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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

February 1996

Critter condos

Properly handled, dead wood on your land can provide a big boost to life.

Mary K. Judd

On a sunset stroll through your woods, you find a forest full of activity – a snowshoe hare bounds for cover, a red squirrel scampers up a balsam fir, a blue jay scolds from a branch above. As the curtain of darkness falls, your senses heighten. Your nose intercepts the pungent, earthy odor of leaf mold, fungi, and rotting wood drifting on the breeze. Your ears snatch a few "who cooks for you, who cook for you all?" notes of a barred owl. Suddenly, you sense movement above. Your peripheral vision just catches the floating form of a flying squirrel as it leaps and glides from a snag.

The smooth, barkless skeleton of this dead tree stands stark and whitewashed in the moonlight. It's then you realize that in your forest, even a dead thing has a life of its own.

The more you know about dead wood and the types of wildlife that depend on it, the better you'll be at providing homes for wildlife on your land. Consider some of the following points and try out some of the techniques on your back lot or woodlot, and you'll be well on the way to becoming a wildlife realtor!


Standing dead or dying trees, called "snags," may look ghostly and uninhabited, but they teem with life. Over 70 kinds of Wisconsin mammals, birds, reptiles and amphibians, plus swarms of insects, spiders, millipedes and other invertebrates use snags for dens, nesting and feeding sites, caching food, perching, preening and courtship.

Just as the style, size and location of housing affect where you live, the style, size and location of dead trees determine what kinds of wildlife you'll find in, on or under them. Snags make good homes for cavity nesters such as woodpeckers, bluebirds, nuthatches and squirrels. Riverside snags, with their tangled masses of gnarled roots, provide shelter for brown trout and burrowing sites for muskrats; their upper limbs may be used for nesting by herons, egrets, bitterns and cormorants.


Snags come in two styles: hard and soft. Both are important to wildlife. Hard snags have rotten centers, a solid exterior and usually a few limbs attached – they make the best den trees. As snags decay, the wood softens and the limbs gradually fall off. Soft snags, with their pulpy wood fibers, make good forage sites for insect-eating birds and excellent nest sites for woodpeckers and songbirds such as black-capped chickadees.

A tree's characteristics determine its snag and cavity potential. For example, sugar maple, elm, black and white oak, hickory and butternut are excellent cavity trees. These hardwood trees grow to large sizes, decay slowly, and produce hard, upright and long-lived snags. The beech tree, common along Lake Michigan and Green Bay, also makes a good cavity tree because it is prone to heart rot.

Softer trees, like aspen and birch, have short lifespans and rot quickly. These rapid growers make superior soft snags of high value to wildlife since they produce cavities more quickly than hardwoods, and provide habitat for swarms of insects that feed many forest songbirds.

Snags of medium value to wildlife come from white ash, basswood, red maple, white pine, red oaks, yellow poplar, box elder, black cherry and black walnut. Coniferous snags generally do not last as long as hardwoods, though pine and tamarack make excellent nest and perch sites for osprey when located next to water.


Human condos come in all sizes and so do critter condos. In general, the larger the snag, the more kinds of wildlife it can host. While small snags are important in their own right, they can only host small creatures such as the red-breasted nuthatch, downy woodpecker, house wren, bluebird and white-footed mouse. But large snags can suit small and large wildlife such as pileated woodpeckers and raccoons. "Wolf" trees – big trees with large sprawling canopies – have great potential for cavities. In addition, wolf trees are often abundant nut and fruit producers.


Snags are most commonly associated with forests. Many a forest mammal, from bat to bobcat, bear, pine marten, porcupine, red squirrel, and gray fox use snags for dens and lookouts. So do many forest birds. Woodpeckers are the primary excavators, drilling out new homes in snags. When abandoned, these cavities become residences for other creatures like saw-whet owls, nuthatches and great-crested flycatchers. Besides woodpeckers, the only other bird that is a primary excavator is the black-capped chickadee. Chickadees lack the powerful chisel-like beak of woodpeckers and can chip out cavities only in soft snags.

