"Hard to believe, but trees can actually provide more habitat for wildlife when they die rather than when they are alive".................. "Standing dead and dying trees, called “snags” or “wildlife trees,” are important for wildlife in both natural and landscaped settings, occurring as a result of disease, lightning, fire, animal damage, too much shade, drought, root competition, as well as old age".................."Birds, mammals, and other wildlife use snags for nests, nurseries, storage areas, foraging, roosting, and perching"................" Live trees with snag-like features, such as hollow trunks, excavated cavities, and dead branches can provide similar wildlife value"............... "Snags occurring along streams and shorelines eventually may fall into the water, adding important woody debris to aquatic habitat"................."Dead branches are often used as perches; snags that lack limbs are often more decayed and, may have more and larger cavities for shelter and nesting"................"Snags enhance local natural areas by attracting wildlife species that may not otherwise be found there".................All trees of all sizes are potential snags"................."Unfortunately, many wildlife trees are cut down without much thought to their wildlife value or of the potential management options that can safely prolong the existence of the tree"..............."Wildlife trees offer a one-stop, natural habitat feature"................"In short, snags “live on” as excellent wildlife trees for all to enjoy!"

https://www.nwf.org/en/Garden-for-Wildlife/Cover/Trees-and-Snags

Trees and Snags

Dead trees provide vital habitat for more than 1,000 species of wildlife nationwide. They also count as cover and places for wildlife to raise young in the requirements for 

Snags - The name for dead trees that are left upright to decompose naturally.

Logs - When a snag (or part of a snag) falls on the ground, it becomes a log—also very useful for wildlife habitat.

By some estimates, the removal of dead material from forests can mean a loss of habitat for up to one-fifth of the animals in the ecosystem.

Dead Wood Good? How Dead Trees Help Wildlife

Wildlife species use nearly every part of a dead tree in every stage of its decay for things such as:

• A Place to Live—Many animals, including birds, bats, squirrels and raccoons make nests in hollow cavities and crevices in standing deadwood.
• A Food Source—By attracting insects, mosses, lichens and fungi, deadwood becomes a gourmet restaurant for wildlife looking for a snack.
• A "Crow's Nest"—Higher branches of snags serve as excellent look-outs from which wildlife such as raptors spot potential prey.
• A Hiding Place—The nooks and crannies of deadwood are put to good use by squirrels and other wildlife looking to store food.
• A Soil Refresher—Mosses, lichens and fungi all grow on snags and aid in the return of vital nutrients to the soil through the nitrogen cycle. Decaying logs on the forest floor also act as "nurse logs" for new seedlings.

Incorporating Dead Trees into Your Habitat

You can create a refuge for hundreds of woodland creatures by keeping snags in your yard (or constructing artificial snags if no natural ones are present).

Despite the importance of snags to wildlife, many modern forestry practices encourage the removal of deadwood from the forest floor in an attempt to control pests and fungi, as well as for aesthetic reasons.

