Bark Eaters

For the indigenous peoples of our region bark was a starvation food, but for many animals, bark is a regular, or at least occasional, component of the winter diet. Most of the thickness of a woody plant stem or branch consists of dead wood and bark cells, but the innermost layer of the bark, located between the wood and the outer bark, is made up of living cells. This stratum, known as the cambium, is responsible for the production of new tissues when growth resumes in the spring. It’s no more than a few cells thick, but for both human and animal consumers these living cells are the only part that provides useful nutrition in the winter.

Bark feeders leave conspicuous evidence of their activity, and this evidence is especially obvious in early spring before it gets obscured by plant growth. With careful observation it is usually possible to pinpoint which species the bark eater belonged to. In the photo below you see the results of cottontail rabbits feeding on downed branches of black cherry. The branches were relatively low to the ground, within reach of a rabbit either standing on the ground or supported by the winter snowpack. The rough appearance of the chews is characteristic of rabbit feeding–the bites penetrate to varying depths, often beyond the cambium into the sapwood. Gouges made by the incisors can be seen in several places.

Rabbit chews on staghorn sumac, shown in the next photo, show similar irregularity in the depth of the debarking. Rabbits seem to take in more woody material than they really need to, perhaps because they can make use of it through coprophagy. Rabbits have an intestinal pouch called a caecum in which partially digested material is fermented by beneficial bacteria. The soft stools produced by this process are passed out of the anus and eaten, allowing the animal to absorb more nutrients before expelling the twice-digested material as fecal pellets.

Chews made by squirrels are usually higher above ground level than rabbit chews and don’t show as much irregularity in depth. The staghorn sumac stems in the next photo were fed on by gray squirrels. Young, vigorously growing stems like these have thin bark and are an attractive meal for a hungry squirrel.

A close-up of one of the stems in the preceding photo is shown below.

The debarked patch on the staghorn sumac branch shown in the next photo was about four feet off the ground. This was an older stem with thicker bark, but gray squirrels had no trouble removing it to get at the cambium.

Deer occasionally feed on bark and leave tooth gouges similar to, but larger than those of squirrels. Their debarking is generally found between two and five feet off the ground. Since deer lack upper incisors, they can only access bark by drawing their lower incisors upward. This leaves thin hanging flaps of tissue like those seen on the staghorn sumac stem in the photo below.

Porcupine chews, like the one on beech in the next photo, have a more organized appearance. They can be anywhere from ground level up, but are often seen high on the trunk or upper branches.

Beaver chews, like rabbit chews, are limited to the reach of an animal standing on the ground or on the snowpack. The tooth marks are organized in regular patterns, sometimes in neat rows like those on red maple in the photo below. The upper incisors made the small digs in the bark and the lower incisors created the long grooves below them.

Our smallest bark feeder is the meadow vole. The buckthorn shown in the photo below was fed on by meadow voles working below the level of the snowpack. Tiny tooth marks can be seen at the edges of some of the dark areas. Meadow voles prefer to stay hidden under the snowpack, so their chews are generally located close to the ground.

In the next photo you see debarking by a pileated woodpecker on a big tooth aspen. Woodpecker feeding is often mistaken for mammal chews, but woodpeckers mine dead trees to get at insects, while rabbits, rodents, and deer chew on living trees to access the cambium. Woodpecker feeding can occur at any height, from downed logs on the ground to trunks and branches high in the canopy. The debarked area is textured by the digs and gouges made by a bird beak rather than mammal teeth.

Deer, meadow voles, and squirrels resort to feeding on bark only when other foods aren’t abundant, but for beavers, porcupines, and rabbits bark is a mainstay of the winter diet. Once the growing season begins the choice of foods becomes more diversified, but in winter and early spring the living cells hidden within the branches and twigs of woody plants can make the difference between life and death.

