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Beavers

The Surprisingly Varied Diet of Beavers

It is common knowledge that beavers are herbivorous but the extent of their herbivory is not always appreciated.  Examining their skull would tell you that their massive four front incisors (as well as the muscles attached to their jaws) are designed to do some serious chewing.  And serious chewing does take place, especially in the fall. Poplars, birches, alders, willows, maples and many other deciduous trees as well as a few conifers are felled in order to reach and consume the inner bark, or cambium layer. (The de-barked logs and branches are subsequently used to repair dams and lodges). Not only do Beavers need to meet their daily nutritional needs but they must cut enough trees to last them through the winter.

However as spring approaches and they can access land, their diet changes from the woody branches they’ve been eating all winter (from their winter food pile under the water) to a diet that consists mainly of herbaceous plants. Ninety percent of their time is spent eating non-woody plants, often skunk cabbage, water lily rhizomes and grasses in the early spring.  As summer progresses, they seek out aquatic plants, ferns, sedges and a variety of flowering plants. Usually it’s not until late summer/early fall that their incisors are once again given a good workout.

The pictured Beaver had the good fortune of having a large patch of tasty Jewelweed (Impatiens capensis) growing on and adjacent to its lodge.

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Wood Ducks Returning To Northern New England

A welcome sign of spring in northern New England is the return of the Wood Duck to wooded swamps and wetlands.  There are seven species of North American ducks that regularly nest in cavities and the Wood Duck is uniquely adapted for doing so.  Its slim body allows it to use Pileated Woodpecker cavities for nesting and the acuity of its large eyes allows it to avoid branches while flying through wooded areas.  Even so, it still tends to make one look twice to see ducks perched up in a tree!

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Beavers Released

Most beavers in northern New England have been trapped inside their lodge for several months, with their only food supply being the branches they cut last fall and piled up on the floor of the pond or river near their lodge. Ice has sealed them in, not only to a life of darkness and dampness, but to a diet of cambium. 

Needless to say, they are quick to take advantage of melting ice that allows them to exit their lodge and make their way to shore to sample fresh food.  One of the first delicacies they dine on, if it’s available, is skunk cabbage.  The rhizomes, leaves and flowers of both yellow and white pond lilies are also favorites.  No longer restricted to woody plants, beavers head for grasses, sedges, ferns, fungi, berries, mushrooms, duckweed and even algae as the water warms.  Their palate must jump for joy with the melting of ice in early March.

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North American River Otters Use Abandoned Beaver Lodges As Dens

North American River Otters use dens (called holts) for giving birth and for shelter from weather extremes.  Den sites are usually close to the water line of rivers and lakes, and have multiple entrances underwater as well as on dry land.  They are often excavated under trees or rocks or in river banks, but otters also use abandoned muskrat burrows and beaver lodges as shelters (cohabitation with beavers has also been documented – see https://naturallycuriouswithmaryholland.wordpress.com/2013/02/22/beaver-otter-cohabitation/).

One of the most obvious and distinctive signs of otter occupancy of a den is the presence of their scat in the vicinity.  It usually has little form; rather, it consists of loose piles primarily composed of fish scales. Pictured is an abandoned beaver lodge that is currently occupied by several otters whose scat in the foreground and tracks and slides in the vicinity confirm their presence. A lack of any beaver sign indicates the lodge has been abandoned by its original inhabitants.

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Muskrats Foraging

Usually at this time of year, Muskrats are restricted to foraging beneath the ice and eating their mostly herbivorous diet inside “push-ups” — huts made out of excavated vegetation that have been hollowed out and serve as protective outposts where they can eat and rest. Because of warmer weather this winter, many ponds have retained open areas where Muskrats (and Beavers) can climb out of the water and enjoy sunshine (if they’re lucky) and fresh air while they eat.

Unlike Beavers, which store their winter food in a pile adjacent to their lodges in the fall, Muskrats forage for food on a daily basis throughout the winter. While occasionally they eat small fish, clams, snails and turtles, Muskrats’ preferred diet is the roots, stems, leaves, and fruits of many water plants, such as cattail, water lilies, and rushes. Equipped with a thick, waterproof coat of hair, they are capable of remaining submerged up to 15 minutes collecting food due to a decreased heart rate and oxygen stored in their muscles.