• When should I remove a snag? Never allow dead wood to rest against your home. Also any trees that may fall on your home (or a neighbor's home) should be removed. In both these cases, however, consider moving the wood to another safer area of your yard.
• What about termites? As long as the snags are a reasonable distance from your home, termites and other pests won't find their way into your home.
• How do I create artificial snags? If there are no natural snags in your yard, you can create artificial ones by trimming branches on live trees of varying sizes and types. Hardwood trees tend to make better nesting habitats while softer wood is better for food foraging. If you do not wish to create snags from living trees, the use of nesting boxes can be a good alternative.
• How many snags should I have? Three snags per acre is a good estimate for most areas, but you should check with your local wildlife management authority to get specific recommendations for your region.
• ------------------------------------------------------
• https://extension.psu.edu/dead-wood-for-wildlife
• Most of us would have little difficulty responding if asked what value trees have for people. Living trees provide shade. Trees filter air and produce oxygen with their leaves. Trees can soften the impact of rain, prevent soil erosion, produce food, and are pleasing to the eye. Harvested trees provide many valuable products for people. When a tree is cut, it can be used to frame, insulate, or heat a house. This publication was written and reproduced on paper made from trees.
  But most of us would have much difficulty relating the value that trees have for wildlife, especially dead trees. Trees do have special value for wildlife. Dead parts of live trees and dead trees, whether standing (snags) or fallen (logs), are particularly important resources.
  Felling a tree for whatever reason alters wildlife habitat. The effects can be beneficial or detrimental, planned or haphazard. Some people believe leaving dead trees in the forest to rot is a waste of resources. However, dead trees offer both shelter and food to many wildlife species. Dead limbs and trees are a natural and desirable part of wildlife habitat. The existence of numerous species depends on the presence of dead trees. A fallen tree becomes infested with fungi and insects. As the tree decomposes, nutrients are recycled into the soil and a microhabitat favorable for the growth of new tree seedlings is often created.
  Insects, salamanders, snakes, mice, and shrews seek refuge in rotting logs. Skunks, bears, and woodpeckers repeatedly return to these cafeterias for easy pickings. Depending on a log's location relative to good cover, a grouse may use it as a drumming site. Some rot-resistant logs have been used by generations of ruffed grouse.
  The accumulation of organic material, including damp, rotting wood and leaves, favorably affects mushroom populations. Mushrooms are food for insects, turtles, birds, mice, squirrels, and deer. During critical winter periods, highly nutritious mushrooms can compensate for nutrient deficiencies in deer's native forage.
  Ruffed grouse and eastern towhees, among other species, nest under partially elevated logs. Depending on their size, hollow logs can shelter a variety of forest mam- mals such as shrews, chipmunks, and bears. Foxes and coyotes also may use logs for dens. For some mammals, including deer mice, chipmunks, and squirrels, log tops are highways over the forest floor. Rattlesnakes often coil next to a log and wait for food to arrive.
  Logs and stumps meet the special habitat requirements of the redback and slimy salamanders. The four-toed and longtailed salamanders hide in moist, decaying wood. The eggs of the northern spring salamander are laid in running water under logs. Greater and lesser gray tree frogs may be found in hollow trees, under loose bark, or in rotted logs during the summer. Seven species of turtles bask on logs that are in or near water. The eastern box turtle may burrow under a log during hot weather. The northern fence lizard is found in log piles and around stumps and hibernates in the rotting wood. Special habitat requirements of the five-lined skink include open woods with logs and slash piles.
  Snakes use logs for shelter and food-seeking activity. Some species, such as the eastern garter snake and the eastern worm snake, hibernate in rotting wood. At least 19 kinds of salamanders and 26 species of reptiles make some use of logs, stumps, bark, and slash piles in Pennsylvania's forests. Ecologists believe dead wood is one of the greatest resources for animals species in the forest.

• Mammals that use tree cavities in the east

  Only the squirrels and perhaps  one or two kinds of bats are  obligate cavity nesters. Other  species may use cavities if they are available.

  Opossum Pipistrel bat Little brown bat Keen bat Indiana bat Silver-haired bat Big brown bat Evening bat Gray squirrel Fox squirrel Red squirrel Eastern flying squirrel Northern flying squirrel Chipmunk Deer mouse White-footed mouse Porcupine Raccoon Black bear Long-tailed weasel

