Insects employ a variety of strategies for coping with winter. Like the winter stonefly, when the thermometer rises above freezing, winter craneflies and the snow scorpionfly crawl about atop the snow.  Their bodies can absorb the sun’s heat and warm up to a tad above the ambient temperature.

Some insects, like the milkweed bug and potato leafhopper, migrate to balmier climes. Monarchs are famous for their epic journeys – populations from eastern and midwestern North America fly south each fall and roost on trees in the Sierra Madre Mountains of Mexico, Honduras, and Guatemala.

But many insects remain in the North Country and survive by entering a period of diapause – which is like hibernation but more complete. During diapause insects live off fat stored during the growing season and their metabolism drops to one-tenth of normal.

Some insects overwinter beneath the snow or under leaf litter, which provides a measure of protection from extreme weather. Others, however, spend the winter above the snow – in leaves, stems, and galls – and are exposed to the brunt of arctic blasts. The goldenrod gall fly can survive temperatures below -58°F.

How do they endure such extremes? Many overwintering insects — including some butterflies, moths, bees, wasps, ants, beetles, flies and midges — increase their cold tolerance as the temperature drops. Water migrates out of their body’s cells and into the spaces between. When the water surrounding the cells freezes, it is not as likely to burst cell membranes and damage tissues. At the same time, concentrated sugars and alcohols form within the cells which lower their freezing point and keep cellular fluids from icing up. This biological antifreeze enables the overwintering larvae of the Arctic willow gall insect – whose winter weight is 20 percent glycerol – to survive polar air down to -90°F.

Some insects elude freezing by lowering the volume of water within and between their cells. Many avoid the “seeding” of ice by overwintering in a dry site and by expelling the contents of their guts so that no solid particles remain on which ice can crystallize. Their pupal cases and exoskeletons are waterproof.  Using these adaptations, ticks, mites, spiders, and many insects won’t freeze if the temperature stays above -4°F.

Each kind of insect survives winter in a particular life stage. Some species enter diapause as adults, including mourning cloak butterflies, squash bugs, and ladybird beetles or “ladybugs,” which cluster under leaves at the base of a tree or rock. This is also true of bumblebee, yellow jacket, and bald-faced hornet queens, which are the only individuals that live past autumn. Cricket and grasshopper eggs rest quietly in the soil. Walking sticks, most aphids, and eastern tent caterpillars also overwinter as eggs. The pupal stage of the tiger swallowtail makes it through the cold season, as well as those of the noctuids and sphinx moths. Cicadas, dragonflies, and damselflies pass the winter as nymphs.

Fritillary butterflies and Isabella tiger moths winter as larvae; the latter are the “woolly bears” that crawl about each autumn in search of a sheltered site to hibernate, sporting felt-like bands of black on each end and reddish-brown in the middle. These endearing caterpillars curl up for the winter but don’t spin a cocoon and pupate until springtime.

When trying to maintain warmth and activity during the winter there is strength in numbers. Ants, honeybees, and termites overwinter in nests. Honeybees, which evolved in the tropics, must store enough honey to provide the heat and energy needed to see the hive through to springtime. By clustering, a bee colony can maintain 48°F (9°C) at the edge of the mass, and 86-95°F (30-35°C) in the center. Termites and ants cope by moving deeper into the soil as the frost penetrates.

Meanwhile, winter stoneflies and others remain active in the frigid air. These fascinating insects possess a low thermostupor point – the temperature at which an animal becomes immobile. I can relate. After enduring three decades of Vermont winters, I’ve finally adapted: my own thermostupor point has fallen by a few degrees. Unfortunately, my general level of stupor has risen noticeably.

Feathers are arguably the most complex of all vertebrate skin coverings and are an outstanding example of evolutionary innovation. Light, strong, and durable, they perform many different functions for birds and the structures of feathers are diversified to suit their various roles. Feathers are shaped as airfoils for flight, fluffed for insulation, or smoothed and overlapped for weatherproofing. The latter two qualities allow birds to survive extraordinary cold, from the minus 25oF of a New England winter night to the unimaginable minus 75oF endured by emperor penguins in Antarctica.