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What Determines The Internal Air Temperature Of An Active Beaver Lodge In Winter?

Regardless of the outside temperature, the interior of occupied beaver lodges has a fairly stable temperature of about 32°F.  This is due to several factors, one of which is insulation. Beavers spend much of the fall collecting and stuffing mud into the cracks between the branches that provide a framework for their lodge (leaving a mud-free air vent at the top of the lodge).  This mud as well as any snowfall that occurs during the winter help keep out the cold and retain the warmth that the resident beavers’ bodies radiate. 

A comparison of the (cooler) interior temperature of bank dens and that of open-water lodges confirms that the temperature of the substrate underlying a lodge also contributes to the air temperature of the chamber.

Lastly, heat produced by beavers raises the temperature of a lodge above that derived from the lodge substrate.

(Photo: beaver lodge on a morning cold enough to show “beaver breath” escaping through the air vent that runs up through the center of the lodge to its peak. Thanks to Kay Shumway for photo op.)

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Beavers Gathering & Caching Winter Food Supply

Beavers are busy reinforcing their lodges and repairing dams as the days shorten and temperatures fall.  Once these tasks are taken care of, they begin gathering and storing all of the food that they will need this winter, for once ice forms they will no longer have access to land.

Their winter food cache is placed as close to the entrance of their lodge as possible, as the inhabitants of the lodge will be swimming out to this pile frequently to obtain food.  After hauling branches and saplings to the lodge, the beaver dives down and jabs the butt end of the branch into the mud at the bottom of the pond.  Additional branches are woven into this base layer until eventually most caches weigh two or more tons. One can often see the pile’s top branches and leaves rising above the surface of the water. 

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Unusual Beaver Activity

Beavers are known for their ever-growing incisors which allow them to cut trees down, eat the cambium (a nutritious layer just beneath the bark) and cut what’s left into pieces they are able to haul and use as building material for dams and lodges. More often than not, it’s straight forward work.

Occasionally not every step is taken – you can find standing trees that had the bottom three or four feet (as high as the Beaver could reach) of cambium removed without the trees being felled.  You can find de-barked logs that have been left where they fell and not carried or floated to the dam or lodge as construction material.  It’s also not unusual to find standing trees where several times a Beaver has attempted but failed to cut all the way through.

Recently John Twomey brought to my attention a tree felled by Beavers unlike any other I’ve ever seen:  one or more Beavers had cut down a Paper Birch and eaten the cambium layer, leaving the tree clean of bark.  At some point they cut into the tree every 18 inches or so, not quite severing the pieces, but leaving them connected by a core of wood that ran the length of the tree. If any readers have seen anything similar to this, or if you have an idea as to why Beavers would have cut the tree in this fashion, Naturally Curious would love to hear from you.  (Photo by Prentice Grassi of his sons investigating said tree earlier this spring.)

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Beavers Foraging

Even in the coldest of winters, there is often a thaw around this time of year that frequently allows beavers to escape the cold (+/- 34°F) dark lodge where they reside during most of the winter.  Our most recent thaw was such that in many beaver ponds, ice didn’t even have to be broken in order for resident beavers to forage for food on land.

When beavers are confined to complete darkness (under the ice) their 24-hour circadian cycle extends to a 28 hour day. During this time beavers sleep for longer periods at a time and thus need less food.  As spring approaches and the days lengthen, the slightest exposure to daylight will reset the beaver’s biological clock back to the circadian cycle. (Leonard Lee Rue, Beavers)  (Photo by Alice Trageser)

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Beavers Feeding Young

In summer Beavers spend considerable time on land searching for, cutting and bringing vegetation back to their young in the lodge.  Their woody plant preference (they eat large amounts of herbaceous plants during the warmer months, but also some trees) tends towards the inner bark (cambium) of willow, aspen, maple, birch, cottonwood, beech, poplar, and alder trees. This beaver, however, has retrieved a Red Oak sapling for its offspring.

