Field Guide to December 2021

Hibernation Station

By Julia Pupko

As the year’s first snowfalls dust Vermont’s woods and fields, many animals are already tucked away, hibernating until warmer weather arrives. Although hibernation is a term we all use, it looks surprisingly different depending on the species in question. In general, hibernation is a loose term, used to define a period of dormancy where an animal’s metabolic, breath, and heart rates are reduced, allowing it to save energy when environmental conditions are unfavorable. Methods of hibernation vary, but many species that remain in Vermont year-round use some form of it. Here is a crash course on types of hibernation, and how they differ.

Mammals:
Hibernating mammals either enter into true hibernation or “shallow” hibernation (also called dormancy). In Vermont, true hibernation is only employed by Groundhogs (Marmota monax), jumping mice (Family Zapodidae), and certain bats (Order Chiroptera). For these species, their body temperatures drop to roughly that of the ambient air temperature, heart rate and breathing nearly stop, and internal organs become nearly inactive. Although true hibernators do not react to outside stimuli, moving them does trigger waking mechanisms, which can be harmful.

Shallow hibernators, such as Black Bears (Ursus americanus) and Chipmunks (Tamias striatus), still experience a drop in body temperature, heart and breath rate, and metabolic function, along with long periods of inactivity. However, these species still move around a bit and may partially or fully wake up before re-entering dormancy. The extent to which their bodily functions are reduced is also nowhere near that experienced by species who enter true hibernation.

Reptiles and Amphibians:
While hibernation may be a familiar phrase, chances are you will not have heard of “brumation”. That is, unless you are a herpetologist. Brumation is the term used to describe the hibernation of cold-blooded species. During brumation, metabolic, heart, and breath rate are all reduced or cease entirely. For example, the Wood Frog actually freezes during the winter—no breathing, no heartbeat, no metabolic activity—with up to 65 percent of the water in its body turned to ice. Reptiles and amphibians essentially enter a state of suspended animation, but many are able to move around on warm winter days if they are heated enough.

This winter, keep an eye out for sightings or signs of different hibernators and be sure to share any observations with the Vermont Atlas of Life on iNaturalist!

The Marten and the Fisher

By Julia Pupko

On chilly afternoon walks in the forest, you will likely notice small dramas written in snow by two- and four-legged creatures. If you are lucky, you may even come across the loping trail of a marten or Fisher. American Martens (Martes americana) and Fishers (Pekania pennanti) are small mammals in the Mustelid family, making them related to weasels and minks.

Once numerous across the state, American Martens experienced a sharp decline in Vermont following colonization and wide-spread deforestation. By the early 1900s, they were considered extirpated from the state. Starting in 1989, American Martens were reintroduced to Vermont and considered to be re-established (yet still endangered). Meanwhile, Fisher populations were also decimated following colonization but have been reintroduced with greater success than American Martens in many regions. Currently, Fishers’ populations in Vermont are much higher than those of the American Marten. Both species prefer older coniferous or mixed forests, especially those with ample downed woody debris and tip-up mounds for nesting and foraging.

In addition to preferring similar habitat, American Martens and Fishers also hunt similar prey, namely small mammals. In winter, resources are limited, and species that already have overlapping niches must figure out new ways to coexist. It appears that differential use of snow features may be a possible method for reducing competition between the two species. American Martens hunt in the subnivean zone, while Fishers do not seem to utilize it. Instead, Fishers hunt for prey on the snow’s surface, diving in only to extract an unlucky critter. While American Martens remain relatively unaffected by deeper snow, some research has indicated that Fishers prefer sites with less snow accumulation.

Aside from differential use of snow cover, it is likely that American Martens and Fishers continue their ecological overlap through the winter months. It is possible that increased homogenization of habitats following colonial clearing and reforestation increased overlap between the two species, and that Fishers have reduced the success of American Marten reintroductions in some regions. Keep your eyes peeled when you are exploring old growth forests, and you may find tracks of both species this winter. If you do, make sure to share your sightings!

The Best Nest

By Julia Pupko
Although we may quickly tire of Stick Season’s greys and browns, this time of year does have its benefits—little jewels that are typically hidden by leaves, like nests, become exposed. Nests can be located anywhere from ground to treetops, depending on the species and surrounding habitat. When looking for these hidden treasures, make sure to search bushes, treetops, and cavities. Here are a few nests to keep a lookout for near or around your house.