When you look at a bird, what you see are the feathers known as contour feathers, the outermost layer of the bird’s plumage. Each feather is composed of a hollow shaft from which sprout on either side hundreds of fine, parallel branches, called barbs. Like a fractal pattern, each barb also has side branches, or barbules. Every barbule is equipped with a series of minute hooks along one side, which lock into a groove in the adjoining barbule. When all the barbules are locked together they create a smooth, water repellant surface, known as the feather’s vane. If hooks and grooves get separated they are easily repaired; the bird simply draws the vane through its beak. Countless overlapping contour feathers keep out the wind and weather and give birds their aerodynamic shape.

But there’s more beneath that smooth surface. Many contour feathers have a second feather attached to their base. These are known as semiplumes, and unlike contour feathers, most of their barbs are loose and fluffy and don’t interlock. Only the tip resembles the vane of a contour feather. The job of semiplumes is to plump out the contour feathers and provide insulation, while their small vanes fill any spaces between contours. Semiplumes also clothe the bird’s skin in tight anatomical spaces, such as the bases of wings, because they are highly flexible.

Down, the highly prized fill of quality jackets, forms the insulating undercoat of a bird’s plumage. Down is radically different from other feathers, having only a tiny shaft topped with a head of fluffy, springy barbs that do not zipper together and thus are able to trap a lot of air. Down is particularly well-developed in water birds like ducks and geese that spend a great amount of time with breasts and bellies submerged. We are familiar with the word “eiderdown,” which has come to mean a down comforter, but originated from the down of the seagoing eider duck. Eiders often nest on exposed shorelines. To keep incubating eggs warm, female eiders pluck out their breast down to line the nest.

The intricate organization of feathers continues at the chemical level.
Feathers are built from a specialized protein called beta-keratin. This is a sheet-like structure bonded tightly together by sulphur atoms, which are the source of the unpleasant smell when feathers are burned. The dense framework of beta-keratin makes feathers strong yet springy and accounts for the fact that a down jacket can be stuffed into a pocket, yet puff right up again when unpacked.

On very cold days, birds look puffy and fluffed-up, yet on mild winter days they look quite sleek. Birds can vary the loft of their plumage because their feathers are connected to sheets of muscle within the skin, known as erector muscles. When the weather calls for extra insulation, the erector muscles pull the feathers into a more upright position. This allows the down to expand and trap more air to increase the insulation value of the plumage.

The complexity of feather structure leads one to wonder why and how they would have evolved. The theory was that feathers evolved for flight. However, in 1997, the fossil of a dinosaur bearing primitive tube-like feathers was unearthed in northern China. Subsequent fossils showed a group of related dinosaurs with feathers ranging from down to contour-like, yet their skeletons were completely unsuited to flight. One can only speculate why earthbound dinosaurs had feathers. Was it for camouflage, or insulation? Maybe, like that puffed-up chickadee at the bird feeder, they survived – and thrived – through keeping warm.

According to estimates by the U.S. Department of Agriculture (USDA), there are about five million free-ranging, feral swine living in the U.S. and they’re wreaking havoc on native ecosystems. If you were a ball-shaped bird, all that destruction might be enough to make you want to launch yourself at the swine with a slingshot.

Feral swine come from two sources: pigs escaped from farms and European wild boars escaped from hunting preserves. Since they are escaped captive species and not native here, they’re called “feral,” not wild.

On the lam, even the farm piggies quickly revert to something tusked and hairy. The young are known as stripers, for their striped tan, white, and black fur. These pigs are more Pumbaa than Porky.

Feral swine are rooting up golf courses and farm fields throughout the American South. They’ve overturned tombstones in a state cemetery in Oklahoma. They gobble up the eggs of turtles and ground-nesting birds. They chow down on salamanders and even the occasional deer fawn. And because they are not native to this continent, they have no natural predators here.

The swine are predominantly a southern problem, buy they’re moving north. In 1982, there were established feral pig populations in 17 states; today, they’re found in 38. There are few hundred of them in New Hampshire. Vermont has the occasional feral oinker – one caused an accident on I-91 in 2010 – but so far, they are not known to breed there.

The feral swine in our area have been here for a while. “I’ve been here for 18 years, and feral swine have been here for all that time, and even before,” says Mark Ellingwood, wildlife supervisor with the New Hampshire Fish and Game Department.