  In addition to location, the nature of the cavity tree is important to wildlife. Some species choose a cavity in either a live or a dead tree; this is not true of all species. The yellow-bellied sapsucker, for example, constructs a new cavity each year in a live tree. The northern flicker, on the other hand, uses or excavates cavities in dead trees. Whether a snag is hard (sound) or soft (plunky) also determines which birds use it. The pileated and hairy woodpeckers choose to nest in hard snags. The brown creeper nests under exfoliating bark of hard snags. The black-capped and Carolina chickadees prefer to excavate nesting cavities in soft snags.
  In addition to the soundness and location of a cavity tree, the following other factors may affect its use by wildlife:
  • The size of the cavity. Will the entrance accommodate a bluebird, a barn owl, a squirrel, a raccoon, or a bear?
  • The diameter and height of the cavity tree. The house wren and bluebird rarely nest in holes more than 12 feet above the ground, while pileated woodpecker cavities are found higher than 15 feet. Generally speaking, the larger the cavity nester, the larger diameter of the tree selected for nesting.
  • The direction faced by the cavity entrance. Screech owls, for example, often choose cavities with north-facing entrances and, consequently, low internal light levels.
  • The relationship to other cavity trees. Cavity trees chosen by gray and fox squirrels are often located near other cavity trees.
  • The nature of the woodlot. Although most species choose stands of deciduous trees or mixed stands including some evergreens, the sawwhet owl prefers stands of evergreens. Whether a cavity tree is located in a woodlot with a dense or open understory also affects its use by some species. Hairy and downy woodpeckers prefer open and dense understories, respectively. Similarly, dense understories favor gray squirrels, whereas more open understories attract fox squirrels.
  • The time of the year. Cavity trees are used for nesting, roosting, winter shelter, escape, food storage, and foraging. One researcher found that amphibian and reptilian use was highest in the summer and early fall, followed by high mammalian use in late fall and winter. Bird use is greatest in spring and early summer. People cleaning bird boxes in early March frequently evict deer mice from the winter apartment.
  The presence of cavities or the possibility of excavating cavities in wood with heart rot or other decay is not the only attraction of a dying or dead tree for wildlife. Snags are a common source of insects and other invertebrates. This food source may be exceptionally important for overwintering birds.
  If snags are houses and cafeterias, they are also airports. Flycatchers use snags for launch sites as they sally forth time and again after flying insects. A snag that borders a field or orchard may be used constantly by hawks and owls while they wait for an errant field mouse. Similarly, kingfishers, ospreys, and bald eagles perch on or fish from dead trees standing in or near water. At least 30 kinds of birds commonly use snags for foraging perches. In addition, the indigo bunting, northern mockingbird, and crow are among species that regularly use snags for singing perches.
  Using dead wood for wildlife rather than fuelwood requires some choices. The fuelwood value of a hollow tree must be weighed against the possible value of the wildlife it attracts.
  Aside from food or dollar values, the recreational value of such species are, for many of us, worth leaving a few hollow trees and logs on every acre. You may be hunting squirrels, wood ducks, or grouse, or trying to take that special photograph of a bluebird. The entertaining chickadee on your bird feeder may have been born in the hollow aspen tree behind your house. These values are not measured by dollars but by feeling.
• The regulation of insect populations is a complex issue. Insects form a major part of the diet of 80 percent of the cavity-using birds in Table 1. Nine of 20 mammals using tree cavities depend on insects for food. Shrews, salamanders, and reptiles that make use of logs, stumps, bark, and slash piles constitute an additional 50 species that forage for insects.
  Insect damage to trees is a significant cause of loss. Insectivorous cavity-nesting birds, in many cases, play an important role in the regulation of forest insect populations. Scientists believe that the most important role of birds is the prevention, rather than the suppression, of insect infestations. The protection of cavity-nesting bird populations by promoting forest diversity and leaving snags and den trees is advocated as an economical means to help prevent insect outbreaks in the managed forest.
  Many people are familiar with the purple martin's ability to consume large quantities of flying insects. A single purple martin may consume hundreds of mosquitoes in one evening, but bats are the champion. They are the only major predator of night-flying insects. A single big brown bat can consume thousands of mosquitoes before dawn. It is evident that woodlots are best protected from insects by a full complement of species including birds, mammals, reptiles, and amphibians. The alternative could be loss of annual tree growth or expensive spraying of insecticides.
• https://wdfw.wa.gov/living/snags/
• Wildlife That Use Snags
  West of the Cascade Mts 39 species of birds and 14 species of mammals depend on tree cavities for their survival. East of the Cascades 39 bird species and 23 mammal species depend on these snags (Pederson, USDA Forest Service). In total, more than 100 species of birds, mammals, reptiles, and amphibians need snags for nesting, roosting, shelter, denning, and feeding (Bottorff, WSU, Snohomish Co. Ext); nearly 45 species alone forage for food in them. Hollow snags and large knot-holes are used by many species of mammals such as squirrels, marten, porcupine, and raccoons.
  In winter when snow covers the ground, northern flickers and other common backyard wildlife depend heavily on insects and other foods found in snags. Brown creepers, bats, and other small animals will roost behind loose bark and bark slits for winter warmth and shelter. Hollow snags are very valuable in winter as they are used by many species such as squirrels, raccoons, owls, and bear for denning and roosting.
  Large snags more than 12 inches in diameter and 15 feet tall offer ideal hunting perches for hawks, eagles, and owls. They function as resting perches for swallows, band-tailed pigeons, mourning doves and other birds; food storage areas for mice, squirrels, woodpeckers, and jays; and song perches for tanagers and flycatchers. Woodpeckers use large dead tree trunks as a way to announce their presence during courtship, hammering their bills against the tree’s resonating surface. Small snags may be used as song posts by bluebirds, hummingbirds, and other songbirds to attract mates and proclaim nesting territories. This high use of snags by a myriad of species underscores the importance of preserving snags and including them in your landscape.

  Click on photo to enlarge Dead Tree/Wildlife Condo You can see where wildlife finds food and shelter if you look carefully at a snag: A snag harbors many insects that are food for wildlife. The outer surface of the bark is where birds such as brown creepers, nuthatches, and woodpeckers eat bark beetles, spiders, and ants. The inner bark  is where  woodpeckers eat  larvae and pupae of  insects. Mammals such  as raccoon and black  bear may tear into these  areas of snags to harvest  the protein-rich insects. The heartwood is where  strong excavators such  as the pileated woodpecker  prey upon carpenter ants  and termites. The space between partially  detached bark and the tree trunk  is where nuthatches, winter wrens,  and brown creepers roost or  search for food. Pacific tree frogs,  several species of bats, and many  butterflies also find shelter there.

