Popular Science Monthly/Volume 33/May 1888/The American Robin and his Congeners
|THE AMERICAN ROBIN AND HIS CONGENERS.|
By Dr. SPENCER TROTTER.
OUR American robin is a thrush—the red-breasted thrush is his proper title—he occupies a high position in the scale of bird-life, and possesses some very interesting records of his family history. When our forefathers first came over they found the frank, hearty bird with the russet breast ready to make friends with them, to stay about the clearings and around their rough cabins, cheering them with the strong, hopeful song that has ever gladdened the heart with its vigor and fullness of promise. With what joy the pioneers must have welcomed the first spring that brought the robins back after the long, dreary winter! To this day the first robin of the spring creates a sensation, coming, as he often does, amid the ice and the snow and the rough wind, and not a leaf on the trees. The early settlers called him "robin" from his red breast, no doubt, and his confiding ways, after the trusty little warbler so dear to their hearts in the old home across the sea. And so it has been "robin" ever since, although our bird is but distantly related to the little robin-redbreast of the Old World, who belongs to the warblers—another branch of the family.
By virtue of being a thrush, our robin enjoys a very extensive range of country for his habitat. From the Atlantic to the Pacific, across the continent, from the shores of the Arctic Ocean to Mexico and Central America, he is found abundantly, breeding throughout the forest limits of this wide area, and building the same nest of dried grass, roots, and plastered mud about every homestead in the land. Although a bird of the woodland, like all the thrushes, he yet prefers the garden and the orchard—even the trees that stand in the midst of the bustling city hold his nest. Insectivorous by nature, but varying his diet largely with the small, wild berries of the woods, the robin has become, since man's invasion, a lover of fruit, keeping pace with man in the cultivation of his taste. The excellence of his taste can not be denied. He takes the biggest cherries of the most approved varieties, and the luscious straw-berries are his delight. Yet for all the fruit he eats he repays the horticulturist double by devouring threefold more of insect-life that would ultimately cover and destroy the trees, leaf, root, and branch. Fortunate it is that we have recognized his valuable service, and protected him by legislation.
The true thrushes—and the robin may be taken as a type—present some very interesting features in their development, characters, and geographical distribution, a study of which throws light not only upon the history of the birds themselves, but also upon several widely different subjects.
The thrushes belong to the most highly organized group of birds—the Passeres—and are farthest removed in structure from the early reptiloid forms. They possess the most complete vocal apparatus—a syrinx—situated at the lower end of the windpipe, with five intrinsic pairs of muscles. The wing has undergone a reduction in the number of its primaries or quill-feathers growing from the long finger, there being ten of these, the first one short and abortive, so that the thrushes may be looked upon as still advancing toward the highest type of wing-structure, that of nine primaries. In conjunction with this, the foot or leg is "booted"—i. e., covered by an unbroken plate of hard, leathery skin, not reticulated and scaled, as in other forms. A decided change has also taken place in the "molt," or shedding of feathers; the autumnal molt being the only complete process, while the spring change is effected simply by the "casting" off of the broken points of feathers in the worn plumage. The young of all the thrushes are spotted in their nestling dress, but never carry it beyond the first autumn, assuming the full plumage of the old birds after the first molt, so that "a bird of the year" in the late fall and winter is scarcely distinguishable from its parents. Any one who is familiar with the young robins hopping about the lawn in the early summer, with their spotted breasts and mottled backs, may have wondered what became of them by October.
The thrushes are migratory birds in the temperate zone, as the nature of their food demands, partaking in the general north and south movement during the spring and fall tides of migration. Though not strictly gregarious, many of the species associate in loose flocks, on the approach of autumn, and forage over the country in quest of food. We are all familiar with the flocks of robins in the fall, scattering overhead, or in the gum-trees feasting on the ripe, black fruit. They are for the most part arboreal, living largely among the trees, but some of the species build nests on the ground, or in the undergrowth just above it, and all of them frequent the ground at times when food is to be obtained there. Being among the most highly organized of birds, the thrushes are consequently rapid livers, possessing a high degree of vitality, and consuming a very large proportion of oxygen. Great feeders, strong of wing and stout of heart, with warm, fast-flowing blood and high temperature, they are, in every sense of the word, alive to their environment.
The robin and his world-wide congeners form the genus Turdus, or true thrushes, comprising upward of fifty well-defined species distributed throughout the forest regions of the globe, excepting Australia and New Zealand, where they are replaced by an allied group.
