Birds and Forests
Why do forests need birds? And what do birds need in a forest?
By Lee A. Pfannmuller and Janet C. Green
Widely distributed in the eastern deciduous forest, the wood thrush has declined steadily during the past 25 years. The cerulean warbler, another inhabitant of deciduous forests, has had a population decline of 4 percent each year since the 1980s.
Statistics like these led scientists to the conclusion that some Neotropical migrants, birds that winter in the tropics and nest in North America, were experiencing population declines. Indeed, radar studies revealed that small bird migration across the Gulf of Mexico in the 1980s was only about half of what it had been in the 1960s.
At the same time, North American forests have been undergoing change. That is especially true in Minnesota, where residential and commercial development are nibbling away at forests, and clear-cut pulpwood harvests, especially in central and northern Minnesota, are steadily increasing and simplifying forest composition.
The apparent decline of forest songbirds and the ongoing transformation of forests they live in have caused scientists to take a new look at the relationship between forests and songbirds. This re-examination confirms not only that forests are critical to the survival of many bird species, but also that the birds' presence is essential to maintaining the health and productivity of the forest. This symbiosis is just one of several reasons forest managers must aim not only to grow healthy trees but also to meet other goals, including protecting and maintaining healthy forest bird populations.
To develop landscape management tools to maintain the state's diversity of forest birds, the Department of Natural Resources and the Natural Resources Research Institute at the University of Minnesota, Duluth, launched Minnesota's Forest Bird Diversity Initiative in 1991 (see "The Great North Woods Medley," Sept.-Oct. 1992 Volunteer). The initiative has collected data from more than 1,200 sampling points throughout Minnesota's forest regions.
Analysis of data from these points and from other monitoring efforts has provided valuable information for forest managers, some of it surprising, some worrisome, and some encouraging:
- Analysis of data from northern and east-central Minnesota shows that most breeding bird populations have been relatively stable and more species have been increasing than decreasing.
- Preliminary results in southeastern Minnesota, however, suggest that several species might be suffering long-term population declines.
- Since 1982, 18 of Minnesota's forest birds have declined, eight of which are Neotropical migrants.
These findings resemble others nationwide. Neotropical migrants (which comprise 44 percent of Minnesota's forest species) have generally remained stable in the West. But in certain regions, such as the Adirondacks and the Great Smoky Mountains, some forest-dwelling migrants are declining significantly.
In the long run, what we learn about the needs and value of birds will help forest managers determine what can be done to help species that are in trouble.
Value of Birds
There are many reasons to place high value on our forest bird species. People enjoy hearing their songs. Many people spend hundreds of dollars a year to feed birds in their yards. Some spend thousands to travel to far-flung locales to view uncommon species.
But few people consider the benefits birds provide in the functioning of forest ecosystems.
One role of these birds is predators of leaf-chewing insects. Almost all forest birds eat insects, primarily larvae of butterflies and moths, resulting in greater tree growth. Some forest birds, such as the black-backed woodpecker, have an impact on pine beetle outbreaks. Birds also help control spruce budworm (see "Spruce Budworm," below). One study attributed an economic benefit of $4,720 per year per square mile to birds preying on the western spruce budworm.
Woodpeckers hasten the decay of dead trees. Feeding activities can turn a strong and sturdy tree stump into a much smaller, more decayed stump in just a few years.
Another big role for the birds is seed dispersal. Oaks depend on acorn-consuming wildlife as dispersal agents. Blue jays have a thick bill to pound and break acorns into smaller pieces and an elastic esophagus that allows them to carry several acorns at a time. Ecologists count blue jays as one of the most important mechanisms for oak dispersal and oak forest succession.
Birds also serve as food for other forest birds, mammals, and reptiles. Eggs, young birds, and adults all are prey for animals such as red squirrels, fishers, mink, hawks, and owls.
What Do Birds See in the Forest?
Birds cannot survive without suitable habitat. Each species has a unique combination of environmental and physiological needs. What vegetational characteristics are important to forest birds? What species should land managers focus on? How do we maintain or produce the habitat features vital to these species?
It's not easy thinking like a bird, particularly when it comes to identifying what makes a forest quality habitat. A big challenge for biologists is to try to tease apart the components of a species' search image: the criteria it uses in finding a place to call home. Given what we know about birds today, birds' search images appear to operate at three levels: the landscape, the forest stand, and the microhabitat.