  Trees that make the best snags.

  Click photo to enlarge Hard and Soft Snags. The snag with the abundant nest cavities and foraging evidence is a soft snag that has been used for many years. This photo was taken four years after the flat-top trees were created”   from live Douglas fir; they have barely started the decay process yet woodpeckers are beginning to work them. Photo Credit: Patricia Thompson Hard and Soft Snags A snag habitat begins to form when a large tree dies and forms a "Hard Snag.” As this hard snag decays it gradually becomes a "Soft Snag.” A partially or recently dead tree is a hard snag. Hard snags tend to have their bark intact while the heartwood (the non-living inner core) and sapwood (the younger, softer, growing wood between the bark and heartwood) are still firm. These kinds of snags are good for cavity excavating birds. A soft snag has considerable decay in its heart and sapwood. Fungi infiltrate the heartwood and the tree becomes soft or hollow in the center. A soft snag rarely has limbs, and its top may be missing. Over the years, a soft snag gets shorter as weather and animal activity weakens it. Eventually it falls over and continues to provide important food and shelter on the ground.

  Tree Species. Snags of both deciduous trees (those that shed leaves in winter) and conifer trees (evergreens) are used by wildlife. The most favored snag species east of the Cascades are: ponderosa pine, western larch, quaking aspen, and paper birch; west of the cascades Douglas fir and western red cedar snags are highly favored and big-leaf maple and cottonwood are also used. Softwood trees such as fir tend to make better food foraging trees, while hardwood trees are sometimes better for nesting cavities. Nevertheless, just about any species of snag tree will be used by wildlife.
  
  Tree Size. Small trees rot rapidly, creating wildlife habitat. Black-capped chickadees nest in snags as small as six feet tall and four inches in diameter. The large conifers such as cedar, fir, larch, and pine, tend to rot more slowly than do deciduous trees such as alder, birch, and cherry. However, large deciduous trees such as cottonwoods, big-leaf maples, and oaks can last many years as snags. Moreover, while alive, they tend to develop cavities in their bulky and dead branches and trunks.
  Decay. The best snags for cavity-nesters are those with hard sapwood (between bark) and decayed heartwood (inner core) making them hard on the outside and soft in the middle. The hard sapwood provides protection from predators and insulation against weather, while the softened heartwood allows easy excavation deep into the snag. Many birds avoid very soft snags for nesting because extremely soft wood can be wet or crumbly.
  Strong primary excavators, such as the pileated woodpecker and northern flicker, occasionally select living trees with decayed heartwood because they can penetrate through the sound layer of sapwood and excavate the nest cavity in the soft heartwood. Generally, the sapwood remains fairly intact and forms a shell surrounding the decaying heartwood. The excavated interior may remain useable for many years by many species.

  Northern flicker in the process of excavating its nest cavity. Note the wood chips flying. The Woodpecker - Cavity Creator Woodpeckers such as the northern flicker create new cavities in snags and are thus referred to as “primary cavity nesters.” They have thick-walled skulls supported by powerful neck muscles, and a beveled, chisel-like bill. A woodpecker’s strong, grasping feet with sharp, curved nails form a triangular base for support in the vertical position along with specially adapted tail feathers. The woodpecker’s barb-tipped tongue and sticky saliva help it get insects from deep crevices. Unlike other cavity-nesting birds, woodpeckers rarely use nest boxes because they are biologically conditioned to dig their own cavities: the physical motions of cavity excavation stimulate reproduction. Woodpeckers excavate several holes each year and rarely nest in the same one in consecutive years, thus creating many cavities for secondary cavity nesters such as bluebirds, tree and violet-green swallows, chickadees, nuthatches, house wrens, wood ducks, squirrels, and owls who cannot excavate cavities themselves. Secondary cavity nesting wildlife are highly dependent upon the availability of these abandoned nest cavities.

  Snags in Your Landscape
  Try to incorporate one or more snags into your landscape keeping old and damaged trees when possible. Retain trees and tall shrubs near the planned snag to protect it from wind and provide a healthier environment for wildlife. In urban areas, tall snags are best located away from high activity areas, where they won’t pose a hazard if they fall. Trees that lean away or are downhill from structures and other areas of human activity present little or no risk.
  Ways to tell a future snag:
  • Sap runs,
  • Splits in the trunk,
  • Dead main limbs,
  • Fungi on the bark.
  • Evidence of animal use, such as woodpecker holes.
  