Most of the feral swine in New Hampshire are found in Grafton, Sullivan, and Cheshire counties. A large game preserve in the area is a suspected source, although Ellingwood reports that the feral swine are now breeding outside of the preserve’s fence, which means that the population is now increasing by the litter (three to 13 piglets), instead of by the occasional adult escapee

“We’ve had animals as far north as Littleton, and in southern New Hampshire as well,” says Ellingwood. “It’s a stretch to claim we know the source of all these animals.”

“Years ago I used to think that the climate would preclude them from surviving in the wild here,” says Scott Darling, wildlife management program director for the Vermont Fish and Wildlife Department. But recently, he notes, breeding feral swine were reported in upstate New York. There are populations in other northern states and in Canada.

Darling says Vermont is vulnerable both to feral swine crossing from New Hampshire, and from the state’s own game preserves, some of which stock wild European boar.

Parker Hall, the state director for New Hampshire and Vermont for the USDA’s Animal and Plant Inspection Service (APHIS), says there is no reason to wait around to see what damage feral swine will do in New Hampshire. They already have a record of destroying cornfields and rooting up people’s backyards.

While his voice drips of Tupelo honey (his previous posting included Alabama and Georgia), his message is pure Dirty Harry: “We know they are bad,” Hall says. “The damage that they do has been documented for years.” The solution, he suggests, is to trap and kill them.

Hall says the state listed Blanding’s turtles, marble salamanders, and spotted turtles are in the feral swine’s crosshairs. Without quick action, Hall explains, the feral swine may eat the last of these vulnerable creatures in the state.

Luckily, he says, years of experience in other parts of the country can be applied to feral swine in the Northeast. Creating a hunting season for them is one idea. However, Hall says hunters can only reduce feral swine populations by 20 percent – too few to make an impact on the population.

The trick, he says, is to trap them in a fenced enclosure using sour corn as bait. The trapping is done in the winter, when the feral swine aren’t chowing down on their very favorite food, acorns.

Who knows, he might be able to teach those angry birds a thing or two.

Like so many cycles in the natural world, patterns of winter raptor invasions are driven by the same things that drive much of our daily lives: sex and food. Okay, mostly food, but reproductive success plays a role for some species. There are five species of arctic and boreal raptors that periodically “irrupt” south into the northern United States: rough-legged hawk, snowy owl, great grey owl, northern hawk owl, and boreal owl. As predators, all of these species are dependent, to varying degrees, upon small mammals (particularly voles and lemmings) as a critical food source. The largest and most powerful of these birds is the snowy owl, a generalist predator of the arctic tundra capable of taking advantage of a wide variety of prey, ranging from small mammals to arctic ground squirrels, hares, ptarmigan, and other birds. At the other end of the spectrum is the tiny boreal owl, which, as its name implies, occupies the boreal forest zone and specializes in preying on small mammals, primarily the red-backed vole.

Across the North American tundra (generally north of 60 degrees north latitude), arctic raptor populations ebb and flow along with the abundance of lemmings, which fluctuate synchronously on a regular cycle. When lemming numbers are high, raptors such as the snowy owl have more prey to feed their young, resulting in years of high reproductive success. For this generalist predator, southward irruptions often follow these years of high nesting success, not because food in the arctic is scarce, but because snowy owl populations (and those of other predators that feed on lemmings) are extremely high. For snowy owls, the result is that many young owls migrate south into southern Canada and the northern United States, thereby avoiding competition with the more aggressive adult birds, most of which remain in the Arctic.

Among specialist owls, however, southward irruptions are primarily linked to years of low small-mammal populations, particularly the red-backed vole, which undergoes a three- to five-year population cycle. Recent work done by Marianne Cheveau of the University of Quebec, along with several colleagues, demonstrated that large numbers of boreal owls move south every four years when red-backed vole densities are at their lowest. The other two owl species (northern hawk owl and great grey owl) show the same four-year irruptive cycle, but they are less pronounced because both species are larger and occupy a variety of habitats, allowing them to prey on a broader range of species.

