jumping Mouse and Woodland Jumping Mouse hibernate. Meadow Jumping Mice use environmental cues of shorter photoperiod (daylight hours) and colder temperatures to spark the beginning of their hibernation process (hibernation lasts from September or October until around May), which includes two weeks of intensive eating prior to hibernation to build up a fat layer. Woodland Jumping Mice also hibernate from around September or October to April or May, with shortening photoperiods in the fall causing the same psychological response that occurs in Meadow Jumping Mice, resulting in heavy eating to build fat reserves for two weeks prior to hibernation. Both are true hibernators, meaning that they enter torpor while hibernating, slowing their metabolic rates greatly.

What are Vermont’s other mice species doing in December, while the Jumping Mice are tucked away in their hibernation burrows? White-footed Mice are found across much of the United States, and have different regional adaptations to the local climate. For example, southern populations of the White-footed Mouse have the peak of their breeding season during the winter, when temperatures are cooler. In northern regions, where the winter is harsh, White-footed Mice do not breed during the winter, as the energetic output of staying warm and finding food would potentially be fatal to both adults and offspring. White-footed Mice remain active throughout the winter, occasionally entering torpor for short periods of time on exceptionally cold days. They will sometimes cache food, and nest communally with other White-footed Mice for warmth. In northern climates, such as that in Vermont, White-footed Mice make thicker, more insulated nests to reduce the amount of energy they need to exert to keep themselves warm while in their nests. Generally, White-footed Mice have been found to be very adaptable, changing their behavior to meet environmental conditions.

The Deer Mouse implements many comparable adaptive measures to the White-footed Mouse in winter to increase chances of survival. The Deer Mouse will create nests both above and belowground, utilizing abandoned burrows for underground nests. They will also nest in groups during the winter for warmth, sometimes in groups greater than 15 individuals. The Deer Mouse is an aggressive winter food cacher, with one individual caching up to 3.2 quarts of food for the winter in hollow logs and other protected areas. The Deer Mouse also has a wide range that spans much of the United States, and also regionally adapts its breeding season to the local climate, meaning the Deer Mice in Vermont stop breeding before December. Much winter travel occurs in tunnels through the snow, rather than on the surface.

These two species can sometimes be found nesting together in burrows during the winter. When a mouse travels on the snow’s surface, you will be able to see marks from their long tails dragging in the snow. As the December snows start to fall, keep your eyes open for tiny tracks in the snow, with the tell-“tail” imprint from their long tails.

A young moose photographed by Ed Sharron in Rochester, VT. © Ed Sharron
A young moose photographed by Ed Sharron in Rochester, VT. © Ed Sharron

Warmer Winters Mean Trouble for Moose

By Emily Anderson

The Moose – northern New England’s unofficial mascot – haunts the imagination of hunters, photographers, and wildlife enthusiasts alike. In the warmer months, these graceful woodland giants forage in areas where nutrient-rich aquatic plants are abundant, at times diving 18ft deep to reach their food. In the frigid months when aquatic vegetation vanishes, moose too must disappear from their watery feeding grounds to pursue woody plant material, such as twigs and bark. These food sources offer distinctly fewer calories than their summer feed, making energy precious and its conservation vitally important.

However, moose are designed for harsh winters. Although often seen as similar to deer, moose cope more easily with deep snow thanks to specially formed joints which allow them to lift their legs straight up nearly to shoulder height. Even in feet of snow, moose can move with relative ease and expend less energy when traveling or escaping predators. However, unlike deer, moose are at significant risk of overheating in the summer, due to their large size and thick fur which help them weather bitterly cold northern winters. As the climate warms, these traits which make them perfectly suited to the icy north will become a greater disadvantage, possibly affecting their survival.

Moose are also facing another significant threat to survival: winter ticks. New England winters are becoming warmer and shorter due to climate change. In the past, freezing temperatures and heavy snows have caused tick populations to sharply decline right when they are searching for a host. In contrast, milder winters fail to dent their populations, meaning that moose can accumulate higher numbers of tick nymphs. Unlike deer, moose do not regularly engage in habitual grooming. Ticks often go through several life stages on a moose and reach their final, most voracious state before the moose notices its passengers and tries scratching them off. By that time, it is usually too late. Studies show that these tick counts can range from ten to one hundred thousand individuals per moose, effectively bleeding them dry. These large infestations lead to increased calf mortality and a decrease in female breeding success.

Tick counts on moose in Vermont are lower than those found in New Hampshire and Maine, likely due to Vermont’s efforts to reduce moose population numbers. However, the likelihood of more mild winters coupled with the reduction in reproduction and calf survival means that New England moose could be facing an uncertain future. For now, biologists are monitoring their populations and working with other state officials to develop the best management path forward.