  

  Cedar snag with top burned out by homeowner adds an interesting and striking feature to this backyard landscape. Photo Credit: Russell Link

  Photo Credit: Russell Link

  Also, note any trees you may want to make into a snag including:
  • Hazard trees--for example, one with a forked top, weak wood, or disease,
  • A shade tree in an area where you want sun,
  • A tree with invasive roots threatening a drainage or septic system,
  • A tree in a group that needs thinning out,
  • A tree in an area where there aren’t any snags.
  Because individual snags may have only one wildlife habitat feature (perch, cavity, etc.), retaining and promoting small clumps of snags throughout a larger property is more likely to provide all of these features. Small dead ornamental and fruit trees can be left in the landscape where they are not a safety hazard because they will be used as perches for preening, resting, foraging, and singing.
  
  
  Creating Snags from Live Trees

  A professional arborist creates an alder tree snag giving it a natural-looking jagged top. You must hire a professional to create these tall snags. It is unwise to attempt this yourself. Photo Credit: Russell Link

  Any snag you provide for wildlife will likely be used. You can even create one from a live tree. Branches or trunk you remove can be added to a brush shelter. Remember, a tree can provide habitat even when just part of it dies. For instance, if a large conifer has a fork in it, you can girdle one of the forks creating an excellent perch. If the trunk of this tree is large enough in diameter, a future cavity may develop as the perch limb dies. In addition, if the tree is not dying after the side branches and top have been removed, some individual side branches can be girdled to create perches and help the tree decline.
  There are several methods for creating snags (See Fig. 1):
  • Remove the top third of the tree and half the remaining side-branches.
  • Leave the top the way it is and remove a majority of the tree’s side-branches.
  • Leave the top and sides as they are and girdle the trunk.
  • Girdle the branches.
  Always hire an expert tree service to remove branches and tops of large trees. Make sure that whoever does the work is licensed, bonded, and insured, and understands your intention to make a wildlife tree. Many certified arborists with the International Society of Arboriculture specialize in wildlife tree creation and maintenance. Check with your local chapter.
  Removing the top third of the tree and half the remaining side branches. This method ensures that the tree begins the preferred inside-out decay process, premium sites for cavity-nesting birds. Leave some shortened branches at the top for perches and make the snag look natural by creating a jagged top (Fig. 1). A jagged top also provides an avenue for fungi infection and other rot-causing organisms. Water and bird feces will collect and speed decay. Sow bugs, earwigs, and other invertebrates will find their way in and assist in the decay process.
  Leaving the top intact and remove about 3/4 of the tree’s side-branches. Douglas fir, hemlock, and pine respond well to this technique. Western red cedar is a tough conifer to kill in this way, but it makes an excellent snag because it is extremely wind-resistant and long-lasting. Keep branch ends jagged and more susceptible to microorganisms and fungi, and more natural looking.

  Figure 1. Ways to create a snag  from a live tree. A jagged top and shortened branches at the top give the snag a more natural look and speed the process of decay.  (Drawing by Jenifer Rees)

  Leave the top and sides as they are and girdling the trunk. Least preferred method. Girdling creates a dead but intact top, providing a taller snag, but leaves it more susceptible to breaking at the wound site. Girdling tends to cause a tree to rot from the outside in, instead of the preferred inside-out. As a result, by the time the rot has progressed far enough for woodpeckers to excavate a cavity, the tree has become fragile and may easily fall in a windstorm. Furthermore, a cavity in a girdled tree may not be safe because the hole is likely to be shallow, which exposes young to weather and predators.
  To girdle a tree, remove a four-inch belt of inner and outer bark around the trunk which stops the movement of water and nutrients. If girdling is done at breast height and the tree falls, this leaves very little remaining snag habitat. Therefore, try to make the girdling cut as high up as possible. Big-leaf maple, aspen, and poplar may send up sprouts, which can be removed or left to grow around the tree as temporary cover. Some tree species, alder for example, are difficult to kill even when properly girdled. A tree girdled in winter may not show signs of decline until well into spring, after it has used its stored energy.
  Roosting Slits

  Brown creeper on a snag with visible roosting slits.

  
  
  Roosting slits for bats and some songbirds, including brown creepers, may be added to created snags that are tall enough and wide enough in diameter to accommodate the cuts. The slits should be at least eight inches deep, one or more inches wide, and angled sharply upward. Bats need to fly up into the slits so the slits should be located in an area free of branches. The higher up the snag they are, the more likely these roosting slits will be used. Some sun exposure warms these roosts and makes them more attractive in winter.