Snags located near waterways and wetlands offer great benefits to wildlife. Wood ducks, hooded mergansers, common goldeneyes and buffleheads need tree cavities for nesting. Herons, egrets, eagles and ospreys build their nests high atop snags standing in or near water. Snags also serve as lookout towers for fish-eating birds, such as belted kingfishers.

Snags located near open fields attract some types of hawks and owls. From high atop their lookouts, these birds use telescopic vision and radar-like hearing to detect mice, rabbits, squirrels and other prey. The flicker, unlike its woodland-dwelling woodpecker relatives, prefers to nest in snags along woodland edges bordering farm fields or open grasslands. Kestrels and eastern bluebirds will often move into old flicker holes. Colorful bluebirds frequently nest in snags along farm fencerows and adapt well to fence posts and nest boxes, too.

Backyard snags can attract house wrens, black-capped chickadees, red-bellied woodpeckers and flying squirrels. If a snag poses no threat of dropping large branches on people, leave it in place.

Let sleeping logs lie

After a snag has toppled over, it doesn't lose its value to wildlife. Downed and rotting logs provide moist and earthy homes for salamanders, moles, shrews, earthworms, millipedes, centipedes and more. Squirrels will cache their food within the soft fibers of fallen trees. Hollow logs can be used by foxes and bears as winter dens. The next time you go camping, watch how chipmunks sprint from downed log to downed log, using them as runways to scurry through the forest. If it's springtime, listen for drumming ruffed grouse performing their courtship dance atop mossy logs.

Toppled trees also are important in forest regeneration. Some rotten logs, known as nurse logs, provide a growing medium rich in nutrients for tree seedlings to get a healthy start in life.

Ever "snag" your line while fishing? As frustrating as it is to lose a lure, every good angler knows that fallen logs in a pond or stream provide trout, bass and other fish with a sheltered, shady place to rest and feed. Downed logs in or near water are especially vital and should be spared at all costs. And while you're cutting your line free, you'll probably startle a few turtles lazily sunbathing on a log.

The loose bark of diseased and fallen trees shelters a multitude of tiny creatures. Just turn over a rotting log and watch the insect activity. The swarming, creeping, slithering tangle of ants, spiders, beetles, worms and slugs provides a nutritious cafeteria lunch for many birds, mammals, reptiles and amphibians.

Pileated woodpeckers actively seek out elm and aspen trees infested with wood boring insects, and black bears lap up ants found in rotting logs. Brown creepers, small brown birds with curved beaks, search for insects hidden under loose bark as they spiral up the trunk.

Other cavity dwellers are voracious insect eaters, though the insects they eat may not live in dead wood. A house wren can feed 500 insects to its young every summer afternoon and a swallow can consume 1,000 insects every 12 hours. In fact, these birds act as natural pesticides and help keep insect populations in check.

Dead wood is good wood for wildlife, and it does not always pose a threat to your woodland. For years, however, this was not the prevailing attitude.

In the past, loggers cut down all dead trees during timber harvests. Dead trees had limited value as timber, harbored forest insect pests, and were potential fire and safety hazards. In recent years cavity nesting bird populations have declined due to a loss of large trees with natural cavities.

Today, loggers, foresters and wildlife managers work together to protect these valuable trees and the insect cafeterias they house. Insects and disease are natural parts of the forest and contribute to the stability, productivity and diversity of life.

Managing dead wood on your land

Consider this: It takes about forty years before completely cleared land becomes suitable for most woodpeckers and other snag-dependent wildlife. It takes about eighty years before trees can support the larger cavity dwellers like raccoons and pileated woodpeckers. So, before you cut firewood or implement a timber stand improvement plan, you should identify existing and future snag and den trees.