In the palæarctic region of the Old World four widely distributed species occur, all having spotted breasts in the adult plumage. The red-winged thrush breeds in the birch region, and throughout the upper belt of pines across Europe and Asia, from the Atlantic to the Pacific. He is rare east of the valley of the Yenisei, but extends his wanderings northward as far as latitude 71° beyond the forest limit. The red-wing winters in southern and western Europe and the British Islands.
The blue-backed thrush, or "field-fare" has a range somewhat similar with the above species; Asiatic individuals migrating in winter to Cashmere, Turkistan, and the northwestern portion of India. The missel-thrush breeds throughout central Europe, ranging eastward to the northwestern slopes of the Himalayas. In such a mild climate as Great Britain offers, he remains the year round, but the majority of individuals winter in southern Europe, Persia, and North Africa. The song-thrush is another palæarctic form, breeding eastward to the Yenisei Valley, and in Norway wandering beyond the Arctic Circle. He has a near relative inhabiting northern and western China, known as Père David's thrush, in honor of a good monk who devoted much time to the study of ornithology.
There are two spotted-breasted thrushes restricted each to a certain island, and found nowhere else: the Anjuan thrush, inhabiting one of the islands of the Comoro group, lying between Madagascar and the African coast; and the St. Thomas thrush, from the island of that name, in the Gulf of Guinea.
In the New World the nearctic, or North American region, possesses several species of spotted-breasted thrushes breeding throughout its forest area. Notable among these are the wood-thrush, whose mellow, rippling music we know and love so well; the hermit, the olive-backed, the gray-cheeked, and tawny thrushes—spring and fall migrants passing through our woods in May and October.
In contrast to the spotted-breasted species, there are a number of thrushes, and among them the robin, which are solidly colored underneath, a few spots being confined to the throat. This difference in color-pattern has undoubtedly arisen far back in the history of the group from some environing influence. The young of these solid-colored thrushes are all spotted like the rest, and, since the young of all animals tend to revert toward ancestral forms and conditions, the spotted-breasted species may be looked upon as representing the more primitive type of thrush. A further proof of this is found in the two spotted-breasted thrushes inhabiting the islands above mentioned, which, from their isolated habitat are undoubtedly of considerable antiquity as species. Our robin is the only representative of the thrushes with solid-colored breasts found in North America, but a host of them occur in South America and some in Africa.
This contrast between the northern and southern continents of the New World, or, to speak zoölogically, between the nearctic and neotropical regions, in the number of species of thrush, has its solution in the peculiarity and variety of physical conditions offered by South America. The ranges of the species inhabiting the great forests of the equatorial zone are in the majority of instances restricted to comparatively limited areas. The varied conditions offered by high mountain-ranges and deep, low-lying forests, tend to the creation of new varieties or local races, which are consequently limited to certain narrow areas, and a particular species is often thus represented by several extreme forms. In temperate North America, on the other hand, the fewer species are kept true by migration, which tends to equalize surrounding conditions.
In northern South America, the valley of the Amazon, and the forests of Brazil, three distinct varieties of the white-throated thrush occur. The common South American thrush, a comparatively widely ranging species, reaching southward to Chili, has an extreme form in the northwest. The Sabian thrush is another species which has split up into several forms under the influence of local conditions. No region in the world is so rich in the number and variety of its passerine birds. In the south temperate portion of the continent the species of thrush are more uniform, and very few varieties are found.
Africa has several well-defined representatives of the solid-colored-breasted thrushes. The Zanzibar thrush and the Abyssinian thrush are eastern forms; in the west, the Ethiopian and Senegambian thrushes are found; while to the southward the olive-thrush, Cabani's thrush, and the Kurichane thrush range throughout the Transvaal, Caffraria and Damara Land, the Bechuana country, and the Cape.
The facts, as they are presented to-day by this interesting group of birds, become very significant when viewed in the light of evolution. The world-wide distribution, large number of established species, and high degree of development which the thrushes have attained, denote their comparative antiquity as a group of birds. Time has been the important factor in establishing the species, and enabling them to live far and wide in harmony with diverse conditions of life. It would be difficult to ascertain the original center of their development—probably one of the great land-masses, as the Euro-Asiatic continent, whence the early forms have spread to other portions of the earth, there to break up into new varieties and species under the action of changing environments.
Where other forms have succumbed in the struggle for life these have lived on, until now, the almost perfect wing and foot; the vital strength that holds the plumage for a year before it is shed, and also enables the mating pair to rear three goodly broods each spring; the vocal development, the omnivorous diet, the abundance and world-wide distribution of species, tell the story of how the robin and his congeners have come to be what they are—a dominant group in the animal life of the earth.