Page:The New International Encyclopædia 1st ed. v. 13.djvu/528

MIGRATION OF ANIMALS. 476 MIGRATION OF ANIMALS. north and >uiitlij arc iiiigraiits, ainl tlic total is a very small part of the entire avifauna of the ^vo^ld. Taking up the character i)f the nii<;ratory birds as a class, it appears, first, that they are such as cither subsist wholly or mainly on soft- bodied insects, larvie, worms, and the like, or give their young such fare; second, such as gain their living from fresh or brackish waters or nuid, which is likel}' to freeze: and third, such as fol- low small birds in order to prey upon them. It is also significantly true that they represent fami- lies whose mass and alliliations are found in the tropics, in many oases only one or two species being known elsewhere. Europe's single cuckoo, our single (Eastern) humming-bird, our few tanagers, orioles, and the like, are familiar and striking examples of this fact. On the other hand, the non-migratory or 'resident' birds of the temperate zones belong to families mainly dis- tributed outside the tropics, and sejjarable, broad- ly speaking, on other grounds. This state of things points to the explanation that the extra- tropical parts of the world, depopulated of birds by the cold, ice, and excessive rains of the Pleistocene or "(ilacial' period, were restocked from the crowded intertropical preserves as fast as the amelioration of the climate permitted plants and animals to occupy the temperate and subarctic regions; and that the reactive effect of the new country steadily checked colonization by selecting only those species adapted or adap- tive to the new conditions. In this light, the seasonal migration of birds must be viewed as an annual excursion, constantly repeated by certain species that have the habit land not by others), outward from equatorial regions to a greater or less distance poleward. Beginning of the Seasonal Movement. As the close of the rainy season ajjproaches in the tropics migratory strangers gradually separate themselves from the resident birds, now beginning domestic cares, and disappear. What starts them off, just as the rains are bringing an increase of both plant and insect food, we do not know. Their ovaries show little pre|)aratory enlarge- ment, and few or none are mated. .s they slowly proceed, keeping pace with the lifting sun and the opening spring, they will gradually concen- trate upon certain highways, or 'migration routes.' The old males take the lead, probably merely through siperior strength of wing: and it is not until the hulk of these have passed by that the females ;ippear, followed, after an interval, by yearling birds. The weather encountered, always uncertain, influences this progress decidedly, warm souther- ly winds encouraging the birds to go forward, while cold spells or northerly storms check them, sometimes for a fortnight or more, and occasion- ally destroy large numbers. When sunshine and southerly breezes again prevail the acoiimulated host goes forward in what observers call a "wave' of migration. Such checks are hical : and larger influences have an effect, so that the movement is nnifonnly earlier in some parts of the con- tinent than in others. Mic.RATiON RofTES. Tt is also true that the movement is not uniformly distribtited. On the eontrarv', there are certain definite routes or paths which birds follow in especially great numbers. The greater of these routes or 'fly lines' are generally recognized and seem to be determined partly by topograpliy. but to a greater degree bv considerations of security and sub- sistence. The most thickly fre(iuented routes are along ocean coasts, river-valleys, or mountain ranges. European specialists, like Palmen and Middendorf, have outlined several such 'fly lines' with great particuhirity. and when sketched upon a map they are seen to coincide in a general way with the valley system of that continent. .Simi- lar highways are traceable in North America. One runs along the Pacific coast, ami another u|) the valley of the Rio (irande and along the connected valleys and parks between the parallel ranges of the Rocky Mountains. East of the plains a horde of spring birds enters the United States along the ea.stern lowlands of Mexico, and by way of the West Indies, and soon divides into definite streams of travel. Parted first by the southern extremity of the AUeghanies into two main currents, one goes to the right up the Atlantic coast and through the Hudson Valley and Xew England, while a second, to the left, ascends the Jlississippi, Missouri, and Ohio, di- verging more and more up tributarv valleys, until all the central and northern parts of the continent are supplied. A little reflection will show how likely, from the nature of the case, are these routes. They are natural bird-roads, without obstacles, and they afford easy guidance, plentiful vegetation, and eonsc<iuent protection against enemies and storms, and the abiuidance of insect food belonging to watercourses. XocTiKNAL Axn HiGii-Fi.viN(i Mkikant.s. a large part of the migration of nianv birds is made at night, especially the natural night-fiiers. like owls and goat-sickers, and the great body of small, timid birds, that in their daily life .seek conceal- ment and obtain their food in shadv. secluded places. It is onlv the day-goers that we or- dinarily see migrating, and these more eoni- monlv in spring th:in fall; hut every observer has note(l how after a favorable night the woods will be full of birds at dawn where none were to be seen the evening before. English and American ornithologists, led by Baird, about 1S7.5. have col- lected at light-houses and other watching places systematic data in respect to night migration. Ol'idance. What guides these travelers on their annual journevs across continents and over seas? This has been an eager question ever since men began to watch the ways of the birds. If anything approaching a rule has lieen dis- closed, it is that diversity prevails rather than imiformity. Binis closely allied in structure, diet, nesting habits, and so forth, vary im- mensely in the extent and manner of their migra- tions. W'hile some travel twice a .year from the equator to near the poles, others of the same family, or geiuis even, never leave warm lati- tudes at all. The increasing perception of this individualit.v in aninuils inclines one more and more to believe that migratory birds are guided by the teaching of their elders, and by their own observation and memorv. rather th;in by any ex- traordinary' faculties or process. Brewster de- clares that the manner of migration of our birds is determined by one. two, or three of the follow- ing considerations, viz.: (I) Habitual manner of procuring food: (2) disposition; (.1) wing- power. Much evidence exists in favor of this simple and practical explanation, hut unfortu- nately contrary and unexplained facts still con-