Forest Birds – Background

In early spring, forests of the Northeast come alive with birdsong. Before the leaves come out on the trees, the songs of the earliest arrivals echo through the bare woods. The sweet, high notes of the white-throated sparrow, the Morse code tapping of the yellow-bellied sapsucker, the ethereal fluting of the hermit thrush, are among the first to be heard. Every week thereafter, the chorus increases in volume and variety as other birds return to the Atlantic Northern Forest to breed. What makes this region so important for songbirds?

Reaching from Maine across northern New Hampshire, Vermont, and New York, the Atlantic northern forest covers some 26 million acres. Within the forest are hills and valleys, wetlands and rivers, mountaintops and coastal plains. It is a place where three major forest types come together: the northern boreal (spruce-fir) forest, the northern hardwood (birch-beech-maple) forest, and the more southerly oak-hickory forest. Such a diversity of plant species in one place makes it an especially rich area for birds and other wildlife.

Forest communities have vertical structure; they are stratified into layers of vegetation. The tallest trees form a canopy of leaves that shade the forest below. Beneath the canopy is the forest understory where smaller trees spread their branches to catch what sunlight filters through, aided by breezes that move the leaves this way and that. Bushy shrubs with woody stems make up an intermediate layer of vegetation, and ferns and wildflowers form a layer of herbaceous (non-woody) plants close to the ground. The forest floor is covered with a layer of leaf litter and a scattering of rotting logs.

Each layer provides homes and food for a variety of forest animals. Spiders and insects occupy every layer of the forest, whereas forest amphibians like wood frogs, toads, and salamanders live exclusively in or under the moist leaf litter. Forest mammals range in size from tiny shrews to huge moose and bears. Whereas shrews spend their lives on the forest floor or tunneling under it, bears and squirrels move from floor to canopy, using tree trunks as convenient staircases. Birds occupy every layer of the forest too, and since they can fly they have no trouble getting from one layer to another.

Some of the birds that live in the northern forest are year-round residents. The black-capped chickadee survives the northern winters by seeking shelter from the wind, storing seeds in every available nook and cranny, traveling in flocks, and fluffing up its feathers against the cold. But many birds are temporary residents, coming to the northern forest to breed and raise their young and then leaving in the fall to return to their homes in the tropics. The number of migratory bird species breeding in the northern forest is greater than anywhere else in continental North America.

The different layers of the forest provide homes and food for many kinds of birds. Some forest birds are ground-nesters like the American woodcock and the ovenbird. The woodcock probes for earthworms with its long, flexible bill. It rocks its body while pressing the ground with its feet to make the worms move – an amusing sight. Other birds, like the wood thrush and Canada warbler, occupy the shrub layer where tangled branches provide good hiding places for nests. Birds like vireos, cuckoos, and woodpeckers make their nests higher off the ground in the forest understory. Here they are safe from most predators, though they must look out for the sharp-shinned hawk that occupies this level as well. The scarlet tanager and black-throated green warbler are birds of the canopy, nesting and foraging high in the treetops. By occupying different layers of the forest, these birds avoid competition with each other over resources.

One characteristic of the northern forest is the emergence of huge numbers of insects in the spring. Because of the harsh winters, insects spend much of the year in a dormant state and then emerge all at once in the spring in order to feed, find mates, and lay eggs before the short growing season comes to an end. The abundance of insects – swarms of blackflies, countless caterpillars, and hordes of beetle larvae, among many others – provides a plentiful source of high-energy food for birds to feed their hungry nestlings. In addition, the longer days in the north allow for more time to forage. It is thought that these advantages led to the evolution of migratory behavior in many species of birds.

Whether migratory or residents, birds play an important role in the northern forest ecosystem because they help to keep insect populations in control. Insects that feed on leaves and buds abound and can damage the health of trees when their numbers become too great. Some of these, like hairy tent caterpillars and gypsy moth caterpillars, are favorite prey of birds like cuckoos and tanagers. Cuckoos consume enormous quantities of these destructive insects, then shed their stomach linings to get rid of the hairy spines.

While other northern forest woodpeckers remain through the winter, the yellow-bellied sapsucker is a summer resident only. Nevertheless, it plays an important role in the northern forest ecosystem. Sapsuckers are aptly named because, in addition to eating insects, they feed on the sap of trees like maples and birches. They do this by drilling rows of small holes through the bark of a tree into the inner bark where sugars and starches are stored so they can lick up the sweet liquid with their long tongues. In winter, when the sap is frozen, sapsuckers migrate south. Numerous other animals feed on the sap from sapsucker founts. This food source attracts hornets looking for high-energy food to start their colonies in the spring, as well as wasps, flies, bats, porcupines, and squirrels, to name a few. The ruby-throated hummingbird is thought to time its arrival in the north with sapsucker activity. Before many spring flowers are in bloom, hummingbirds can get a head start on nesting by drinking the tree sap. Butterflies like the mourning cloak, which overwinters as an adult, depend on the sap for energy when they first emerge from their winter rest.

The sapsucker is also important because it excavates its nest in partially rotted trees. A standing snag is a magnet for a host of other animals, and sapsucker nest holes are often used in subsequent years by other birds, mammals, or insects. Many of the cavity nesters that inhabit sapsucker nest holes also feed on insects. The sapsucker is called a “keystone” species because it provides food and nests for numerous other animals which, in turn, help to keep the forest insect populations in control.

Forest birds must be able to find mates, defend territories, and stay in contact with their families. Living in this shadowy world, most forest birds use song to communicate, their voices carrying through the dense foliage. In some species, the males display striking colors that can be seen among the leaves. An experienced birder learns to recognize forest birds by their field marks, special characteristics that are easy to spot and unique to each kind of bird, like a white wing-bar or stripes on the bird’s crown. To some, learning the songs and calls is an easier way to identify birds in the woods, since birds can be hard to see among the leaves. Keeping records of forest bird species helps us to track the health of their populations, which is important to forest health as well.

The Northern Forest is a habitat rich in plant and insect species, making it an ideal nesting ground for many songbirds. Whether migratory or resident, birds play an important role in the forest for they help to keep insect populations under control. A pleasure to see and hear, these jewels of the forest are a valuable part of the forest ecosystem.

Suggested Reading

Kricher, John. Guide to Eastern Forests. Peterson Field Guide Series. New York: Houghton-Mifflin Company, 1988.

Peterson, Roger Tory and Virginia Marie Peterson. A Field Guide to the Birds of Eastern and Central North America. 5th ed.  New York: Houghton-Mifflin Company, 2002.

Thompson, Elizabeth H., and Eric R. Sorenson. Wetland, Woodland, Wildland. Hanover, NH: University Press of New England, 2000.

http://www.birds.cornell.edu/bfl/speciesaccts/canwar.html

http://www.nationalzoo.si.edu/ConservationAndScience/MigratoryBirds/Featured_Birds

http://www.vt.audubon.org

Developed in partnership with Audubon Vermont, 255 Sherman Hollow Rd., Huntington, VT  05462 (802-434-3068) www.vt.audubon.org.

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