Looking On The Bright Side

The leaves are down, and the colorful spectacle of autumn is behind us. The forest has gone from a kaleidoscope of color to a narrow spectrum of browns and grays. But wait, what’s that pale streak glinting among the tree trunks? If you look closely you can see it in the center of the featured photo. Moving closer we can see that it’s a buck rub, bright wood laid bare by a hormone-driven male deer. This is rutting season for whitetail deer, and the bucks are roaming the landscape seeking receptive does. They leave their calling cards on living trees–anything from very young saplings to substantial trunks 8 inches or more in diameter. To make a rub the animal lowers its head and rakes its antlers up and down against the stem. Rough areas around the bases of the antlers work like files to abrade the outer bark down to the light colored sapwood.

The photo below is a close-up of the rub in the first photo. Rubs are usually between one and four feet above the ground, and their edges are often rough or stringy. Gouges made by the short tines near the antler bases are often present–look for them just above the debarked area. The brightness of the freshly exposed wood is what attracts our attention, and it may do the same for deer. But buck rubs also carry scent messages, deposited when the animal rubs its forehead against the newly bared surface. We’re not equipped to detect these chemical signals, but to a visiting doe they convey a wealth of information about the age, health, and even individual identity of the rub maker.

The light colored areas in the photo below have also been denuded of bark, but this wood was exposed by feeding rather than by rubbing. A porcupine climbed these yellow birch trees and chewed through the outer bark to get at the cambium, the living cells that produce both bark and wood during the growing season. There’s no mistaking this example for a buck rub, but porcupine chews are sometimes found close enough to the ground to be confusing. In both cases the light wood stands out against the bark, but there are several clues that distinguish rubs from chews.

Instead of a smooth surface, wood that has been exposed by porcupine feeding is textured by tooth grooves, and the margins are more irregular, as in the photo below. The tooth marks are just deep enough to reach the nutritious tissue, and are organized with a neatness that speaks of feeding efficiency. Along the margins of chews there are often tooth marks instead of the stringy fibers that mark the edges of rubs.

Beavers, like porcupines, rely on the cambium of woody plants for much of their winter diet. Being larger than porcupines, beavers’ wider incisors give their chews a more robust appearance. And rather than climb to access food, beavers bring the food down to their level by felling trees. The beaver that felled the log in the photo below stood on its back feet to feed, anchoring its upper incisors in the bark and drawing its lower incisors upward to scrape up the cambium. It moved systematically along the log, leaving the row of shallow upper incisor digs in the bark and the longer lower incisor marks below them. Like the porcupine, the beaver penetrated just deep enough to scrape up the nutritious cambium.

Not all bark chewers show this kind of efficiency. The sumac stem below was chewed by a rabbit, and its ragged appearance contrasts with the more orderly work done by beavers and porcupines. Rabbits only feed on small stems, and their chews show varying depths of penetration with projecting splinters of bark and wood. Like beavers they are limited to what they can reach from the surface they’re standing on, but if there’s a deep snow pack or heavy snow that bends branches down, rabbit chews can be found in some surprising places.

Here’s another kind of feeding that might catch your eye in the autumn woods. Woodpeckers worked on this standing dead tree to get at the insects in the outer layers of wood. The beak strikes left pits, partially lifted slivers, and gouges (best seen on the right edge of the tree). This kind of woodpecker work can be located at any height, and may even be found on downed logs, but unlike the previous examples, it only occurs on dead trees.

Here’s a final example of eye-catching brightness. As the weather gets colder, squirrels leave their leafy tree-top dreys and make nests in hollow trees or other protected places. They gather fibrous bark for nest lining, and in the process, leave freshly debarked wood for us to find. The dead, fallen branch in the photo below was stripped of its fibrous inner bark by a squirrel. Although there’s a vague resemblance to a buck rub, the position of the branch and its non-living status indicate squirrel work rather than deer.

When squirrels harvest fiber from woody plants they may leave another clue. In the photo below you can see the paired marks of a squirrel’s incisors. Much of the bark removal is done by pulling up long strips, but occasionally the squirrel leaves a bite mark as it grasps the bark with its teeth.

