Managing Forest Habitats for Migrant Songbirds
Washington
State University
Woodland Fish and Wildlife Publication
MISC0198
Neotropical
migrant birds play an important role in the forest
ecosystem. They help control insects, disperse seeds, and
some species may be important pollinators for plants. They
also are a major food resource for predatory birds such as
owls and hawks. Because Neotropical migrants are typically
sensitive to habitat disturbance, they are often viewed as
indicators of overall forest health. In Pacific Northwest
forests, many Neotropical birds are closely associated with
riparian zones,
vegetation
adjacent to lakes, streams, creeks and wetlands.
For the past several years, breeding populations of several
eastern North American Neotropical migrants have been
declining. Many factors, including predation, failed
reproductive efforts and stresses of winter and migration,
contribute to their natural mortality, The factor most
strongly implicated in the decline of forest-dwelling
Neotropical migrants is the fragmentation and destruction
of forested habitat.
Data for the western United States is not complete.
However, population trends for some western Neotropical
migrants are becoming apparent. Although the western U.S.
breeding populations seem to be managing better than their
eastern counterparts, some western species may be at risk.
Habitat Features
What is it
about a particular area that attracts birds? Factors that
affect habitat selection occur on two levels: (1) the local
habitat and (2) the individual forest stand. Influential
components of local habitat are distribution of plants
across the landscape and the existence of special features
such as cliffs, snags, and streams. Selection features at
the individual stand level may include the successional
stage(s) represented and the site’s disturbance
history, as well as the nature of surrounding habitat.
To understand more about conservation of Neotropical
migrants, it is important to realize how birds depend on
the quality of habitat. These birds apparently select
habitats that increase their reproductive potential. To
reproduce successfully, food abundance and nesting habitat
are important. Neotropical migrants, like most species of
animals, are sensitive to change. Dramatic changes, such as
intense forest fragmentation, clearcutting, and the removal
of trees from riparian zones alter the structure of the
habitat. Forests may lose structural diversity (i.e.,
variability in vegetation size and height) if they are
intensively managed exclusively for wood production. Loss
in habitat diversity, especially reduction in forest
vegetation of varying heights, diminishes nesting
opportunities for many Neotropical species. This
transformation may eventually affect the composition of the
year-round bird community.
Snags
Dead trees, called snags, are important to both migratory
and resident birds. Snags and dying trees supply
cavity-nesting habitat for Neotropical migrants such as
swallows, swifts, sapsuckers, small owls, kestrels, and
wrens. They also provide homes for insects and other
invertebrates that are food for many breeding Neotropical
birds. Studies suggest that larger diameter snags enhance
both of these habitat features.
Riparian
Areas
Many Neotropical migrants live in riparian zones because of
the high productivity and diversity of the vegetation.
Streams, lakes, and creeks are characterized by dense
vegetated areas that provide water, cover, and a wide
variety of food sources. Riparian regions are especially
important in the drier landscapes of eastern Oregon and
Washington.
Canopy
Gaps
During the breeding season, many Neotropical migrants
prefer to nest in areas with both coniferous trees and
deciduous shrubs. Openings in the forest canopy can provide
this structural and vertical diversity. When light is
allowed to penetrate to the forest floor, there is
increased growth of deciduous shrubs. These shrubs help
create small patches of habitat important for birds
attempting to escape from predators and for young birds in
the process of dispersing. New nesting and feeding
opportunities may also arise within these habitat patches.
Stand
Age
The age of a forest is a strong influence in the
distribution and composition of bird species. Each
successional stage, from a clearcut to mature, old-growth
forest, has a different community of Neotropical migrant
birds associated with it. As one stage develops into
another, the species composition of the bird community also
changes. An increase in one species and a decrease in
another may occur at the same time. Because of this
relationship, most studies suggest that stands with a mix
of as many age classes as possible provide the optimum
habitat for these birds.
Nesting
Habitat
A mix of trees and shrubs of different heights creates
habitats for many different birds and increases the
diversity of the overall bird community. The structural
features of good nesting habitat are important on many
scales, from the area immediately surrounding a single nest
site to the entire forested landscape. The success of a
single nesting attempt may be highly influenced by the
foliage in the immediate vicinity. The denser the foliage
surrounding a nest, the less likely a predator will be able
to locate it. However, when predators hunt, their success
depends on more than just physical barriers. The efficiency
of their search for prey may be influenced by the
structural complexities of a larger habitat patch that
contains many potential nest sites. For example, the hermit
thrush prefers to nest in small white fir
(Abies
concolor). As the
density of white fir increases, the odds of predation go
down for each individual hermit thrush nest. Domestic cats
prey on wild bird populations and can reduce the population
dramatically.