The most reliable winter raptor visitor from the north is the rough-legged hawk. A nomadic species that nests in the arctic tundra of Alaska and Canada, rough-legged hawks are migratory, moving south each year to winter in open habitats in southern Canada and the northern United States. It is unclear why, as a migrant to our region every year, this species occasionally exhibits “irruption” years. Intuitively, you would expect these invasions to correspond to years of high reproductive success, like the snowy owl, when there are simply more birds around. But the pattern appears to be more complex, fluctuating regionally across the wintering range, with “invasions” occurring in areas where small rodent prey are abundant. This has led some to theorize that rough-leggeds, like European kestrels (and probably many other hawks), can visually detect areas where rodents are abundant by seeing their scent markings, which are visible in the ultraviolet spectrum that many birds are sensitive to.

The best places to look for these wintering birds of prey are areas of extensive open habitats, reminiscent of their arctic tundra breeding grounds. In Vermont, the fertile agricultural lands of the Champlain Valley, especially around Dead Creek Wildlife Management Area in Addison, provide the most consistent sightings of rough-legged hawks and snowy owls, while in New Hampshire, open marshes and beaches along the seacoast produce reliable results. But, keep in mind that during invasion years, sightings can and do occur almost anywhere across the twin states, and Boston’s Logan Airport is a well-known hotbed for wintering raptors from the far north.

But the price of this peace of mind turns out to be high when everything is counted. Rusted out cars, corroded bridges, weakened parking garages, contaminated wells and streams, and a lot of sick roadside vegetation are among the many costs of a safer ride.

The best news on the road salt front in this area is the increasing use of salt brine instead of dry salt. Gil Newbury of the Vermont Agency of Transportation compares it to the non-stick stuff you spray on a frying pan. Applied just before a storm begins, it keeps snow from sticking to the road, making it easy for plows to wing it off the roadbed. It’s estimated that up to one-third of the dry salt that leaves a spreader bounces right into the roadside ditch before doing any melting at all, whereas brine stays put. Like any salt, it becomes less effective as the temperature drops, but the addition of molasses – yes, molasses – makes the brine work when it’s nearly -4°F, while salt alone loses almost all of its melting power at around 15°F.

Fifty or so years of enthusiastic salt use has taught us a lot, and vehicles and highways are now built to resist salt, at least to some extent. Cars are made of more rust-resistant materials, reinforcing in concrete bridge decks is epoxy coated, and there are better paints for steel bridges. Though woody roadside vegetation cannot be modified so simply, it turns out that some species have considerably more resistance to salt than others. (Black ash, cottonwood, tamarack, northern red oak, balsam poplar, gray dogwood, staghorn sumac, choke cherry, and serviceberry are all highly tolerant.)

Unfortunately, this list does not include many of our most common trees; sugar maple, beech, and white pine are among the worst choices for salty roadsides. And, alas, some of those aggressive non-native species that we love to hate do rather well in brine: non-native honeysuckles, autumn olive, common buckthorn, and Norway maple.

Salt enters trees and shrubs either by being splashed onto the above-ground parts or by way of the roots. Curiously, plants that resist salt at one entry point may be highly vulnerable at the other. Northern red oak is a champ at keeping soil-borne salt out, but the leaves of seaside oaks splashed with salt spray by hurricanes turn brown within a few days.

In solution, salt separates into sodium and chloride ions and it’s the chloride ones that cause more damage to plants at the cellular level. Salt tolerance is based more on a plant’s ability to exclude chloride than on any intrinsic ability to tolerate it. Severely damaged plants may have chloride levels of 1.25 to 2 percent. Tolerant species growing under the same conditions will have levels of 0.5 to 0.9 percent.

Though sodium ions enter plant tissues more slowly, they are the major culprits in soil compaction – yet another method that salt uses to kill vegetation. Sodium ions change the way that soil particles aggregate, leading to a loss of normal spatial distribution. Severely compacted soil restricts access to water and oxygen, and the symptoms resemble those caused by drought. Plus, sodium travels along the same shuttle system as potassium and magnesium – both essential to making chlorophyll – and if excess sodium messes up the system, potassium deficiency results.

The amount of salt used to melt last winter’s near-record 122.8 inches of snow on state roads in Vermont was 37 percent less than in the similarly snowy winter of 2000-2001. Plus, last year, the state used almost no sand, which meant almost no hydrocarbon-soaked sand clogs in our stream beds. New Hampshire, too, is dissolving salt before applying it to the road. From a tree’s perspective, this trend is overwhelmingly positive.

