Birds are an integral part of the American landscape, and their numbers and kinds vary as the landscape varies. Our lofty mountains with their adjacent rolling hills, our wide valleys with their meandering rivers, grasslands, and fields of grain, our sculptured coastlines and sandy beaches—it is to these panoramic wonders that birds add color, song, movement, and, above all else, life itself. The cry of sea birds along New England’s coastline, the clatter of wintering geese on the back bays of Virginia’s lowlands, the statuesque herons and egrets atop the cypress and the mangroves, the raucous call of crows above an Ohio cornfield—these things are as symbolically Americana as the physical terrain upon which they dwell.
America is especially blessed with birds, both in numbers and in varieties. Mostly, this population is not a static one, but one that changes with the seasons and with the environment.
As the cold days of winter settle upon the land, the song sparrows, catbirds, and towhees give way to the white-throats and juncos. The field sparrows and bobolinks no longer feed upon the seed heads of foxtail and bluestem grasses; in their stead, we hear the tinkling of tree sparrows and redpolls. The daily preachings of the red-eyed vireo no longer resound from his woodland pulpit. The bluebirds (except the pair in the sumacs) have gone. The nest in the box elder is wind-blown and deserted; but the tree’s winged seeds, hard-fast to the supple twigs, rustle their defiance at the winter winds. Their fate is sustenance for evening grosbeaks and purple finches. Only now, in the quiet bleakness of winter, can we hope to see the snowy owl, or his miniature counterpart, the saw-whet.
Springtime brings a change in mood and tempo. Life is breathed into the earth once again, and it responds with a crescendo of song and the beat of birds on the wing.
All movement is northward. Flocks of tree swallows swirl in cyclonic fashion as they leave the sedges and shrubs where they roosted for the night. Gradually the flocks thin, and flight is resumed; they will feed as they travel. Waves of warblers progress from treetop to treetop as they rest and feed; their long flight will be resumed in the darkness of night. Everywhere, all across the land, birds are on the move, and the tide rolls ever northward.
In the fall, this great tide reverses itself; the flow of life is southward. The sandy spits along our sea beaches overflow with migrating shore birds. Thousands of ducks leave the sloughs and potholes of the Midwest and prairie provinces of Canada. And the warblers—their thin lisps in the stillness of the night reveal their myriad numbers—are on the wing again. But come the dawn and daylight hours, they can be seen flitting about the treetops in search of insects to replenish their energy for the coming night’s long journey.
But it is during the warming months of summer, when birds are at home, that we can observe the excitement of their living. All activities seem to take on a hastening pace; there is so much to be done before chill winds once again force the departure of most species. For some, there is no need to hurry. The red-bellied woodpecker can spend days drilling his hole in the hard dry stub of a pine; he will not move with the cold. The goldfinches can remain in their social flocks; the milkweed pods will not release their down and seeds until early July. But for those who will leave in the fall, time is of the essence. There is territorial homesteading to be done, and a mate to be found. Hours will be spent in singing and warding off intruders. A hundred searching trips or more must be made to find appropriate nesting materials. Eggs must be laid and brooded, and thousands more forays made in search of food. The young must be taught to fend for themselves, and then, only then, may the pace of living be slowed. And slow it does, for there is a mysterious quiet about the sun-baked gardens and woodlands during the hot days of late summer. Old Red-eye now preaches to an unresponsive congregation.
Environmental changes also help keep much of our bird population in a state of fluctuation. Natural succession is slow, but nevertheless effective. A pond in the open, edged mainly with water lilies, may appeal to the pied-billed grebe and a few dabbling ducks. As grasses, sedges, cattails, and loosestrife begin to dominate the edges, these few species will be joined by others such as red-winged blackbirds, swamp sparrows, marsh wrens, and Virginia rails. The appearance of shrubs—willows, black alders, and buttonbush—will bring yellowthroats, song sparrows, catbirds, yellow warblers, and other shrub-loving species. If this natural succession were allowed to continue undisturbed, the forest would eventually be the permanent habitat of the pond’s borders. Birds of the previous pond-edge communities would be gone; chickadees, titmice, vireos, nuthatches, ovenbirds, woodpeckers, and other woodland birds would be dominant.
When man enters the scene with his monstrous machines, environmental changes are apt to be radical. With a few swipes by a bulldozer or backhoe, a sand spit, a marsh, or a fencerow can disappear from the face of the earth. The sad part of such intrusions is this: there is little recuperative power in a habitat of concrete and steel.
But change is the essence of bird watching’s appeal. From season to season, from mountain to valley, from field to forest, no matter when or where you go, there is a constant newness each time you are afield. Each type of habitat supports a different variety of birds. Birding knows no season, requires no license, and is of interest to all age groups. Birds belong to us all—free for the watching.