Forest
Fragmentation
Forest fragmentation is believed to be one of the greatest
threats to Neotropical migrant survival. In the eastern
United States, many studies have examined the effects of
forest stand size on the species composition of
Neotropicals. These studies have found fewer breeding bird
species in stands of less than 10 acres in size. This
reduction in species may be a function of limited habitat
availability (i.e., increased competition for food and
nesting sites), or it may be a result of the species’
sensitivity to stand size. Some bird species, such as the
hermit thrush and the red-eyed vireo, are considered
area-sensitive because they require large tracts of land
for their territories. There is concern that these types of
birds may become locally extinct in isolated forest
fragments where the stand is not big enough to maintain
natural populations.
The most dramatic reductions in the numbers of Neotropical
migrants have been noted for those birds that use interior
forested habitat on both the wintering grounds and the
summer breeding areas. Many Neotropical migrant birds that
breed in North America spend the winter months in areas
less than one-fifth the size of their breeding grounds.
Deforestation of wintering grounds thus affects large
numbers of birds. The decline of many Neotropical migrants
in the eastern United States corresponds to the extensive
clearing of the forests of Mexico and Central America,
which began in the late 1970s. When winter resources become
limited, the remaining habitat can support fewer and fewer
birds, potentially reducing the breeding population.
Edge
When a large tract of forest land is broken up into smaller
patches, edge is created. Edge is the area of transition
between two different vegetation communities, such as
forest and meadow. Any serious bird-watcher knows that the
greatest variety of birds is found along edges. But too
much edge can greatly affect those Neotropical migrants
that nest primarily within the forest interior. When an
edge is expanded, the amount of edge relative to the amount
of interior is increased, and the effects are felt deeper
within the interior. Although these edges may support a
greater variety of birds, they also attract a large number
of predators such as crows, hawks, raccoons, and house
cats. As the edge widens, predators venture farther into
the interior. Many Neotropical migrants build nests low to
the ground, making them extremely vulnerable to predation.
If a clutch of nestlings is eaten or destroyed, many birds
will not nest again until the following year and will have
lost an entire breeding season.
Brood
Parasitism
Another danger Neotropical migrants face with increased
edge is brood parasitism, a phenomenon that occurs when one
bird species lays its eggs in the nest of another. The host
birds often raise the brood parasite’s offspring at
the expense of their own. Some birds are able to
distinguish between the parasitic eggs and their own, and
will destroy the alien eggs. Others, such as the vireos and
many of the warblers, have developed no defensive reactions
to brood parasites, and are experiencing dramatic
population declines. The best known brood parasite in
Oregon and Washington is the brown-headed cowbird. The
cowbird is a Neotropical migrant that is not dependent on
interior forest cover, favoring riparian areas and
centering its feeding and breeding activities along forest
edges. The smaller the fragment of forest, the closer to
the edge many interior birds are forced to nest. This
increases their vulnerability to brood parasitism by
brown-headed cowbirds and the risk of reproductive failure.
Brown-headed cowbird populations increased as forests
across the eastern United States became fragmented. They
have since extended their range into the northwestern
states.
Forest
Management and Succession
There is a relationship between wildlife existence and the
composition of trees, shrubs, and other forest vegetation.
Diverse habitats can be enhanced or created as part of
forest management activities. Proper forest management
allows for both forest products and wildlife, including
Neotropical migrants.
An important concept in wildlife management is succession,
the orderly, predictable change in the kinds and numbers of
trees, shrubs, forbs, and animals that inhabit a given
place over time. For example, if you wish to maintain
populations of birds typically found in the early stages of
succession, you must periodically disturb the forest by
logging or fire. Conversely, other Neotropical migrants are
adapted to older forest habitats. Birds select only those
stages of succession to which they are adapted. As stands
develop over time, the habitats change, and so do species
using those habitats. Many successional stages are
relatively short, and the presence of the birds associated
with them is equally short-lived. Changing an environment
to benefit one species may affect a different species in a
negative way. This may also result in changes in
successional stages present on a landscape needed by other
animals. Thus, by managing for a variety of habitats, a
landowner can expect to increase the variety and general
abundance of wildlife.
Age
Structure
Most forests in Washington and Oregon are even-aged.
Similar development with different species occurs east of
the Cascades in the absence of fire. Even-aged stands,
intensively managed for timber, are generally low in
habitat diversity. Such diversity can be increased if these
stands are partly harvested or thinned, and if some
defective or dead trees (snags) and some healthy or live
trees (green or recruitment trees) are left standing.
Recruitment trees are those trees left to become snags at a
future date.
