The matter of procurement of food is probably a greater factor than timidity in inducing nocturnal flight. Digestion in birds is rapid, so that food must be obtained at regular intervals to main tain the activity especially characteristic of smaller species, whose tiny bodies have necessarily smaller reserves of energy than their larger brethren. Stomachs of small birds killed at night by striking lights or other obstructions are nearly always entirely empty, which, though an entirely natural thing, in some cases has given rise to belief that the individuals in question were in the throes of starvation. If tiny migrants flew long distances by day they would arrive at some distant destination with empty stomachs and de pleted energies, perhaps almost exhausted, and because of darkness would not be able to procure food until the following morning. Such circumstances would lead to delay in further flights through need for recuperation or, if accompanied by cold or storm, might prove fatal because of the lowered vitality. By arrival at day break it is possible for migrants to rest for a time and then search for food, and so to recuperate that they may continue the follow ing evening if desired. At the same time there is often detected among these smaller species an indication of continuation of migratory flight by day, as in feeding they often tend to move in the general direction towards which the seasonal flight trends.
It was thought formerly that migrants flew at great altitudes above the earth, there being a somewhat hazy notion that rarefied atmosphere in some uncertain way facilitated flight. Modern observations from aeroplanes show, however, that this belief is unfounded, and that the bulk of migrant birds travel at less than 3,00o ft. above the earth. It is unusual to meet with birds above 5,000 ft., though exceptionally aviators have recorded shore-birds at io,000 and 12,000 feet. In many instances birds pass at very moderate altitudes, particularly above the sea, where they may barely clear the waves. At night, the calls of migrants may be often heard, apparently only short distances above the trees.
There has been much discussion of the method by which birds direct their courses in flights to distant lands, without arrival at definite explanation. Memory of routes previously travelled, some magnetic sense, courses laid by the positions of the heavenly bodies, as a mariner directs his navigation, the direction of regular winds, telepathy, television and hereditary memory have all had their champions. It may be said that there appears great proba bility that young birds of the season, on occasion, migrate south ward without the guidance of others that have previously made the journey, that sea-birds return across apparently trackless oceans to remote islands where they nest, and that birds have returned to their breeding stations when removed forcibly to waters that they do not ordinarily frequent. This may, perhaps, be explained in the somewhat general terms, by supposition, that the birds are directed by some special sense of direction, but must be regarded at present as something of which there is no definite explanation.
One of the most interesting facts connected with migration is the almost unfailing regularity that accompanies the movements of birds in the temperate regions. Through records kept over a long period of years, the average dates of arrival in spring of the common birds are now known, and it is found that they appear with almost uncanny regularity, often on the average day and always within a range of a few days earlier or later. Among small birds the seed-eaters generally migrate earlier in spring and later in autumn than insect-eaters, as they find their food with greater readiness. The first arrivals in spring or fall are usually few, and
it may be days or weeks before the mass of individuals appear.
The length of the journeys made by individual birds varies greatly. Inhabitants of high mountains, in a flight of a mile or two, descend to some warm valley, where they may spend the winter in comfort, or the migration flight may entail a movement to a great distance, as from central Europe to southern Africa.
The Arctic tern is supposed to have the longest migration route known, as it nests in the far north and spends the northern winter near the shores of the Antarctic continent, with r r,000 m. in an air-line separating its northern and southern homes. Incidentally, this tern probably enjoys longer hours of daylight than any other living creature, since it lives under the midnight sun in both northern and southern hemispheres, and only in its travels through the equatorial belt does it meet extended periods of darkness.
There are numerous shore-birds, such as the American golden plover, that nest in the Arctic and spend the northern winter on the plains of Argentina. Many birds nest in Canada or in the northern United States and winter in the Gulf States, Mexico, Central America, the West Indies, or northern South America. Likewise, many pass from northern Europe to Africa, some going as far as Cape Colony.
There is almost infinite variety in the migration routes and travels of the many forms of birds, yet in each continent it is found that there is a tendency to converge in great lanes of migra tion that carry the bulk of individuals, though random birds may cross anywhere in the intervening area. The flight-lines often fol low river valleys or coast-lines, which seem to afford assistance in directing the flight. The observer interested in bird life who is located on one of these great air-roads is fortunate.
Through prolonged journeys birds encounter constant perils and dangers, so that many are destroyed. Storms, unfamiliar coverts, with consequent exposure to the attacks of enemies, and wandering from the direct course in crossing broad reaches of water annually exact a toll of unknown thousands of individuals, as has been indicated in a previous section. To enable mainte nance of their proper number migratory species, therefore, are required to produce a sufficient number of young to permit this toll, and at the same time leave a pair to continue the species at the next breeding season. On the average, the migratory species of the north and south temperate zones produce from three to six eggs in each nesting, four or five being the rule, with from one to three families each season, while the more strictly resident form of the Tropics may produce only two or three eggs and have only one nesting annually. In spite of apparent prolificness the more northern species do not increase inordinately. The song sparrow family, in August, at the close of the breeding season, if there has been no mortality, may have ten or 14 individuals, de pending upon whether two or three broods have been reared, yet by next spring does not show any appreciable increase in abun dance; so that there has been a tremendous wastage in individual life during the migrations and the intervening winter.
In final consideration, it may be said that residents in Europe and North America are often prone to consider migration as something peculiar to birds breeding in the northern hemisphere. It must be noted, however, that in South America, southern Africa, and Australia there are native species that, at the approach of the southern winter, travel north toward the Equator, to return at the proper season to their breeding-grounds.