Road salt in some form is here to stay, at least until global warming takes a giant leap forward. There’s no future in asking drivers to slow down, stay home, or risk sliding into each other just in order to keep white pines from turning brown. Although the harm to the environment does keep building decade after decade, we’re pretty much stuck with trying to slow the rate of increase in the damage inflicted.

To escape an icy demise, insects in northern latitudes employ many tactics for winter survival, such as overwintering as freeze-resistant eggs, or fortifying their bodies with natural antifreezes and hiding in protected crevices. 

Not so the honeybee, a familiar, non-native insect that made its way to the Americas via settlers in 1622. Honeybees are native to Africa, and adhering to their warm-latitude origins, remain active all winter. Individually, they’d stand no chance against months of subfreezing weather, but as a collective, they’ve developed several extraordinary ways to survive in cold northern climes. 

In the Northeast, a honeybee’s preparation for winter begins with the first flowers of spring, but this process accelerates as summer wanes. The collection of fall nectar, most notably from goldenrods and asters, can be essential to the colony’s winter survival.  The average hive may need 60 or more pounds of honey to get through a northern winter.

Another crucial late-summer process is the production of winter bees, bees with a different blood protein profile and greater body fat than their summer bee counterparts. The winter bees get the colony through winter, while the summer bees, the ones that so diligently foraged for nectar and nursed the developing winter bees, die off in the fall. Beginning in September, the colony also rids itself of drones, male bees whose only role is to mate with new queen bees. All drones are forced out to starve. Lastly, the colony stops producing new bees, as eggs and larvae require temperatures of 90oF to 96oF to develop.

As flowers disappear and temperatures fall, bees stop foraging and remain in the hive. When the mercury falls below 64oF, bees begin a behavior known as clustering, where they gather together to form a ball that extends through several honeycombs in a typical hive. The cluster consists of an outer mantle of tightly-packed bees which surrounds an inner core of bees that are more loosely-packed and free to move about. The all-important queen bee is sequestered at the center.

As the temperature drops, the cluster becomes tighter and tighter, shrinking to one-fifth of its original size. At their most tightly-packed, the mantle bees form a layer that approaches fur in its insulating qualities. Bees become comatose if they chill below 43oF; thus, when their thorax temperature falls to about 54oF, the mantle bees exchange places with bees from the core. Mantle bees that get too chilled to return to the core drop off and die.

Once the temperature in the hive falls below 50oF, the cluster can only maintain its life-supporting interior temperature of 64 oF (at the periphery) to 90oF (center) by actively producing heat. Like mammals, the core bees begin to shiver by pumping their large flight muscles. This is why honey stores are critical. The bees eat honey and transform it into heat through metabolism. After they empty one honeycomb, the cluster slowly shifts sideways and upwards towards a full one.

This survival mode, however, has some inherent hazards. If there is not enough honey, or if it gets so cold that the bees cannot move to full honeycombs, the colony starves.

Eating honey, especially if it is high in indigestible material, leads to a need to void feces. Usually, this is taken care of when winter days warm above 50oF, allowing bees to leave the hive for cleansing flights during which they dot the snow with tiny yellow splashes. If prolonged cold stops bees from leaving the hive, they eventually defecate inside, and if enough bees do this, it leads to the death of the entire colony.

To make matters worse, metabolism produces water vapor as a byproduct, and this must be allowed to escape from the hive. Savvy beekeepers drill a hole near the top of the hive for this purpose. Otherwise, the vapor condenses and ice-cold water drips onto the bees, causing them to freeze to death.  In the particularly bad and long winter of 2000-2001, many beekeepers in the northeast reported losses of 50 percent of their colonies. Some suffered losses of 90 percent.

One of nature’s marvels is the temperature achieved within a mere ball of bees in subzero weather. So take off your winter hat and put your ear to the thin wood of a hive; you’ll hear the humming of bees by the thousand. The sound is enough to transport you to summer meadows, even in the cold, dark depths of winter.