Snags are critical habitat components. At least three hard
and three soft snags (see Glossary) of the larger sizes
should be retained per acre to maintain a cavity-nesting
bird population. Snags should be of the largest available
diameter, preferably over 20 inches at breast height (BDH).
Snags and recruitment trees are particularly valuable to
cavity-nesting birds, and are used by over 40 species. They
need not be highly valuable commercial trees; in fact,
large wolf trees with many branches are ideal because they
are worth less at the mill or for export. Preferably, leave
the least vigorous trees, so that they will die more
rapidly. In riparian areas such trees should be located so
that they shade the water surface, and should have the
potential of falling into the stream channel to provide
fish habitat, thus directly benefiting aquatic resources.
Uneven-aged stands have three or more age classes. These
are managed by selectively harvesting individual trees or
small groups of trees at periodic intervals or by allowing
natural processes to occur over a very long time. This
system is best suited to those areas where the dominant
tree species are generally shade tolerant, since
reproduction must occur in the partial shade of older
trees. Ponderosa pine forests east of the Cascades, for
example, can be either even- or uneven-aged in structure.
Silvicultural systems that perpetuate uneven-aged stands
have generally been more successful on the eastside. In
low-elevation westside forests, wind-thrown trees, unwanted
shrubs, severe stem and root rot, and tree species shifts
to alder or maple have rendered the system mostly
unworkable. If poorly applied, the system can quickly lead
to high-grading, the repeated harvesting of the best trees,
leaving the poorer ones to regenerate the forest.
Vertical
Development
Vertical development of tree and shrub species is affected
by shade tolerance, stocking, site quality, and disturbance
factors. Over time, competition for sunlight and soil
nutrients influences the height of shrubs and trees. This
difference eventually results in overtopping and the death
of species that cannot compete. Many Neotropical migrants
thrive when there is a greater diversity of plant species
and growth forms. If you wish to maintain diverse
Neotropical migrant populations, use silvicultural systems
that develop well-defined canopy layers from the tree
canopy to the forest floor.
Management
of Mature Stands
For mature stands west of the Cascades, use clearcut or
green-tree retention systems to create diversity. On the
eastside, use shelterwood, patch clearcut, or selective
logging systems.
In the shelterwood method, about one-third of the trees are
removed in the course of three entries carried out over a
5- to 10-year period. This results in approximately half
sunlight and half shade on the forest floor. Allow the area
to get “brushy,” using herbicides sparingly.
Patch clearcuts can be used if the patches are kept to less
than 10 acres. Analysis has shown that leaving large
patches unharvested for interior bird species, and grouping
harvest patches for early successional and edge bird
species, can accommodate all birds and return the same cut
of fiber. Consideration should be given to what other
owners have done in your region. Locate patches so that
habitat transitions occur between stand types. Patch
clearcuts are often called for if you have predominantly
shade-intolerant species such as red alder or lodgepole
pine, or if bark beetle or root rot problems are severe.
The aggregated green-tree retention (AGTR) system is
experimental, but it holds promise for Neotropical
migrants. In this system, an undisturbed portion of the
forest is retained as a kind of “island” in a
harvest unit. This aggregate is maintained throughout the
life of the next rotation to provide biological
“legacies” for the future. Over time these
areas should provide late successional habitats of
increasing wildlife value. Generally, the size should be
large enough to include the biological diversity of the
existing stand. Multiple islands in midslope and riparian
areas are desirable. These islands must provide enough
undisturbed area to prevent displacement of wildlife.
Because of limited trials and research, use of AGTR on the
eastside is not well understood. However, the concept
should still apply. Higher-elevation mixed conifer forests
hold the most promise for this new technique.
In the selective logging system, more commonly applied on
the eastside, trees that are the oldest
in
the stand, not
necessarily the largest, are harvested
at approximately 10-year intervals. Shrub management is
important to maintain canopy-layer diversity and allow for
adequate natural regeneration of the trees. Too many shrubs
will hinder the natural regeneration of the trees; too few
shrubs may result in harsh environments for young trees.
Management
of Immature Stands
A common practice in immature stands is thinning. Some of
the trees in the stand are removed to provide additional
light and nutrients to those that remain. Thinning is a
fundamental technique that promotes increased tree health
and vigor, and can also create small openings in the
forest canopy for
light to reach the forest floor. Whenever light reaches the
forest floor, new plant diversity is stimulated.
For timber
management, the smallest or
most defective trees are cut to promote a healthier stand.