Squirrel stripping is also found on living stems–I’ve seen it on honeysuckle and red cedar–and these are more likely to be mistaken for buck rubs. But areas shredded by squirrels are often in places a deer wouldn’t be able to reach, higher on a trunk, within multi-stemmed shrubs, or on stems guarded by projecting branches. Deer prefer sites with straight stems and unobstructed approaches, and any small branches or twigs are usually broken off by the vigorous action of making a rub.

I love this time of year–the leaves are down, and I can see for greater distances through the trees. Many signs of animal activity are hidden by fallen leaves, but others have become more visible. And every once in a while a bright patch shining among the duller tones draws me in and opens up a new and interesting discovery.

Incisive Communication

Communication is an essential part of life for all animals. For squirrels and their kin spring is a time of intensive communication, as they select nest sites and prepare to birth and raise young. And the leafless canopy of early spring affords good visibility, so it’s a great time to spot the messages left by these animals.

The strong, sharp incisors possessed by all rodents–two in the upper jaw and two in the lower–are perfect tools for inscribing messages. In the photo below bright gouges show where a squirrel bit into the sides of a narrow cleft in a tree. The cleft was at eye-level, so the reddish marks of the animal’s incisors were easy to see. After making the bites the animal may have rubbed its cheeks on the bark to leave a scent message. Squirrels have a well developed sense of smell, so the saliva left in the bites and the scent from the cheek rubbing may have been even more meaningful than the visual marks.

Squirrels also create “stripes” on tree trunks. In the next photo gray squirrels–probably more than one–made many bites in an elongated zone between four and six feet off the ground along the trunk of a white oak. If you look carefully you’ll see that the individual bites vary in color, from bright reddish (the most recent) through grayish red to dull gray (the oldest). The varying age of the bites indicates that this stripe has been worked in the same way over several years. The individuals making the bites probably also did some cheek rubbing, so the bark would have been perfumed with an abundance of scents.

Marks like the ones pictured above are usually found in the general vicinity of nesting or feeding areas, but sometimes the bites seem to indicate a claim to a particular nest site. I’m not talking about summer nests, the leafy dreys seen high in the branches of large trees. In areas with cold winters, both winter and birthing nests are located in safe, weather-proof sites like hollow trees or underground cavities, and the supply of good sites may be limited. The hole shown in the photo below was located about 20 feet up in a large tree. There was probably a perfect nest cavity inside–the opening looked well used, and the varying intensity of the bitten areas suggested that the site had been used for at least several years. By marking the opening, the resident squirrel was able to establish ownership of its chosen refuge.

Squirrels are not the only creatures associated with tree holes. The openings shown below were made by pileated woodpeckers. From a distance the bright margin of a woodpecker hole may look like the chewed edges of a squirrel hole, and the size and shape may be about right. But it’s easy to tell that these holes are not squirrel holes. The first clue is their rough, splintery margins. Another clue is the number of holes–in this case there were five similar openings distributed along the trunk. These holes were made for feeding, and they’re distributed up and down the trunk because the insects the woodpecker was seeking–probably carpenter ants–had colonized much of the tree. Woodpecker holes are often more irregular in shape, and when that’s the case it’s easy to tell that they weren’t chewed on by squirrels.

Whether a mark was made by a red or a gray squirrel is often hard to determine–both are known to make stripes along trunks, and both probably mark the entries to nest cavities. Flying squirrels may mark in similar ways, but I haven’t been able to find any mention of that in the tracking literature and I don’t have any examples to share. If anyone knows the location of a definite flying squirrel nest hole, I’d love to hear about it.

Chipmunks tend to use underground refuges rather than tree cavities, and I haven’t seen anything in the literature about chipmunk marking. But a few weeks ago I came across the hole in the photo below, located a little above my head in a small tree. It puzzled me at first because the chews looked rougher and more irregular than the typical squirrel chew. But the mystery was solved when I stood on my tip toes to get a closer look. Out of the hole came the frantic chittering of a chipmunk. The animal sounded so upset that I left quickly, but I thanked it for showing me what a chipmunk can do with its teeth.