Figure 8.- The wound on this small sugar maple was caused by red squirrels gnawing the stem. The wound could be the start of a future cavity.
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Effect of Degree of Decay
Depending on their size, degree of decay, cause of injury or death, and location, cavity trees are used by various wildlife species for several purposes (Table 1). As a tree slowly dies and decays, it goes through several successional stages that are used by different wildlife groups in turn (Fig. 9). After a tree dies, the bare branches provide perch sites for predators such as hawks and other raptors as well as flycatchers. Perches that project above the surrounding forest canopy or exist in clearings are used as lookouts for prey by birds such as American kestrels (Falco sparverius), Cooper's hawks (Accipiter cooperii), sharp-shinned hawks (Accipiter striatus), and broad-winged hawks (Buteo platypterus). The existence of such perches is a major factor in the use of a woodland by these birds. Flycatchers, likewise, use strategically located perches: Eastern phoebes (Sayornis phoebe), kingbirds (Tyrannus tyrannus), and great crested flycatchers (Myiarchus crinitus), least flycatchers (Empidonax minimus), and olive-sided flycatchers (Contopus borealis) catch or "hawk" flying insects, returning after each sally to the same perch. These perches, whether above or below the forest canopy, are important habitat components as they largely determine whether these species are present or absent.
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Further decomposition of the tree results in a transitory "loose bark" stage. This sloughing bark provides the nest site for the brown creeper (Certhia americana) and roosts for bats (Tables 1 and 2).
Primary excavators-essentially the woodpeckers-usually excavate cavities when decay is present in the stem. Trees with central columns of decay resulting from stem stubs are readily excavated by woodpeckers, especially downy, hairy (Picoides villosus), and pileated (Dryocopus pileatus) woodpeckers and northern flickers (Colaptes auratus). All species of woodpeckers that occur in the Northeast, except the yellow-bellied sapsucker (Sphyrapicus varius) and black-backed woodpecker (Picoides arcticus)-which use only live trees-excavate nest cavities in live or dead trees. But decay columns in live trees, especially those resulting from stem stubs, seem to be preferred. The hard exterior wood protects the easily excavated nest cavity. These excavated cavities are subsequently used by the so-called secondary cavity nesters-the birds that we commonly attract with nest boxes (Table 1).
After a standing dead tree has decayed to the point where most of the branches have fallen, it is called a snag if it is at least 20 feet tall, a stub if shorter. The soft, punky snag or stub in the final stages of decay is used as a foraging site by many insectivorous birds, and as a nest site by black-capped chickadees (Parus atricapillus), that do not chisel cavities as woodpeckers do, but merely pick out the soft punk to form a cavity.
Once the rotted stub falls, invading carpenter ants and other insects provide an important food source for pileated woodpeckers. Many species of amphibians and reptiles live and forage in and under the moist, soft, rotting wood. Many small mammals are also associated with down logs (Table 2).
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Cavity Tree Characteristics and Wildlife Values
The preceding section describes how the degree of decay affects wildlife use. Other factors affecting wildlife are a tree's size, location, species or type (deciduous or coniferous), and how it was killed or injured. There are predictable groups of birds and other wildlife that use cavity trees depending on these interacting factors.
A general rule of cavity tree management is that bigger is better. This is so for several reasons. Large birds need large trees in which to excavate nesting and roosting holes-for example, the pileated woodpecker needs at least 20 inches in diameter at breast height (d.b.h.). Also, small birds can find nest sites in large trees, but not vice versa. And a large dead tree or snag will usually stand longer than a small one, and so be available longer. Table 3 is a guide to cavity tree or snag sizes and numbers needed by various woodpeckers for cavity excavation. Emphasis is placed on woodpeckers because habitat management for viable populations of these species will also provide nesting sites for the secondary cavity users.
Location is another factor that determines cavity tree use. Some species prefer such trees in the open, others within the forest. The flicker is a woodpecker that prefers to nest in open habitat: cavity trees, snags, or stubs at woodland edges, in pastures, or in clearcuts are preferred. Secondary users of flicker cavities are kestrels and eastern bluebirds (Sialia sialis), among others. Birds that prefer to excavate nests in more concealed trees are the hairy and pileated woodpeckers and the yellow-bellied sapsucker. Others are intermediate, using open stands or scattered trees: the red- headed (Melanerpes erythrocephalus) and red-bellied woodpeckers (Melanerpes carolinus) are examples.
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Cavity trees at the banks of streams or edges of ponds or lakes seem to be preferred by virtually all primary cavity excavators. This may be explained partly by the security provided-perhaps fewer nest predators can approach over water-and partly by the fact that trees near water may be larger and do tend to lean toward the water. Woodpeckers will usually excavate their nest hole on the underside of a leaning tree. Protection of the nest from rain and other elements seems to be the obvious function. Wood ducks (Aix sponsa) commonly use old pileated woodpecker holes excavated over water.
In addition to these factors, the extent and pattern of interior decay and the type of tree injury are probably very important factors in cavity trees selected by primary excavators. The ideal nesting substrate for most woodpeckers is a stem or limb (of suitable minimum diameter and height above ground) that is sound on the outside but that contains a central column of decayed wood In which the cavity will be excavated. This condition can result from several causes: decay spreading throughout the tree, decay spreading from a snapped off limb or top, or a compartmentalized area of decay from a wound.
The presence of decay that has been developing for a number of years can usually be detected by spore-bearing structures such as mushrooms or bracket-shaped conks on the trunk. Trees with compartmentalized decay or wounds high on the trunk or in the crown are more difficult to detect. When found, they should be marked for retention because many cavity-nesting birds use live trees.
Dead snags and stubs, while not always common in stands, are usually obvious; they should also be left for wildlife if they are at least 6 inches d.b.h. Snags and stubs that show a history of woodpecker use-new holes excavated sequentially lower as the upper portion breaks away-are especially valuable. Woodpeckers will usually continue to excavate cavities so long as relatively hard or firm outer wood supports a decay column the appropriate height. Finally, only the rotted stub will remain. The hollow formed in the tops of large broken stubs provides a nest site for barred owls (Strix varia), hooded (Lophodytes cucullatus) and common (Mergus merganser) mergansers, and turkey vultures (Cathartes aura). Smaller stubs are used by bluebirds and chickadees (Figs. 10 and 11).
Cavity Tree Management
Maintaining existing cavity trees and snags, creating them when absent, and ensuring their continued future availability are wildlife management objectives. Table 3 provides a guide to the number of cavity trees per 100 acres needed to maintain maximum wood-pecker population levels. Recall that larger trees can substitute for smaller ones.
On smaller ownerships, one should consider the availability of cavity trees on surrounding forest land. If adjacent lands have few cavity trees, those on a small parcel could be critical to cavity-dependent wildlife.
At each stand entry, maintaining cavity trees, and snags and stubs that are in the proper size classes and diameter should ensure a continuing supply of cavity trees in the condition required by the various wildlife users. These can be trees of inferior form or even noncommercial species. Foresters are able to identify trees with exterior signs of advanced internal decay-presence of fungal conks, and so on. Retention of these trees is the key to ensuring that snags and stubs are present in the future.
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