However, this “low thinning” may be detrimental
to some Neotropical migrants, especially those that use the
mid-canopy regions. If your forest management goal is to
increase Neotropical migrant habitats leave some dead
standing trees and some nonmerchantable live trees that
occupy the mid-canopy areas. Even small snags (less than 8
inch DBH) will have value for wildlife and should be
retained during a thinning. It is a good idea to vary the
leave-tree density as well. Small groups of leave trees
will provide some protection from windthrow. Commercial
thinning done during the winter will minimize the
disturbance to Neotropical migrants.
On the westside, some landowners thin repeatedly to
maintain and develop multiple age and size classes in
predominantly Douglas-fir stands. This approach is more
likely to be successful on sites where root rot and soil
compaction are not serious concerns. The long-term effects
of this practice on biological diversity and soil
properties are not known.
In eastside forests, multiple-entry thinning has been used
by some landowners as a viable commercial harvest
technique. This method allows them to adjust species
composition for both habitat and economic reasons. A high
degree of skill is required to implement this technique
successfully.
Recommendations
for Each Stand Type
If your goal is to provide for the broad needs of a variety
of Neotropical migrant species, try to attain the best
possible mix of the stand conditions discussed
below. Take into
account the habitat provided by land adjacent to
yours. As stands
develop over time, their habitats change. And as these
habitats change, so do the Neotropical migrant species.
Grass
and Forb Type
If your property already contains some open land, you can
maintain a healthy mixture of vegetation types by
establishing grasses and forbs by seeding, periodic
burning, grazing, or mowing. This kind of habitat can be
created by making openings in existing stands, using small
clearcut patches, and seeding the disturbed area with a
grass or legume mix. On the eastside, forest-land grazing
is common. However, grazing is sometimes incompatible with
ground nesting Neotropical migrants and should be monitored
carefully. Protection of riparian areas from grazing is
desirable whenever possible.
Shrub
Type
Shrubs provide many wildlife species with food, cover, and
shelter. You can meet the cover needs of Neotropical
migrants by leaving existing brush areas unplanted or by
thinning the tree canopy to stimulate shrub growth. Desired
shrubs often require direct sunlight. Shrubs can also be
planted to create habitat, and specialized nurseries have
appropriate planting stock. Try to plant and retain native
species that produce berries. If herbicide use is necessary
to control competing vegetation, consider leaving some
areas untreated to retain shrub habitat. Leaving
wider-than-required no-spray buffers along waterways and
wetlands is also recommended. On the westside, consider
cutting alder rather than spraying, when possible. Cut in
summer after the alder is in full leaf to avoid
resprouting. Alder can sprout even if it is less than 1
inch in diameter at the ground line during the dormant
season. Be sure to cut below the lowest lateral branch.
Immature
Second Growth Conifers
Thin these stands periodically to maintain a healthy,
vigorous condition. Patch thinning is highly recommended in
older stands to provide more light to the forest floor.
Leave some standing dead and defective live trees as well
as cavity trees or nesting trees. Unfortunately, wildlife
diversity in these monetarily valuable stands is often low.
Selective thinning helps maintain trees and shrubs in many
canopy layers. Retain recruitment trees for the next
generation of snags. On the eastside, favor
shade-intolerant or intermediate shade-tolerant species
such as western larch and ponderosa pine whenever possible.
Mature
Second Growth Conifers
West of the Cascades, use a shelterwood system, patch
clearcut, or aggregated green-tree retention system. Use
the shelterwood system on the eastside. The selective
logging system can be tried east of the Cascade crest. If
you use this system, take into account tree vigor, species,
location, and health, as well as age. Do not high-grade.
Select trees to harvest based on their age, species, and
location. Harvest trees that display insects, diseases
— especially bark beetles, root and stem rots, and
dwarf mistletoe. If possible, minimize the period of
disruption by limiting silvicultural activities to small
areas, and complete them in a short time. Simultaneous
logging of adjacent drainages should be avoided. Retain
snags whenever practical. Try to avoid logging between late
February and mid-July in pine forests because of the
increased likelihood of Ips
(pine engraver)
beetle attacks.
It is best to seek advice from a competent wildlife
biologist and or forester before implementing silvicultural
treatments to improve Neotropical migrant habitats.
Checklist
of things you can do to improve
habitat for Neotropical migrant birds in your
forest:
• Maintain habitat
diversity. Retain or
create a variety of tree and plant species, diameters,
heights and spacings. Leave uncut patches, and vary
thinning densities. Consider habitat on neighboring
properties when planning your activities.
•
Protect riparian vegetation along waterways and
wetlands. Restrict
harvesting, herbicide applica-tion, and grazing in these
critical habitat areas. Restore riparian vegetation in
areas where it has been removed or damaged.
