Northward Movement in Let us now consider the actions of the real migrants, who have been spending the winter in tropical America, on the arrival of spring. Nothing that we can see compels them to move, yet they abandon the delights of their winter home and proceed northward as soon as the proper time comes. This "proper time)) seems to have no relation to the weather or to food-conditions there in the tropics, which are almost change less, hut is determined by the weather and food conditions the bird will find when it ar rives at its northern destination, if the season there be an average one. This varies with the requirements of different birds, so that some start much earlier than others. Thus the ducks and geese, which ask only that the rivers and ponds in the north shall be free from ice, come to us much earlier than do the warblers and flycatchers that must wait until flowers are in bloom and insects numerous. As the last are the most numerous they come in crowds soon after the leaving-out of the north ern woods and orchards.
Not all, however, pursue the same route, although each species keeps to its traditional path until it arrives in the district suitable to it, when it scatters. The configuration of con tinents, narrowing into a mere isthmus be tween North and South America, permits only a very narrow land-path for the migrants be tween their winter and summer resorts, yet most of those who winter south of Panama crowd along this narrow neck. Certain shore birds, confident of their strength, strike straight north from Brazil to Nova Scotia, and a few species follow the line of the Antilles from Venezuela to Florida. Birds whose destina tion is California and northward follow the western coast of Mexico, and those aiming at summer homes in the Rocky Mountain region pass straight north through central Mexico and across the desert, or skirt the eastern coast of Texas and the plains. All these are habitual routes for certain species. The great body of migrants, whose songs are later to be heard in the eastern United States and Canada, fly straight across the Gulf of Mexico from Yucatan to landings on the northern shore all the way from eastern Texas to Florida. Ar rived there — and as a rule this is about day light, the journey from Yucatan having taken but • one night — they spread northward along two main channels, one up the coast eastward of the Alleghanian Mountains (diverging into these uplands at each river valley) ; and two up the Mississippi Valley, separating into bands that ascend every tributary, and rapidly cover the whole country, while those bold species, sur prisingly many in number, who are content only in the subarctic zone, hasten on to Alaska and down the Mackenzie Valley. The point to keep in mind in this general sketch is that every species pursues the same route every year, and sometimes it is far from the most direct one. The speed of travel is not great as a rule. The birds must feed by the way, and this food must be found as a rule by daylight; therefore, most species travel during the night, and are often delayed by foggy, rainy nights or by cold storms. The swallows, swifts, night jars, hawks and some others that capture their prey on the wing feed as they go and prob ably rest at night. Those bound for the far north must and do move more rapidly than the southern breeders; and Cooke has gathered some very interesting statistics on this matter.
Autumnal The autumnal re turn of the birds presents some very different features. It begins in the far north before any change in weather or food suggests the necessity of departure, and is led by male birds, after whom the mothers and young fol low as soon as they have strength to travel.
i The movement is far less direct and hurried than the spring flight. They make a long stage hy night, flying sometimes a mile or more above the earth, and drop down at dawn to feed and rest, then loaf along. The members of each species gather gradually into the regular route, which in many cases is quite different from that followed northward in May; and those that are inclined to flocking at this season form large companies that go on together, striking out at last from the Gulf shore to cross in a night that space of dark water, or disappearing in the Mexican forests.
How do Birds in Migration find Their Way?— We have seen that they follow definite routes; also that these in places lead across wide spaces of water; also that the routes in some cases differ according to season; and had we been able to give more details it would appear that often these routes are very eccen tric. Moreover, they travel mostly at night; and finally certain species cross areas of ocean hundreds of miles wide, and far from land, as when golden plovers fly from Nova Scotia to hundreds of miles wide, and far from land, as in the case of the curlews that migrate be tween Australia and New Zealand. It is also pleasingly evident that birds return year after year to the same grove, dooryard and nest ing-place. How do they do it? It was formerly taught that they followed landmarks, such as coast lines, ranges of mountains and large river-courses, which are visible even on clear nights from a great height, and doubtless these are aids to the day fliers and when the sky is clear. But many lines of migration cut across such landmarks, instead of follow them, and others stretch across wide plains and vast water-spaces. Other theories, as of magnetic influences, etc., are without value. It appears plain that birds are guided by an innate sense of direction. This need not be esteemed miraculous, for in a lesser degree it is possessed not only by various other animals, but by wild men, espe cially those who dwell in a forested region, where in following game they would become lost daily had they not a faculty for orientation. The direct testimony to such a faculty in savage mankind, and comparison of the evi dent ability of many animals in this direction, makes its presence in the minds of birds easily credible, the more so as the sense is by no means infallible, since birds sometimes be come completely bewildered when buffeted about at sea by high winds. Add the ele ments of observation and memory and a suffi dent explanation is at hand of how migratory birds find their way.
Bibliography.— An early treatise on this subject of much value is by Alfred Newton in his 'Dictionary of Birds) (London and New York 1893-96). A more recent review is a part of the introduction of Dr. Frank M. Chapman's 'Handbook of Birds' (2d ed., New York 1912), which includes an extensive bibliography. The latest and most important results of study are in the writings of W. W. Cooke, summarized in Bulletin 185 of the United States Department of Agriculture (Washington 1915).