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Muskrats & Spatterdock

6-12-19 muskrat eating water lily flower bud1B0A0488Cattails, sedges, rushes, water lilies and grasses make up the bulk of a Muskrat’s diet although these aquatic rodents have been known to occasionally dine on fish, crustaceans and freshwater clams.

Muskrats typically don’t eat their food where they find it – they usually bring it out to a feeding platform in the water, which provides them with some protection from predators. However, they make an exception for Bullhead Pond-lily flower buds (Nuphar variegata), also known as Spatterdock, which they often devour on the spot wherever they find them (see photo). Beavers, Porcupines (yes, Porcupines can swim), White-tailed Deer and waterfowl also dine on the leaves, rhizomes, buds, flowers and seeds of Bullhead Pond-lilies.

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What Goes On When We’re Not Looking

5-17-19 beaver and ducks_U1A0432A blind is a wondrous thing, allowing you to observe natural behavior in a natural setting. Having watched beavers for over 50 years, I thought I had seen most of what there is to see regarding Beaver behavior, but this particular morning I was witness to a new activity, namely a game of tag.

The resident Beaver spent the better part of half an hour chasing ducks around its pond. A pair of Mallards were subjected to this annoyance first. When approached, the Mallards would swim away together, with the drake quacking loudly, but obviously weren’t put out too much by this game as they tolerated it for about ten minutes before taking off (undoubtedly in search of a more serene body of water).

No sooner had the Mallards left than a pair of Hooded Mergansers arrived. The Beaver greeted the newcomers and proceeded to chase them around and around the pond, occasionally catching up to them, and then restarting the game all over again. Eventually the mergansers, too, departed, leaving the Beaver king/queen of his/her castle. (Thanks to Mike Keating for photo op.)

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Canada Geese Nesting On Beaver Lodges

4-29-19 c. goose on beaver lodge _U1A7241If you are fortunate enough to have a beaver pond near you, you should give the lodge more than a cursory glance this time of year. It is common to find Canada Geese nesting on beaver lodges, for obvious reasons – safety from most land predators. While Common ravens have been known to raid Canada Goose nests for eggs and goslings, the overall rate of survival of the goslings of lodge-nesting geese is very high.

A Canadian study showed that ponds with beaver lodges (and therefore Beaver activity which warms the water and thaws the ice) thaw at least 11 days sooner than ponds without Beavers, allowing early access to water for Canada Geese returning for the spring nesting season. Battles between pairs of geese vying for these coveted nesting sites are not uncommon.

Canada Geese have much to thank Beavers for. Not only can geese get an early nesting start on beaver lodges, they have a relatively safe spot to incubate their eggs and raise their young.

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Muskrats Cohabit With Beavers

4-24-19 muskrat_U1A7137Muskrats, or “rats,” as they’re sometimes derogatorily called, are semi-aquatic, mostly plant-eating rodents that live in ponds, streams, lakes and marshes. During the winter they seek shelter in lodges that they build out of grasses, reeds, cattails and sticks. Muskrat lodges are much smaller than Beaver lodges, which are constructed out of mud and sizable branches, sticks, stones and mud.

In the spring Muskrats often build nests by burrowing into a stream or pond bank, which they enter under water. Muskrats are also known to set up residence in active Beaver lodges. After dining on aquatic vegetation, the pictured Muskrat made a beeline for the beaver-occupied lodge nearby, and dove under as it approached it. Beavers and Muskrats tolerate each other’s presence in the same pond (and lodge) even though they both consume much of the same vegetation. Unlike Beavers, Muskrats supplement their diet of plants with frogs, crayfish, clams, snails, and fish. It may be that when cohabiting a lodge, they may help one another keep an eye out for predators. (Photo: Muskrat eating pond vegetation)

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Beavers Seeking Green Food

3-22-19 beavers 935Although spring is arriving a bit later than usual this year, there are beaver ponds where the ice has started to melt and resident beavers have wasted no time in exiting the water. (Where ice is thinning, they are known to bump their heads up against it in order to break through.)

Having had a diet of bark all winter (from the underwater supply of branches they stored last fall), beavers head for a greener diet if it can be found. Skunk cabbage, where it’s present, is the first fresh food of the season for beavers. The rhizomes (underwater stems) of water lilies, as well as the leaves and flowers when they appear, are also favorite foods. As spring advances into summer, grasses ferns, sedges, and all manner of herbaceous flowering plants are eaten.