•
Retain and create snags and wildlife reserve
trees. Preferably,
select large diameter trees of little or no commercial
value; including damaged, deformed, and large-limbed
“wolf trees.”
•
Establish native trees, shrubs, and seed
mixtures known
to be beneficial to birds.
•
Protect mid to lower canopy level vegetation,
including
berry-producing shrubs, during forest man-agement
activities.
•
Limit herbicide use. Consider
manual control of competing vegetation where practical.
Leave untreated areas, especially those with
berry-producing shrubs, if herbicide application is needed
to control competing vegetation.
•
Install and maintain nest boxes as a short-term habitat
enhancement where preferred
snag and reserve trees are limited or not available.
•
Limit forest management activities during breeding
season. Most
Neotropical migrant birds breed during spring and early
summer (April 1 - August 1).
Copyrighted
by Washington State University Cooperative Extension,
Oregon State University Extension Service and the U.S.
Department of Agriculture, 1998.
Make Your Land More Appealing to Wild Birds:
Maintain Native Plants!
University
of Washington, College of Forest Resources
Wildlife Science Program
Fact Sheet #10, November 2000
The population
of Washington State is expected to double over the next 50
years. As a result, urban and suburban development will
increase dramatically, especially in the Puget Sound
region. Conservation of birds and other wildlife will
become increasingly difficult as natural habitats are lost
(Figure 1 [deleted]). Successful conservation will require
creative partnerships involving regional and local
landowners who manage the land for the benefit of birds.
One way we can begin this process is to increase the
suitability of both our yards and parks as habitat for
native birds. Areas less than an acre or as large as
several hundred acres need special components to be
attractive nesting areas for birds. University of
Washington College of Forest Resources researchers suggest
important resources all landowners and land managers can
provide to increase the suitability of their lands for
nesting birds.
Why
do I need to know where birds nest? Although birds
nest in many different species of plants, they have
preferences. Knowing what types of plants native birds are
likely to nest in can help you choose shrubs and trees that
are both attractive to nesting birds and are suited to the
conditions of your property.
Where do birds nest? Researchers at
the University of Washington have found birds nesting on
the ground, in shrubs, in the lower portions of trees, in
the canopy of the forest, and in dead trees (Figure 2).
Within these zones, birds are more successful at producing
young where dense vegetation obscures the nest. Because
birds nesting in the canopy are very difficult to study,
this fact sheet focuses on birds that can be found nesting
in shrubs and on the ground.
How
can I provide nesting opportunities for ground and shrub
nesting species? Birds will nest
in your yard if provided with the right habitat. However,
birds often have requirements for nesting that are not met
by the average yard. A highly manicured yard with
non-native plantings is not attractive to most species. To
attract nesting birds, plant native species that are suited
to the conditions of your yard (Figure 3). If you want to
maintain some lawn or exotic shrubs, you can provide
habitat for nesting birds by devoting a specific area of
your yard to native shrubs. However, this area must be
fairly large, and protected from predators such as cats
(well fed-cats kill millions of birds each year). A patch
of salmonberry 15' by 15' may very well attract a pair of
American Robins or Swainson's Thrushes—especially if
set against a fence or forest that provides additional
cover. Juncos, Song Sparrows, and Spotted Towhees will use
a similar-sized patch of sword fern or salal. Birds also
nest in larger, but very narrow, patches of vegetation. A
simple solution is to plant a strip of native vegetation
about six feet wide along one side of your property or,
better yet, around the perimeter.
Are there any unusual nesting locations that I should
consider? Yes! Dead upright
trees (snags) and downed logs are important for many
species, both as a source of food in the form of insects
and as nesting sites. Woodpeckers use snags both for
feeding and for nesting. Many other birds such as owls,
chickadees, and Winter Wrens use old woodpecker holes as
nesting sites. Downed logs are also used by many species
for feeding and nesting. Another important nesting location
is among the roots of upturned trees. This is a favorite
nesting spot of Winter Wrens and Pacific-slope Flycatchers.
Although unsightly, please allow dead wood to remain on
your property.
Is providing a nest site good enough? It takes more
than a good nest site for birds to be successful. If you
choose to provide nesting habitat for birds, you must also
be willing to protect the birds you attract. It would be
better to have a yard devoid of birds than to attract them
to a place where their nesting attempts are doomed to
failure. Provide cover to decrease the chance that
predators will find the nest. If cats or rats frequent your
yard, remove them or don' there. Nest failure is also
caused by human disturbance. If you know the location of an
active nest, avoid the area as much as possible until the
young have left the nest. In addition to nesting sites,
birds need food and water. Just allowing your property to
produce natural foods and to hold water during the dry
season can help bird populations.