When Europeans first arrived in the New World, mourning doves probably existed only in scattered locations throughout North America. But that would change. As the settlers modified the land to suit their needs, they ended up suiting the mourning doves’ needs as well. Both humans and doves like open and semi-open habitats: neighborhoods, parks, open woods, grasslands, and farms.

Today, the mourning dove holds the distinction of being the only native North American bird to breed in every state, including Hawaii. Their U.S. population is estimated at more than 400 million. Despite their numbers, their lives tend to be short and difficult. In any given year, more than half of the adults and two thirds of first-year birds will die. Nationwide, hunters take more than two million birds annually, though the mourning dove is not a legal game bird in Vermont or New Hampshire. Around here, predators and bad weather are the limiting factors.

While observing the birds, it is possible to tell the difference between males and females, although the difference is subtle. Males are a little larger, their breasts are rosier, and their heads are a more iridescent and brighter blue-gray. If you’re watching a nest, note that males do most of the incubating from mid-morning to mid-afternoon, while females typically take to the nest in the early morning, evening, and night.

As is the case with most members of the dove family, females lay two eggs. Both male and female provide their hatchlings crop milk, a rich mixture of cells sloughed off from the crop wall. Crop milk is the consistency of cottage cheese, and is extremely nutritious, having more protein and fat than mammalian milk. On crop milk, the young grow quickly fledging in about 14 days. But what may be more interesting than what is fed to hatchlings is how dove hatchlings eat. Instead of mom and dad placing food into the hatchlings’ gaping mouths, the opposite happens. Parents open their beaks, and babies stick their heads into the open mouths to consume food right from the parent’s crop. Young doves feed this way on both crop milk and seed. In the Northeast, mourning doves may raise up to three broods a year, although two is more common.

While mourning doves are common at the bird feeder all year round, the doves you see in winter are not the same as the ones you see in summer. Mourning dove’s migration is a complicated affair called “differential” migration and is related to a bird’s age and sex. They begin to move south to the mid-Atlantic and southern states in late August and early September. The young leave first, then the females, and finally the males. Some birds, most of them males, don’t migrate at all but remain in the north. If you look closely at the mourning doves at your winter feeders, you will find that they are predominantly males. It’s worth it to these males to brave bad weather and frostbitten toes to get a head start on establishing a good breeding territory early in the spring.

If you’ve ever startled a mourning dove, you undoubtedly caused it to blast off into the air from its perch, making a whistling sound as it goes. This high-pitched whistle – sometimes called a whinny – does not emanate from the bird’s syrinx; rather, the high-pitched noise comes from the bird’s powerful wings. It is believed that the whistling is a built-in alarm system, warning others that danger may be near, while simultaneously startling a would-be predator (and giving the dove the precious seconds it needs to make its escape).

The more I learn and the more I look, the more I see that the common mourning dove is not so common at all. This winter I’ll be watching them very closely; there may yet be more secrets to learn.

Ask any young child to draw a Christmas tree and chances are he’ll draw something close to a triangle. If the kid is particularly talented, she’ll draw a cone. Indeed, the cone-shaped tree is as traditional as the holiday itself. Sure, there are Charlie Browns among us who will settle for a less-than-perfect Christmas tree. But most of us look for a fir or spruce with just the right taper, symmetry, and conical form.

That conical shape is certainly the norm at most Christmas tree farms, and the short explanation for it is that the tree farmer shears the trees to look that way. But even if you’re wandering afield in search of a wild Christmas tree, far from any shears or knives, you’ll still find plenty of classic cone-shaped specimens out there. In winter, these conical evergreens stand out against the more rounded and leafless birches, beech, and maple.

Why is your Christmas tree conical (though your spouse might say comical)? Like so many of nature’s designs, trees – including spruce and fir – take shape according to a system. Their branches don’t grow randomly in various directions. Rather, there’s an intriguing pattern to this story. It’s a story of dominance and control – not exactly traditional holiday themes, I’ll grant you, but a good story nevertheless.

Look at your Christmas tree. At its top is one vertical shoot that should be longer than all the other shoots around it. This shoot is called the leader, and it exerts control over all the shoots below it. This mechanism has evolved, presumably, to ensure that the tree’s main thrust is upward toward its source of energy, the sun. That it makes an excellent stem upon which to place a star is just a bonus.