If you want to learn more about moose and ticks, visit the VT Fish & Wildlife Department or check out this episode of Outdoor Radio. If you see moose sign when out exploring backcountry trails this winter, make sure to share your observations with the Vermont Atlas of Life project on iNaturalist.

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Red Crossbill at Marsh-Billings-Rockefeller NHP. © Marv Elliott.
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A spectrogram created from an audio recording by Kent McFarland of two different Red Crossbill Types calling simultaneously, can you spot the difference?

Recording Red Crossbills

By Nathaniel Sharp

Just in case you missed the late breaking news in last month’s Field Guide, one of the bright spots of the winter of 2020 so far has been the appearance of irruptive winter finches in droves across the Northeast and as far south as Florida. One of these finches, the Red Crossbill, has been making an early push into conifer stands across Vermont, with reports piling in on Vermont eBird over the last few months. It’s a storm of crossbills compared to the light drizzle of last year.

Unlike many of the winter finches visiting Vermont right now, Red Crossbills won’t be pigging out on black oil sunflower seed at your backyard bird feeder. Instead, look for Red Crossbills perched high in the tops of conifers or flying in loose flocks overhead. If you’re lucky, you might even get the chance to watch them use their namesake crossed bill to pry apart the scales of hemlock, spruce, pine, and fir cones to access the nutrient-rich seeds within. You may also find Red Crossbills perched alongside or directly on dirt and gravel roads, where they can be seen ‘gritting’ – ingesting small pieces of rock and gravel to aid in the digestion of tough conifer seeds.

One of the most fascinating things about Red Crossbills, besides their bizarre bills, parrot-like behavior, and nomadic tendencies, is their diversity. Red Crossbills occupy a unique rung on the tree of life shared with only a few other finches. The diversity of their food sources, from tough Pitch Pine cones to tiny, delicate Eastern Hemlock cones has created recognizable groupings of Red Crossbills that specialize on a certain type of cones and can be recognized by their unique vocalizations. While this may sound like I’m describing multiple, evolutionarily divergent species, we’re not quite there (yet).

These groupings of Red Crossbills that differ in food source specialization and vocalizations are known as Types. There are currently ten recognized Red Crossbill Types in North America, five of which have been documented in Vermont. Types 1, 2, 3, and 4, known as the Appalachian, Ponderosa Pine, Western Hemlock, and Douglas-fir Types, respectively, are represented by only a handful of records in Vermont, while the Type 10 or Sitka Spruce Type Red Crossbill is by far the most commonly encountered. Interestingly, one former Red Crossbill Type was recently elevated to full species status. The Cassia Crossbill, hailing from the southern mountains of Idaho, represents the first of potentially many ‘splits’ of the Red Crossbill species, perhaps one day in the future there will be a new species of crossbill in Vermont!

If you encounter Red Crossbills this winter, you can make a recording with a handheld recording device (a.k.a. your cell phone) and upload it to Vermont eBird. By listening to your recordings and viewing the spectrogram provided by eBird, you can actually determine what Type of Red Crossbill you recorded. Check out the amazing resources provided by the Finch Research Network for more information on Red Crossbill Types. You can also email the recordings in your Vermont eBird checklist to one of the Red Crossbill researchers with the network for identification to Type, both to help with their research and to satisfy your own curiosity.

9298, , heart leaved birch, , , image/jpeg,, 1536, 2048, Array, Array © K.P. McFarland
Heart-leaved Paper Birch (Betula cordifolia) along the Appalachian Trail in Pomfret, Vermont. © K.P. McFarland
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Paper or White Birch (Betula papyrifera) in the southern Green Mountains, Vermont. © K.P. McFarland

The Paper Birches Among Us

By Kent McFarland

Paper birches are beautiful in winter, but many people don’t realize that two species of paper birch trees grow in northeastern North America – Paper or White Birch (Betula papyrifera) and Heart-leaved Paper Birch (B. cordifolia), once considered a variety of White Birch. As its name suggests, the latter species has distinctive heart-shaped, many-veined leaves, and it is restricted to mid- to high-elevation Appalachian and northern forests.

The primary means of distinguishing Heart-leaved Paper Birch from White Birch include:


We know surprisingly little about the exact range of these two species. How low in elevation does Heart-leaved Paper Birch grow? How high does White Birch climb into the mountains? Do their ranges overlap in some areas? And how will these species respond to climate change, or have they already? Observers adding records to Vermont Atlas of Life on iNaturalist, are helping to map these (Heart-leaved Paper Birch map versus White Birch map) and many other species. We hope you will add your observations too.

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