Many species of cavity-nesting birds have
declined because of habitat reduction. In the eastern
United States, where primeval forests are gone, purple
martins depend almost entirely on man-made nesting
structures. The hole-nesting population of peregrine
falcons disappeared with the felling of the giant trees
upon which they depended. The ivory-billed and
red-cockaded woodpeckers are currently on the endangered
list, primarily as a result of habitat destruction. The
wood duck was very scarce in many portions of its range,
at least in part, for the same reason and probably owes
its present status to provision of nest boxes and
protection from overhunting.
Some 85 species of North American birds excavate nesting
holes, use cavities resulting from decay (natural
cavities), or use holes created by other species in dead
or deteriorating trees. Such trees, commonly called
snags, have often been considered undesirable by forest
and recreation managers because they are not esthetically
pleasing, conflict with other forest management
practices, may harbor forest insect pests, or may be fire
or safety hazards. In the past such dead trees were often
eliminated from the forest during a timber harvest. As a
result, in some areas few nesting sites were left for
cavity-nesting birds. Current well-intentioned
environmental pressures to emphasize harvesting large
dead or dying trees, if realized, would have further
adverse effects on such ecologically and esthetically
important species as woodpeckers, swallows, wrens,
nuthatches and owls.
The majority of cavity-nesting birds are insectivorous.
Because they make up a large proportion of the
forest-dwelling bird population, they play an important
role in the control of forest insect pests. Woodpeckers
are especially important predators of many species of
tree-killing bark beetles.
Several of the birds that nest in cavities tend to be
resident (non-migrating) species and thus more amenable
to local habitat management practices than migratory
species. Nest holes may be limiting for breeding
populations of at least some species. Bird houses have
been readily accepted by many natural cavity nesters, and
increases in breeding density have resulted from
providing such structures, an indication that management
of natural snags should be rewarding.
Removal of snags is also known to reduce populations of
some birds. For example, removal of some live timber and
snags in an Arizona ponderosa pine forest reduced
cavity-nesting bird populations by 50 percent.
Violet-green swallows, pygmy nuthatches, and northern
three-toed woodpeckers accounted for much of the decline.
A previously high population of swallows dropped 90
percent, and a low woodpecker population was eliminated.
On an adjacent plot, where live trees were harvested but
snags were left standing, cavity-nesters increased as
they did on a plot where live trees and snags were
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