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Beavers Breeding

2-6-19beaverimg_3683Beavers are in the middle of their breeding season, which means copulation takes place under the ice, undetected by human eyes, making information about their reproductive habits hard to come by. What is known about the breeding practices of North America’s largest rodent is that beavers form permanent breeding pairs and are monogamous through consecutive breeding seasons. Should a beaver’s mate die, a new one will take its place. In a colony, only the adult (individuals which are at least three years old) pair breeds. Mating usually takes place in January or February, and typically occurs at night in the water. Gestation is roughly 128 days and three or four kits are born in May or July.

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Coyotes Investigating Beaver Lodges

1-21-19 beaver lodge img_6186Over the past century beaver trapping has declined and beavers have returned to many of their former habitats. Wolves also have come back in a few areas (not the Northeast yet) — but most places where beavers now live remain free of wolves. As a result, the beaver population has continued to increase, limited only by a few predators, primarily humans and Eastern Coyotes.

Coyotes are major beaver predators and have established themselves throughout the Northeast partly because of the abundance of prey and partly because of the absence of wolves, who keep coyotes out of their habitat. During most of the year, coyotes can take advantage of beavers that leave their pond to feed on land. When they are in their lodges, however, beavers are fairly safe from coyote predation, especially if their lodge is surrounded by water. Come winter, when ponds freeze and beavers remain in their lodges, coyotes can easily approach an inhabited lodge by walking over the ice. Thanks to the lodge’s two to three-foot-thick walls of frozen mud and sticks, the beavers within are safe. (Photo: signs showing a coyote’s attempt to access a beaver lodge)

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Beavers Eating & Grooming

7-11-18 beavers 049A5253

This photograph conveys the essence of a beaver’s summer – eating and grooming… more eating, more grooming. During the summer months, beavers feed on non-woody vegetation (grasses, ferns, aquatic plants, etc.) 90% or more of the time. (During March/April and October/November, their diet switches to 60%-90% tree bark, and during the winter, bark from trees stored under water composes 100% of their diet.)

When beavers are not eating, much of their time during the warmer months is spent grooming, both themselves as well as each other.  Combing debris out of their coat (with the help of a split nail on both hind feet) and applying oily material from their anal glands to waterproof their fur consume much of their waking hours, both at night as well as at both ends of the day. (Castoreum, produced in castor sacs, differs from anal gland secretion, and is used primarily to mark territory.) (Thanks to Roger and Eleanor Shepard, and Sara and Warren Demont for photo op.)

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Beavers Up and Out

4-16-18 beaver l 690Although most of the Northeast is snow-free, parts of northern New England still have a while to go before bare ground is visible. Most beaver ponds, however, are open and the beavers have been taking advantage of their ability to access fresh food after a long winter of water-logged woody plants. While this particular beaver is dining on bark, most beavers head for any green plants, buds, grasses, etc. that are poking their heads above the ground. A particular beaver delicacy this time of year are the leaves and blossoms of skunk cabbage.

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Beavers Breaking Ice

11-13-17 beaver and ice IMG_4410

We’re right on the verge of when beavers will no longer be able to smell fresh air, see the sun and obtain fresh bark. Until the temperature drops to around 16 degrees F. they continue to break through the thin ice covering their pond. Once the temperature remains in the teens or lower for several days, they no longer try to break through the ice and are sealed under it until spring, unless there’s a mid-winter thaw.

Once beavers are confined by the ice, their activities outside the lodge are minimal. Beavers leave their lodge in winter primarily for three reasons: 1) to swim out to their winter food supply pile and retrieve a branch which they bring back into the lodge to eat, 2) to defecate in the water, and 3) to mate in January or February. Other than these excursions, they spend most of their days in the dark, enduring life in a lodge that has a temperature of about 34 degrees F. (Thanks to Kay and Peter Shumway for photo op.)