Almost all trees begin their life this way. However, while a cone shape is retained in conifers, it is soon lost in most hardwoods as the tree grows and the single, central stem gives way to lateral branches that often grow at least as fast as the leader.

In spruce and fir, the leader outgrows the lateral branches below it, resulting in a conical and a well-defined central trunk. The leader exercises its dominance with help from a hormone called auxin in the top bud, which inhibits the elongation of the shoots below it. They still grow, but not as fast as the leader. The pattern goes like this: in any given year, increases in length are greatest at the top of the tree, and these increases diminish downward and inward toward the bottom. The reason the lower branches are longer is that they are older. They’ve had more years to add growth. But in any given year, they don’t grow as much as the upper branches. Thus, the tree takes a conical form and maintains it indefinitely.

Or at least until the leader’s dominance is interrupted.

This occurs, for example, if an insect destroys the bud at the top of the leader. In such cases, the lateral shoots just below the leader – newly freed from its control – resume growth at their full potential, vying to outpace each other in a race for dominance. The fastest growing of them becomes the new leader, and dispatches its hormones to suppress the others. The result is a crooked stem, as seen in so many white pines whose leaders have been killed by the white pine weevil.

There is an interesting irony to this story of conics, though. Although the dominant-leader phenomenon results in the conical shape sought after in Christmas trees, Christmas tree growers actually try to overcome it by shearing. Without shearing, the tree does take on a nice conical shape, but an unsheared tree is very open. That is, it tends to be airy and not very dense. Shearing, on the other hand, stimulates the opening of many buds lying dormant along the branches, and encourages the shoots within them to elongate. This results in a fuller, denser tree.

May all your Christmas trees be just the way you want them.

These northern visitors make cold-weather birding interesting, but one bird – the northern shrike – stands out because of its unusual hunting habits. The bird’s Latin name, Lanius excubitor (“watchful butcher”), gives some clue as to what comes next, but we’ll get to the gory details in a moment.

The shrike is an attractive bird, with grey on its head and back, a white chest and throat, black patches on its wings and tail, and a black mask-like band across its eyes. From a distance, you might mistake it for a blue jay.

But what makes the shrike notable is the fact that it’s a predatory songbird. Like hawks and owls, it hunts and kills for a living. But unlike most hawks and owls, the shrike is small – about the size of a robin – and unlike the raptors, it kills not with its talons, but with its sharp, notched beak.

The shrike hunts by perching atop a tall shrub or tree at the edge of a field where it surveys the surrounding area for songbirds, insects, and small mammals. From its high perch, it watches and listens for movement. When it sees or hears prey, it swoops down and uses its strong, hooked bill to dispatch its meal.

Chip Darmstadt, director of the North Branch Nature Center in Montpelier, Vermont, notes that the shrike’s bill is adapted to kill quickly and precisely. It is equipped with a tomial tooth – a small triangular projection on the beak’s upper mandible that helps shrikes surgically sever the prey animal’s spinal cord with a single well-aimed bite. The only other birds whose beaks feature a tomial tooth are falcons. This time of year you might watch a shrike swoop down from a treetop perch, land in the snow, and with a single movement of its head, grab and kill a hidden vole.

The northern shrike probably evolved this hunting technique on its far-northern breeding ground. It nests and breeds near the Arctic, on the natural edge between boreal forest and open tundra. There too, it habitually perches high atop a prominent tree or bush, scanning the tundra for prey. Its nest is a bulky cup of twigs and roots, woven together with feathers and hair. And it is so deep that when the female shrike is incubating, she is usually completely out of sight except for the tip of her long tail.

Northern shrikes migrate south in the winter to the northern U. S. and southern Canada. In some winters, they are more plentiful here than others, possibly because of fluctuations in the number of voles and small birds wintering on their home range, year-to-year.

The bird’s folk name is the “butcher bird,” and we don’t know which came first, the folk name or the butcher reference in the Latin name. But both names reflect the fact that if prey is available, the shrike will kill more than it can immediately use. It stores the extra meat, often skewering the tiny corpse so that it hangs, butcher fashion, from a thorn or a fence wire. Darmstadt has watched a northern shrike tear a mouse into chunks of meat and systematically hide them in the crotches of a small tree.