We think of a bird’s habitat as being the place or particular type of area in which it has a habit of living. Actually, it is much more than that; it is a combination of interacting physical and biological (community) factors that produce an environment to which an individual species has become best adapted through innumerable generations. Each species remains in, or returns to, the type of habitat in which it was born. Although a certain habitat is often home to a variety of birds, we speak of each species as being best suited, and adapted to, a specific habitat. When all plants and animals within a given area are considered, we refer to the area as a community.
Most species of birds are so instinctively bound and so physiologically adapted to one type of habitat that they cannot tolerate environmental changes, nor can they survive by moving to a totally different habitat. Other species, however, are ecologically tolerant in varying degrees. In my home state of Florida, the Everglade kite faces possible extinction because of its complete dependence on a delicately balanced fresh-water habitat. It feeds exclusively on the fresh-water Pomacea. This snail breeds and thrives in the flooded shallows of the Glades. But man has entered the scene. Hundreds of miles of straight-line drainage ditches (ostensibly for flood control) and the indiscriminate use of pesticides have narrowed the habitat in which the snails and the kites can survive. By contrast, the robin is a most tolerant species; its food habits are not so specialized. I have seen it nest in the gardens and parks of the Carolinas, along the shrubby fencerows and woodland borders of New England, and in the openings of Quebec’s northernmost forests. Still other species have a moderate tolerance for environmental changes. If we are to know where to find and watch birds, we should have a knowledge of these tolerances and of the dominating forces within the bird community. Why are the birds there? What attracts them? What are the relationships between species and selected home sites? An understanding of the functions within the bird community will make our trips afield more successful and more meaningful.
The bobolink returns to its home in a Wisconsin hayfield. It welcomes spring from the bobbing tip of a dried goldenrod stem, its song bubbling forth in a series of alternating high and low metallic notes. The wood thrush announces its homecoming from the deep shade of a Pennsylvania forest with a song of the flute—clear, mellow, and ending with a vibrato that fades into the shadowy depths. The American bittern claims its territory by “driving stakes” along the marshy borders of a remote lake in Maine. Everywhere across the land, birds return home in the spring to a particular biological niche in the outdoor community.
The bobolink may return to the very field in which it was hatched slightly less than a year ago. It does this not by choice, or by any calculated evaluation, but by instinct alone. The young bobolink is the culmination of all the inherited reflexes, responses, habits, and abilities of its parents and countless generations of its ancestors. Have they not made the great flight down the east coast of our continent and across the Bahamas to Jamaica? And then another five hundred miles across an islandless ocean to the shores of South America? Have they not spent a hundred thousand winters on the pampas of Argentina? Have they not returned to sing from a hundred thousand golden-rod stems? Have they not known fear at the screaming of a hawk, or the quiet passing of a fox?
Yes. All the experiences of living to be encountered by the young bobolink have been known countless times before. But unknowingly, and in an infinitesimal way, it will contribute to the inheritance that will assure the survival of future generations.
The wood thrush senses security in the cool shade of the forest. Its large eyes have developed a special keenness in the subdued light; it avoids the bright sunshine. It experiences little competition as it forages about the forest floor, flipping dead leaves aside with its bill, seeking crickets, grubs, spiders, ants, flies, and earthworms. Most of its woodland neighbors either are seed eaters or catch their insects at higher levels. The wood thrush is an understory specialist—it feeds low, perches low, and nests low. And always, its tawny back blends with the protective browns of the forest floor. Just as were its progenitors throughout the eons, it is a creature of the forest community. It takes, it gives, and it survives.
And the bittern, has it not always known the wetness of the marsh? Over the great span of time past, it has been so much a part of this wetland habitat that the striations of its plumage now mimic the reeds and grasses in which it lives. Unlike most herons, its way of life is solitary and secretive; it does not feed in flocks, nor does it colonize for nesting. It is a loner—a master of stealth and concealment.
And so it is with the ducks and the geese, with the hawks and the falcons, and with the warblers and the sparrows. And so it is with the plovers and the sandpipers, and with the terns and the gulls. Each species instinctively fulfills its destiny as an active member of the particular community to which it is irrevocably bound.
To the human eye, the outdoor community in which birds live presents a deceptive façade of harmony and tran-quillity. In reality, it is a composite of dynamic forces that function interdependently in the never-ending struggle for survival. It is not unlike the community in which man himself lives; it has its basement dwellers, street-level residents, and high-rise occupants; it has its own factories, shopping centers, police force, garbage collectors, robbers, and parasitic welfare cases. The community functions as a circuitous chain of events propelled by the energies of birth, competition, and death. The green leaves that have survived the competition for sunlight and are structured by the carbon they have taken from the air may satiate the appetite of a ravenous caterpillar; the life juices of the caterpillar may flow in the bloodstream of a newly hatched cuckoo; the cuckoo may strengthen the sharp-shinned hawk who, through his own demise, will eventually release the carbon for use by other green plants. Life within the community beats with a fundamental rhythm of natural laws. Birds are an essential part of this rhythmic beat.