Otter Brown-Out

9-28-17 otter brown-out 049A5342North American River Otters are not territorial in the classic sense of marking territorial boundaries. Instead they mark prime resource areas within their territory. Often, the site is near their den or a productive food area. They visit these sites repeatedly to urinate, defecate and roll around on the ground – so much so that the surrounding vegetation is often dead or dying and is referred to as a “brown-out.”

If an otter has been eating fish, its scat is often just a pile of fish scales. However, if it has been dining on crayfish and it is fresh, the scat can be tubular. No matter what form otter scat takes, a tell-tale sign (in addition to fish scales and/or crayfish exoskeletons) is the presence of clear, white or yellow mucus (scat-jellies). It is not always deposited, but occasionally you do find it. The origin of this mucus is not known – most likely it’s from the otter’s intestinal tract or its anal glands. Research shows that the presence of mucous deposits in some otter species indicates reduced prey availability or reproductive state.  (Photo: Tubular otter scat is circled in red. Mucus is on right side of photo. Thanks to David Putnam and Natalie Starr for yesterday’s and today’s photo op.)

Reasons why Mystery Photo was not

       Black Bear: Scat consists primarily of crayfish remains.

Beaver: Beavers defecate only in water, and individual pellets consist of tiny woody fragments resembling sawdust.

       Raccoon: Raccoons have latrines where multiple scat is deposited, similar to otters. However, only otters deposit mucus.


Beavers Preparing For Winter

9-12-17 beaver2 20160905_5707The lazy, hazy days of summer are dwindling, and beavers’ internal clocks are telling them it’s time to batten down the hatches and prepare for several months of life below the ice. This entails adding a significant amount of mud to the outside of their lodge. The mud freezes and creates an impenetrable barrier between them and predators such as coyotes that, thanks to ponds being frozen, will have access to beaver lodges. This mud is anchored by the addition of debarked branches and logs that have provided the beavers with meals of cambium during the summer.

A beaver can transport its own weight in material (roughly 45-60 pounds). Retrieving debarked pieces of wood in many cases involves carrying them over both land and water, using only jaws and sometimes a shoulder for support.  A beaver’s short, muscular neck and its powerful lower jaw muscles make this possible. Try lifting one of the larger logs on a lodge or beaver dam sometime. Then imagine carrying it any distance in your mouth with no assistance from your hands. (This feat rivals that of a moose carrying two 25 – 30 pound antlers around for several months.) While there are recorded cases of beavers felling trees 150 feet tall and 5 feet in diameter, logs of this size are not used as building material for dams and lodges, but rather the bark and upper branches provide them with food.  (Thanks to Roger and Eleanor Shepard and Sara and Warren Demont for photo op.)


Beavers Mating

2-17-17-beaver-img_3766Under ice-covered ponds and lakes in dark, cold water, sometime between December and March, beavers mate. Latitude and climate affects the length of the breeding season, which is shorter and later in colder, more northern locations and longer and earlier in warmer, more southerly regions. February is the peak mating season for New England beavers.

Beavers are classified as monogamous, as once a bond has formed, they remain as a pair throughout their life until one of the pair dies, at which point a new mate is found. However, this does not mean they don’t stray. In a study of beaver colony genetics, researchers discovered that more than half of the litters had been sired by two or more males. So much for monogamy.

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A Beaver’s Winter Quarters

2-1-17-beaver-lodge-interior-img_4899What exactly is it like inside an active beaver lodge in winter? It’s dark, damp and around 32°F. The living chamber inside usually has a ceiling no more than two feet high with a diameter of 4 to 6 feet, depending on the number of individuals in the family. (A typical beaver family is composed of an adult male and female, 2 to 3 yearlings, and 2- 4 kits that were born in the spring.) Fresh air enters and carbon dioxide leaves through a central vent (where mud is not applied) and through small holes that remain under logs on the side of the lodge. When there isn’t much snow and the outside air falls well below zero, the temperature inside may drop to a degree or two below freezing, but if the sun is out, it warms right back up again during the day.

The dampness is due to the beavers’ repeated need to enter the water both to retrieve sticks from their nearby food supply pile and to defecate. Upon returning to the lodge, the humidity inside increases due to the water draining from the beavers’ fur. No small wonder that if a January thaw permits, beavers will exit their pond for some fresh air, food and a little bit of sunshine.

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