Earlier observers, noting that the shrike sometimes kills more than it immediately needs, termed the bird “bloodthirsty.” But the behavior is actually a form of caching, a precaution against times of lean hunting. The skewering serves a practical purpose, as well. A shrike’s small feet don’t allow it to properly grip large prey while it feeds, and so it impales its food to hold it in place while eating.

In all its actions, the shrike is a charismatic bird that exudes a fierce energy, even when perched watchfully atop a tree or bush at the edge of a clearing. Hunting, they often pursue smaller birds closely and intently, twisting and turning through the air like a fighter pilot.

Shrikes have what scientists call “site fidelity” – that is, they may regularly return to hunt from the same location, even the same field or treetop, winter after winter. Consequently, birders hoping to see one should regularly check out known shrike haunts and likely-looking open fields, edged by trees. Habitually scanning treetops, utility wires, and other high perches on the edge of pastures, marshes, or other wild clearings can be fruitful. Watch for a flash of white patches on gray wings as the shrike shoots across a clearing, then dramatically stalls and swoops up, landing lightly on its high perch. Once seen, the bird’s grace and power are unmistakable.

Tree cavities provide essential habitat for a variety of wildlife, from birds and mammals to insects, reptiles, and even amphibians.There’s good reason for this, of course. Cavities provide excellent protection from harsh weather and temperature extremes, they are relatively safe from predators, and they can be used for multiple years with little or no maintenance.As you might imagine, competition among the dozens of species that use tree cavities can be quite fierce, with more aggressive species often evicting the smaller or more timid ones, while occasionally two species will share a single cavity.

Aside from the day-to-day drama of eviction notices and cohabitation, cavity users fall into one of two basic groups: builders or squatters.Most of the builders, known as primary cavity nesters, are woodpeckers,while chickadees, titmice, and nuthatches can only excavate cavities in wood that has been softened by decay.The squatters, on the other hand (also called secondary cavity users), include a variety of other birds, small- to medium-size mammals, some snakes, tree frogs, and insects, all of which depend on cavities but are incapable of excavating them.Primary cavity nesters provide a critical function for secondary cavity users.Moreover, studies have shown that there is a significant relationship between the abundance of primary and secondary cavity-nesting birds, such that declines in the former lead to declines in the latter.

In Vermont and New Hampshire, we may be seeing an example of this relationship among three cavity nesting birds associated with open woodlands, forest edges, old fields, and agricultural areas.The American kestrel, eastern screech owl, and northern flicker are all showing long-term population declines.While there may be many factors contributing to this, several studies indicate that the flicker is the primary cavity excavator for both kestrel and screech owl, so that a decline in flickers could limit nest site availability for the two birds of prey.

Kathy Martin, a professor of forest ecology at the University of British Columbia in Canada, has been studying the ecology of cavity-nesting communities for many years.Along with her graduate students, Dr. Martin has found that both the northern flicker and pileated woodpecker are keystone species in forest communities – species that have a much greater impact on their community or ecosystem than would be expected based on their relative abundance.In the case of these two woodpeckers, it’s because they provide critical nesting and roosting habitat for such a wide range of species.Although there are nine woodpecker species that excavate cavities in British Columbia forests, most of the 32 species of secondary cavity users utilize holes made by northern flickers.And while pileated woodpeckers are much less abundant than flickers, they add to the cavity-nesting community by creating large, durable cavities that provide breeding and roosting sites for cavity-nesting ducks, raptors, and many mammals.In fact, both Barrow’s and common goldeneye ducks only use cavities excavated by pileated woodpeckers or those that occurred naturally.

Martin’s studies also revealed that excavators prefer quaking aspen over all other trees because of its susceptibility to heartwood rot, which provides a soft interior that is easily excavated, while the outer sapwood remains solid.In fact, within her study sites, aspen is considered to be a keystone species as well, because without it the cavity-dwelling community could fall apart.This has major implications for forest management in British Columbia.

In the Northeast, where forest communities have a greater diversity of tree species that share the cavity-friendly characteristics of aspen, including basswood and red maple, there is less reliance on a single tree species to provide the majority of cavities.So the next time you’re out for a walk in the woods, consider grabbing a walking stick and knocking on a few “doors.”You never know who might be home.