There are numerous and varied physical and biological factors associated with each type of bird community that largely determine the numbers and varieties of species it can support. These factors, when divided into their innumerable components, associations, and relationships, form the basis for detailed ecological studies. Obviously, that is not the purpose of this book, but we should be concerned with the major and more recognizable factors of community structure. This will help us determine where we have the best chance of finding certain species and give us a working knowledge of what species we can expect to find in a particular habitat.
Plants—not birds or other animals—are the structural backbone of the community. Either directly or indirectly, they are the source of all energy necessary to maintain the lives of all organisms. In most communities, they provide food, home sites, and protective cover. From the minutest plankton to the most stalwart of trees, all plants are in some way involved in maintaining the rhythmic beat of life within the community. Plant growth, as we know, is subject to such climatic and ecological conditions as air temperature, precipitation, humidity, exposure, wind, light intensity, and soil composition. Plant groups that tolerate these conditions in various combinations determine the numbers and species of birds a particular community will attract and support. When a degree of uniformity is reached through a certain combination of these conditions, a corresponding degree of uniformity is reached in the plant life and in the animal life. These factors, along with topography and geographical location, constitute the basis for a specific type of habitat. These principles apply in both terrestrial and aquatic communities.
The importance and contributions of birds to a balanced environment are difficult to evaluate. Their associations with other animal and plant members of the community are so numerous and often so obscure, but nevertheless important, that ecologists may never unravel all the ramifications involved. However, we do know enough about these relationships to realize that birds are a significant part of the intricate web of all living things, including man. We know, for example, that such species as the bobolink, field sparrow, and junco consume tons of weed seeds every year. As far as community balance is concerned, the amount of seeds actually consumed has less significance than the thinning out and distribution of the various plant species from which they came.
In a forest, an elm tree succumbs to disease and the shade of a beech-maple canopy. Woodpeckers chip away the bark in search of grubs and insects; they drill nesting cavities in the softer limbs and trunk areas. In doing so, they hasten the processes that will eventually return the tree to the soil. A limb breaks at a weakened spot and falls; rain water fills the cavities and helps the process of rotting; eventually, a wind storm tumbles the tree to the ground. Here it may serve as a drumming log for the ruffed grouse, or it may provide a temporary home for a chipmunk or a deer mouse. But now the bacteria and fungi take over, and gradually the elm is returned to the soil, releasing its carbon and other elements to be used once again.
Birds might be looked upon as the “middlemen” in the community’s pyramid of numbers. Certain songbirds may rear a dozen or more young in a season, but the seeds, insects, and smaller animals upon which they feed are produced in tremendous numbers. Along with weather, disease, and other natural deterrents, birds act as a “lid” over these exploding populations. Vertebrates higher up on the pyramid aid in the same repressive controls over the songbird population through predation. Species such as vultures, crows, and gulls serve as community scavengers.
Aquatic communities tend to be less stable and not as easily defined as the terrestrial varieties. Nevertheless, the same ecological principles govern the survival of species, and birds are involved in many ways. A duck flying from one pond to another may carry a number of tiny floating duckweed plants on its body. In doing so it helps distribute and perpetuate its own food supply. A bittern feeding in the marsh may spear a minnow infested with tiny parasitic grubs, some of which will remain, live, and lay eggs within the bittern’s mouth. As the bittern feeds, the eggs are washed into the water, where they hatch, feed, and eventually infest another minnow, thus continuing an intricate aquatic life cycle.
By now we know our bird population is widely distributed among a variety of habitats. We also know that feeding habits, nesting requirements, physiological adaptations, and other factors tend to limit many species to a certain type of habitat. This is especially true during the nesting season. During migration, when birds are concerned chiefly with travel routes, feeding, and resting, they can often be observed outside their normal nesting areas. Also, we should remember that the more pronounced natural communities seldom have definitive boundaries; more often than not, there is a transition zone (known as an ecotone) from one community to another. These eco-tones often support more numbers and more species of birds than either adjacent community. For example, the transition zone between an uncultivated field and a hardwood forest may consist of the hardier field plants, a variety of shrubs and vines, and a number of young encroachment trees from the forest. This mixture of vegetation provides an abundance and variety of foods, nesting sites, and protective cover. In the eastern part of our country, this type of ecotone would attract such species as the field sparrow, song sparrow, cardinal, catbird, brown thrasher, yellow-breasted chat, rufous-sided towhee, indigo bunting, prairie warbler, and chestnut-sided warbler.
It now becomes quite obvious: the greater the variety of habitats we visit, the greater the variety of birds we can expect to find.