The factors that a bird takes into account when selecting a piece of real estate for raising its family are, in many ways, similar to those you and I consider when we look for a new home. At the landscape level, we take into account general regional concerns: Do we wish to live in the suburbs, the city, or a rural community? Then we look for neighborhoods where the schools are good, community services are accessible, and transportation to and from our work is efficient. Finally, at the smallest scale, we look for amenities in a home: the number of bedrooms and bathrooms, the size of the lot.
It is easiest to begin examining bird-habitat relationships at the forest stand level because it is the level traditionally used in forest management. It is also the level at which most research on habitat requirements for birds has been done. At this level, the most influential factor for the birds is the density and number of vegetation layers.
Each stand generally has four layers: ground, shrub, subcanopy, and canopy. Different birds specialize in foraging and placing nests on different layers. Ovenbirds, for example, nest and forage on the ground. Canada warblers prefer the shrub layer. Flycatchers, such as the eastern wood pewee, sally out into the open space between shrubs and the canopy to catch insects. A good example of a species that requires more than one layer, the red-eyed vireo forages in treetops but nests in the shrub or subcanopy. The larger the number of vegetational layers in a forest, the larger the number of bird species the forest will support.
Birds also select habitat based on plant species available. The single most important predictor of a species' presence is whether the forest stand is coniferous or deciduous. Minnesota's forests support 42 bird species that require conifers either as contiguous forests or isolated trees or groves.
Many birds do not exhibit strong habitat relationships. The least flycatcher, for example, resides in lowland conifer and lowland deciduous stands as well as in upland conifer and upland deciduous stands. The hermit thrush can be found in lowland conifers and both young and mature upland conifer and deciduous stands. Common species are often generalists, which is probably the reason for their high numbers.
A cautionary note: Individual birds exhibit considerable variability in their habitat selection. In a long-term study in New Hampshire, 100 percent of ovenbird nests were found on the ground. But nests of the veery were found on the ground 36 percent of the time and in shrubs 64 percent.
In addition to stand-level considerations, birds take into account microhabitat features within the stand, such as the amount of dead woody material and mixtures of several tree species. The downy woodpecker and 31 other Minnesota forest species rely on tree cavities and snags for feeding and nesting. A deciduous canopy that also has some scattered mature white pine or white spruce might harbor conifer-dependent black-throated green warblers. Small balsam firs in an aspen stand might attract magnolia warblers. Cedar waxwings might look for berry-producing shrubs in the understory. Ruffed grouse need woody debris on the forest floor for drumming logs. Winter wrens seek roots of upturned tree stumps for nesting.
he third lens birds use when selecting a home is how the habitat looks at the landscape level. At this large, panoramic scale, several factors come into play: the degree of fragmentation, the size of the forest patches, the proximity to edges, and the patterns of vegetation composition.
Forest fragmentation occurs in landscapes with distinct contrasts between land uses, such as between wood lots and farms. Habitat fragmentation, on the other hand, occurs when one contiguous habitat type, such as a conifer-dominated forest, is fragmented by logging, converting it to another type, such as aspen regenerating after clear-cutting.
Most landscape-level bird research has been done in the heavily fragmented forests of the eastern United States where researchers have focused on one major consequence of forest fragmentation -- edge effect. Within a fragmented landscape, forest patches are more likely to be small and have more "edge" per unit area of forest. Edge habitats are ideal for many nest predators, such as crows, raccoons, and one of the most insidious predators, cats, both wild and domestic. Many songbirds nesting in small forest patches experience very low reproductive success because of the edge effect.
Brown-headed cowbirds, which lay their eggs in the nests of other birds, search for adoptive nests in trees at the edge of a forest stand. Many studies have identified parasitism by cowbirds as detrimental to songbird nesting success. A female cowbird can lay up to 40 eggs a season in other species' nests. Cowbirds need open short-grass habitat, such as pasture, in which to feed. In forested areas managed by clear-cutting, cowbirds are not a problem unless agricultural land or rural cottage development are present. Cowbirds are a big concern in areas of mixed land use, such as the fragmented forests of southeastern Minnesota.
