[CHAPTER XXVI]
PASSERINE BIRDS
We have now run through the list of all the orders of birds except the last and largest—the "passerine" birds, the ordinary songsters of the fields and woodlands of the northern hemisphere. There are fifty families contained in the order. Here, among our North American migratory birds are to be found the kingbirds, pewees and other "tyrant" flycatchers; the larks of our western plains and eastern seashore; that sprite of the Rocky Mountain brooks, the ouzel; the waxwings, the butcher birds; the pretty greenish vireos that build those exquisite, cup-shaped hanging nests made of grapevine bark and spider's silk; and the swallows that become so friendly every summer about barns, paying rent by diligent service in insect killing. Then there is that interesting little group of small and cheerful climbers, the nuthatches, chickadees, and creepers, that rid trees of hosts of injurious insects which they dig out of crevices of the bark as they scramble up and down the trunks, some of them continuing the good work all through the winter. These have their counterparts in Europe, for in respect of our common song birds, as of the birds of prey and game birds, the avifauna of Europe and North America is virtually one. The differences are mainly in the few representatives of tropical groups that visit northern countries in summer, those of Europe partaking of the African or Indian families, while we have wandering species from groups that are properly inhabitants of Mexico and southward. Such, in fact, are our few humming birds, hundreds of species of which belong to the American tropics (and none to the Old World), our two tanagers, members of a very large tropical family, and our blackbirds and orioles, far more numerous in species south of the United States.
While we have many delightful vocalists, the best singers of all our birds are no doubt the thrushes, and that is true of thrushes elsewhere, for the European blackbird and mavis, the celebrated nightingale, the solitaire—both that of the West Indies and that of our northern Pacific Coast—and several noted musicians in the Orient, are of this melodious family. Which is the best singer of them all will never be settled, for the citizen of each country likes best that to which he is most used; but to Americans nothing can be better than the evening carol of the wood thrush, the serene hymnlike music of the hermit, or the sweet and wavering call of the veery. Yet in the South, where these northern thrushes are rarely heard at their best, the palm is given to the mocking bird, which, like the northern brown thrasher, rivals all in turn by simulating their notes in a liquid melody that, especially when heard in the calm of a moonlit summer evening, seems of surpassing beauty.
[CHAPTER XXVII]
THE BEASTS OF THE FIELD—SOME PRIMITIVE TYPES
We have now arrived at the highest rank in the scale of animal life—the four-footed, hair-clad, milk-nursed denizens of our woods and fields—the subclass Mammalia, mammals.
These are the "animals" of popular speech, but accuracy requires a more distinctive expression, for every living thing not a plant is an "animal." Unfortunately no such distinctive term exists in our language, and hence we must borrow from the Latin the word "mammal" for this group. It is correct, easy to remember, and there is no reason why it should not be used popularly as well as scientifically. It is good, because it is exact, and expresses the one great distinction which separates mammals from all other animals—the feeding of the young on milk secreted by the mother. The milk-producing glands were called in Latin "mammæ," whence our word "mammal" and the technical term Mammalia—animals that suckle their young.
Another peculiarity of the group is the coat of hair—persistently growing threads of horny substance produced from the skin in greater or less abundance and of varying quality and color. Its chief purpose appears to be that of keeping the body warm; and, as in the case of the feathers clothing birds, it enables the blood to rise to and maintain a temperature much higher than that of the air; hence the mammals are "warm-blooded." This condition, gradually acquired, stimulated their activity and hence their brain development, the result of which is a higher degree of intelligence than is manifested, as a class, by any other animals, and a moving cause of their progress to the highest plane of organic evolution.
The history of the evolution of the Mammalia may be traced back to obscure beginnings in the Triassic, the oldest of the three divisions of the Secondary or Mesozoic era. Just preceding that time there flourished a group of reptiles, the Theromorpha, whose skull, teeth, and forelimbs were very like those of a modern beast of prey; and zoölogists consider it "altogether probable" that the origin of the mammalian branch must be looked for among their number. It is not doubted, however, that true mammals, although very small and inconspicuous, existed throughout the whole Mesozoic era, despite the fact that the world at that time was filled with ravenous reptiles. Indeed, it is believed that their steady development was an important agency in destroying the reptile population, largely by eating their eggs. At any rate, before the end of the Mesozoic era the two grand divisions of Mammalia, Prototheria and Eutheria, had become established; and also the two primary divisions of the latter, the Marsupials and the Placentals, had been separated. Then came that extraordinary change in the physiography of the globe that marked the end of Mesozoic conditions and introduced those of the succeeding era named Tertiary. In the broader and higher land areas and the drier and more invigorating climate that followed, producing a vegetation tending constantly to become like that of the present, mammals found increasingly favorable conditions, and became the dominant race of animals.