Snags in advanced stages of decay are easy to identify – they stand out like skeletons. Big wolf trees also stand out prominently. Diseased trees are a little harder to spot. Look for signs of injury or a rotten core. Dead branches, rotting branch stubs, fungal growth, old wounds, scars and discolored or soft bark are all signs of a dying tree. Also, look for woodpecker holes. Woodpeckers actually seek out trees with rotten cores. When a woodpecker begins pecking away on a tree, it may be in the early stages of decay. Mark it as a future snag tree.

The best den trees, which can be either living or dead, are 15 or more inches in diameter at breast height (DBH), with den openings of four inches or more. If you have large den trees on your property, keep them!

If you want to manage your land for larger birds and mammals, you'll need to preserve some of the larger trees. Just let them die of old age. The pileated woodpecker, Wisconsin's largest woodpecker, needs a tree at least 20-22 inches DBH in order to excavate a nest cavity. Pine martens also need very large trees.

If you find only a few snag or den trees on your property, you can create more yourself using some simple techniques:

1. To create a snag, select a living tree that's over a foot in diameter – the bigger, the better. Also, try to select a tree that's either diseased or severely deformed, or select one that's crowding more valuable trees. Good trees for creating snags include sugar maple, black oak, white ash, elm and basswood. Use an ax to cut away a 3- to 4-inch band of bark around the entire circumference of the trunk. Make sure you remove the bark and cut well into the sapwood. This technique is known as girdling. It disrupts the flow of nutrients within the living layer of the tree found just underneath the bark. Cut off from sustenance, the tree gradually dies.

2. To create a den tree, cut off a 4- to 6-inch limb about six inches from the trunk, or chop out a section of bark 6 by 6 inches at the base of a suitable wolf tree. These open wounds allow fungal disease to enter the tree and start the decay process. A natural cavity will form over the years. Elm, ash, box elder, maple and basswood are especially prone to forming natural cavities.

Have patience!

It takes several years for these practices to create suitable nest and den cavities, so you may want to build and place nest boxes for birds and mammals until trees become available. To increase the chances that nest boxes will be used, locate them carefully and don't neglect regular cleaning and maintenance. While some species like chickadees, house wrens, wood ducks and bluebirds will readily take to nest boxes, others, like woodpeckers, simply prefer to wait for the real thing to rot!

Cut timber, save snags

If you are managing your woodland both for wildlife and timber production, keep the following rules of thumb in mind:

  • Preserve about one to six hard snags per acre and as many soft snags as possible.
  • For every 20-acre woodlot, leave the following: four to five snags or den trees over 18 inches DBH, 30 to 40 snag or den trees over 14 inches DBH, and 50-60 snags over 6 inches DBH.
  • Save at least one tree of any size per acre showing potential for den or snag tree development, especially those with broken tops, woodpecker holes or wounds.
  • Never cut a wolf tree; they make excellent den trees.
  • Check for wildlife before cutting a tree; avoid cutting inhabited trees.
  • Leave most snags evenly spaced, but include a few patches where they are clumped together.
  • Leave fallen snags on the ground to provide food and cover for wildlife.
  • Cut green rather than dead wood for firewood, and cure for several years.

The life of a dying tree

The decline of a tree begins when heart rot fungi invade the tree through a wound. The core begins to rot slowly – barren branches appear, perhaps a good site for an eagle nest or a perch for broadwing hawks and flycatchers. Insects and beetles feast. The bark loosens and woodpeckers soon follow, in search of food and potential home sites.

Using their specialized bills, woodpeckers chip away at the softened wood to create a cavity large enough for nesting. They raise their young, feed on the insects harbored within the decaying wood, and move on. The empty cavity then becomes home to another creature – perhaps an owl, squirrel, bluebird or bat.

Eventually the battered ghost of a tree topples, or remains as a soft stump, maybe half its original height. Carpenter ants invade – a healthy lunch for a passing bear or raccoon. Salamanders, snakes and mice move in while a nearby hawk perched high upon a neighboring snag takes note.

The tree decays further until new plants and mushrooms sprout in the remaining organic matter. So life goes on...and on.

Mary K. Judd is a DNR wildlife education and communications specialist stationed in Madison, Wis.