Gray Squirrel (Sciurus carolinensis):
Gray Squirrels will often build their nests either in cavities or among tree branches. Nests built in branches are typically around 20 feet off the ground, 12 to 19 inches in diameter, ball-shaped, and composed of leaves, bark, and twigs. You will likely notice them situated in the fork of a large branch to increase nest stability.

House Wren (Troglodytes aedon):
These little cavity nesters are known for constructing their nests on or near houses. For the best chance of finding them, make sure to check cavities around your property—in living or dead trees, on or around your house, and even in old boots that may have been left outside for a long time! Within the cavity, you will see a pile of twigs with a cup in the center. The cup is usually lined with a small amount of grasses, feathers, hair, plant material, plastics, string, snakeskin, spider egg sacs, or other soft material.

Eastern Phoebe (Sayornis phoebe):
Eastern Phoebes either nest on cliffs or directly on the sides of buildings. They typically seek out a sheltered location, such as under the eaves, and are often less than 15 feet off the ground. Nests are cup-shaped and can consist of mud, moss, leaves, grasses, and hair.

Don’t forget to upload photos of nests that you find to iNaturalist! If possible, photograph the nests from different angles and make notes about the plant species or feature it was found on.

Winter Wrens

By Julia Pupko

Due to climate change, migration patterns have been shifting and many species (especially short-distance migrants) have been leaving late, or not at all. Take some of our wren species for example. Winter Wrens (Troglodytes hiemalis) are late migrants to begin with and usually head south in October. However, in years with milder autumns, they can migrate even later. Many birders have found Winter Wrens in low numbers in New York and Massachusetts, just south of us, year round. Several of the most recent eBird sightings of a Winter Wren were on November 29th!

Likewise, warming temperatures have allowed southern species to increase their ranges northwards. Take Carolina Wrens (Thryothorus ludovicianus), who remain in their territory year-round. Carolina Wrens were not always found in Vermont; harsh winters in the past would have killed any individuals that ventured too far north. However, milder winters in recent years, combined with more people providing food (bird feeders), have proved a game-changer for this tiny bird. Just 10 years ago, only eight percent of people in Massachusetts reported seeing Carolina Wrens at their feeders during the winter. This number has now increased to 73 percent.

As the climate continues to warm and weather patterns become more erratic, chances are we will begin noticing more newcomers and variations in migration patterns. You can help scientists better understand how climate change is affecting birds by keeping track of the species you see in eBird!

Why Do Some Trees Keep Their Leaves In Winter?

By Jason Loomis

Oak (Quercus), Beech (Fagus), and Hornbeam (Ostrya) trees keep their leaves in winter. While these trees technically meet the definition of deciduous—with leaves that fall off seasonally—they are unlike most deciduous trees. This has earned them a mouthful of a moniker: everciduous. Their leaves remain attached well into winter, when they are mechanically removed by wind, or remain until spring, when they complete the process of abscission or programed leaf senescence.

To better understand why some deciduous trees retain their leaves, it is useful to understand the order in which trees evolved. Coniferous trees evolved first, with deciduous trees following, likely in response to changing climactic conditions. The large surface area of deciduous leaves increases photosynthetic capacity, at the expense of increased vulnerability to winter damage. Leaf abscission was the solution, conserving energy and preventing damage as snow begins to accumulate.

If senescing seemingly-useless parts in winter is an advantage, then why do these trees keep them? Botanists hypothesize that Oak, Beech, and Hornbeam are less evolved than their fully-deciduous brethren—specifically, their abscission feature is half-baked. Nonetheless, to survive despite this juvenile evolutionary trait, it must also confer some advantage. Because these trees persist today, they must out-compete their rivals under some conditions.

One of the proposed benefits of dead leaf retention over the course of the winter for Oak and Beech species relates to their predominant habitat: they are often found in poor, dry soils. By waiting until spring to drop dead leaves, they delay leaf composting until their awakening buds demand nutrients from soil-bound roots. This shift in the timing of nutrient supply could explain how this behavior confers a survival advantage by providing better soil conditions during the growing season.

Another idea posits that leaves which remain attached at the base of the tree in winter help the tree accumulate a deeper snowpack, providing a bolus of water from snowmelt to start the growing season. Yet another says that leaves protect new buds from being eaten by browsing fauna during winter.

It is fun to imagine how this curious evolutionary feature might work in the favor of everciduous trees, and nice to be reminded of it when, as Stick Season deepens and snow stays on the ground in early winter, we encounter them on walks in the woods. It is comforting to know that diversity engenders survival for ancient evergreen, youthful deciduous, and everciduous trees!