Research in the eastern deciduous forest has also identified an area effect associated with forest fragmentation. Some species require large blocks of habitat to successfully reproduce. Minnesota area-sensitive species include the northern goshawk, pileated woodpecker, and palm warbler.
An important question is whether forest management that creates edges is as detrimental to such species as permanent forest loss caused by urban development and agriculture. Much remains to be unraveled here. However, research suggests that edges created by forest management do not negatively impact forest birds that nest in the shrubs or canopy, but they do adversely impact ground nesters, such as the ovenbird.
The abundance and diversity of birds in our forests presents both public and private forest landowners with a variety of challenges. Not all species respond the same to changes in the forest at the stand, microhabitat, or landscape scale. In the fragmented forest landscape of southern Minnesota, strategies for benefiting breeding birds can vary considerably from those in the extensively forested landscape of northern Minnesota. Depending on management goals and options, land managers have many choices. All involve trade-offs. Some bird species will benefit, and some will lose. Our challenge as stewards of Minnesota's forests is to maintain a healthy and diverse forest ecosystem that can continue to provide habitat for a wide diversity of birds for generations to come.
For a free copy ofPlanning for the Birds: Things to Consider When Managing Your Forest, contact the DNR Information Center. If you own 20 acres or more of forestland, you can learn more about managing forests for the birds by contacting the DNR Forest Stewardship Program, also through the DNR Information Center.
Minnesota Birds and Forests is available from Minnesota's Bookstore, 651-297-3000 or toll-free 800-657-3757.
Lee A. Pfannmuller is Chief of the DNR Section of Ecological Services. Janet C. Green is a Forest Conservationist and the author of Minnesota Birds and Forests.
Budworm larvae feed on buds and new needles of spruce and fir trees, weakening the tree and making it more vulnerable to other insects and diseases that ultimately kill the tree. It usually takes three years of infestation to kill the trees. The budworm is an integral part of the ecology of the boreal forest and predation by birds is an important component. Several species of warblers, including blackburnian, Cape May, and bay-breasted warblers, are well-known budworm specialists.
A novel experiment conducted in the western United States demonstrated how effective birds are in controlling the budworm. Fir trees enclosed in plastic netting to prevent birds from feeding on larvae had six times more budworms than did uncaged trees. The conclusion of this and many other studies: Bird predation can lessen the seriousness of endemic budworm infestations. However, when an outbreak of epidemic proportions occurs, the population of larvae grows so rapidly that the birds become satiated before they have an impact on the budworm population. Some research has shown that, by keeping the endemic populations in check, birds lengthen the time between spruce budworm outbreaks.
Although the relationship of birds and budworm in coniferous forests has been most intensively studied, an innovative study in Missouri demonstrated the contribution of birds to the productivity and health of white oaks. Saplings were caged to prevent access by forest birds. The caged saplings harbored significantly higher numbers of insects than did the uncaged control saplings. The following year researchers compared the leaf, twig, and total biomass of the caged oaks to that of the control saplings. All three measures of growth were less on the caged saplings. This led the scientists to conclude that forest birds enhance oak growth by consuming forest insects. Forest management practices that promote the conservation of these birds are essential for maintaining forest productivity.
--Lee A. Pfannmuller
Why Focus on Forest Birds?
Of the 421 bird species officially recorded in Minnesota, 150 depend on forests, trees, and shrubs, including six species of thrush, six of vireo, and 29 of warbler. Forest birds comprise nearly 70 percent of all vertebrate species in the state's northern forests.
Minnesota's diversity of forest birds is of national significance. The forest belt that extends from the maritime provinces and New England states west through the Great Lakes and across northern Minnesota supports a greater diversity of breeding birds than anywhere else in North America. High breeding bird diversity occurs along the compositionally diverse transition between the boreal forest to the north and the deciduous forest to the south and east.
Researchers have chosen to focus on birds because they are visually and aurally more conspicuous than other animals, so they are easier to study. The male white-throated sparrow's loud and distinctive Poor Sam Peabody-Peabody-Peabody is recognizable to nearly everyone who spends time in northern Minnesota. Field ornithologists skilled in bird song identification can quickly collect important data to monitor songbird populations.
Finally, birds have a public constituency that is rapidly expanding. People value birds for their contribution to the overall quality of our natural environment.
--Lee A. Pfannmuller