A Mystery of Migration
I
AN old mystery begins to take on a new interest; and one of its more recent illustrations particularly concerns inquirers on the two sides of the Atlantic.
There appeared a year or two ago on the coast of Wales and South Britain an unheard-of number of broad-winged visitors from America, though none was seen in France or Spain. They were monarch or milkweed butterflies, whose gorgeous wings have a power of flight that may compare with the birds’. Some of the greatest authorities believe that they flew the Atlantic on their own wings, aided only by the wings of the wind.
Since this surprising event, what is virtually a new branch of natural history has come into being, and the would-be solvers of the mystery are already very far-flung. They are found in Asia and Africa as well as Europe and North and South America. Communication has so greatly improved of late that it has become easy for men of science to pool their knowledge without delay and to let one another know what sort of coöperation they desire. Hence the quick success of the new inquiry.
Before describing the curiosities of this research into the migrations of insects, which mimic the migrations of birds, I may quote an example of the help that quick transport gives to the research worker. A famous German doctor believes that he is on his way to discover a great secret in medicine. It is connected with electrical manifestations in the bodies of animals. The first inspiration came from the firefly. One capacity is common to the various insects — mostly in the tropics, but some in temperate climes — which show a bright light at night. They can produce light without heat — a feat still denied to our electricians — and in this regard have achieved almost a hundred per cent efficiency. The study of the technique of this marvelous power was necessary to the medical professor, and thanks in part to the aeroplane, in part to easy communication of his needs, he was enabled to acquire specimens with little delay from many distant parts of the world.
So in this other inquiry into an insect secret, information and specimens pour into the head office of the inquirers with such rapidity that one person is needed for the sole purpose of tabulating and collating the news, and is often unable to keep pace with the material. A great deal of information comes from Egypt, and there is some evidence that a regular migration route of a beautiful and rather small blue butterfly lies across the Red Sea and the Suez Canal, following the Mosaic route.
In the course of the inquiry I have myself received from Kenya, in East Africa, a vivid account of a flight of white butterflies which lasted for fourteen days. The butterflies were so numerous that they suggested a snowstorm. At the place of observation, butterflies seldom stopped, and the few that rested on tree or bush flew on again after a short rest. This vast company, which must have numbered many tens of thousands, flew consistently in the same direction — almost directly south. Some east and west migrations have been observed, and in Europe there is one regular route which starts in a southerly direction from mid-Scandinavia but turns southwest; and its followers cross both the North Sea and the Straits of Dover into Britain.
Nevertheless, though such routes are known, the movement in the great bulk of examples is in a northerly and southerly line. The greatest and most regular voyager of them all, the monarch or milkweed butterfly of both North and South America, flies habitually north and south. Its eccentric appearance in Britain seems to be an accident due perhaps to the disturbance of a gale. When carried out to sea by a strong westerly wind, it must needs fly on.
Birds too, for the most part, move in a north and south direction. This is true of birds in Europe, Africa, and America. The wild geese which Mr. Jack Miner has marked with conspicuous success in his sanctuary in Ontario fly very much in the same direction as the swallows which nest in May and spend the winter in South Africa. Perhaps the most surprising record that has been duly authenticated concerns a pair of swallows which over a period of six years nested on the same building in Aberdeen and were traced to Griqualand in South Africa. These birds must have flown each year a distance of some twelve thousand miles.
Such an immense effort for a purpose that seems altogether inadequate must continue to stir our wonder and to stimulate our curiosity. As the directional regularity (so it may be called) becomes more and more apparent and certain, the theory of a sixth sense in birds — and perhaps in insects — has become more insistent. It is now suggested that birds have a power of responding to terrestrial magnetism, and fly their great distances with perfect truth of aim under the guidance of magnetic and electric currents. They feel the motions of the sphere if they do not hear the music of the spheres.
Be this as it may, an irresistible compulsion is put upon the bird, whether young or old, to fly off at a more or less definite date in a very definite direction.
The compulsion is felt whatever the conditions. I will give two examples from my own knowledge. A man of wealth in England brought over from the United States a few pairs of the species known as the robin in America and as the American thrush or American robin in Britain. The eggs from these birds were placed in wild birds’ nests and hatched well. The young flourished, and their relations with their cousins, the English blackbirds and thrushes who were also their foster parents, proved ideal. The parents are for the most part stay-athomes, but the robins are notable migrants. They flew about happily in an English park till the beginning of October. Then they flew off, not leaving a single straggler, in a southerly direction and were no more seen. None ever returned and none was ever traced. The experiment was carried on for a number of years successively, always with precisely the same result. Only the few pairs kept behind in large aviaries remained or were seen again.
These particular caged birds did not seem unhappy, but in some species and individuals the instinct is almost as strong when they are caged as when they are free. One example has become almost classical since it was first made public. That expert in evolution, Mr. Benjamin Kidd, kept a young cuckoo in his study and gave me at the time an account of its ways. One autumn evening this bird went into a sort of coma and stood with his wings moving an inch or two up and down for hour after hour. He was deaf to such calls from his master as were usually answered promptly and eagerly. Some mysterious force drove the bird into this strange and persisting trance, in which he imagined himself, we may guess, flying on and on over sea and land to the beckoning warmth of his southern home.
It would seem that this compulsion to fly south and later to fly north is a force independent of the more obvious causes of migration, such as the need of warmth and particular sorts of food. Virtually all birds nest at the most northerly point of their springtime journey. ‘True and tender is the North’ in a peculiar and particular sense.
II
The question arises: Do insects feel an influence of the same sort? There are sound economic and social reasons for the journeys of birds, though other reasons remain mysterious. For the long journeys of the butterflies only the mysterious reasons can be involved. Some of them seem, if one may say so, to be aggressively purposeless. For example, there is a glorious butterfly common in England called the red admiral. It is regarded by most people who have not studied its ways as a properly British resident. It appears in good quantity each year, both early and late in the year, but especially in the autumn. Its wings have a depth of black that would make even Velasquez, that artist in luminous blackness, envious; and the black is set off with brilliant patches of scarlet. Thanks to its salience and size, every gardener is familiar with it. Nevertheless, in spite of the number of observers, this butterfly has never been known to winter in England.
Some butterflies and moths lay eggs which live through the winter and hatch out in the next spring. Some cover themselves with tough silk cases and, so protected, slumber throughout the winter to emerge in a new form in the spring. Some hibernate in the proper sense — that is to say, the perfect winged insect hides itself in some snug crevice and sleeps through the cold months, coming forth to propagate its species early in the next spring. Some butterflies adopt all these three methods. A certain so-called tortoise-shell butterfly, which is the nearest rival to the red admiral in popular regard, is one of these. You see it often on the same flower head as the red admiral, and the two have a certain likeness; but how different their habit!
The red admiral would vanish totally from Britain if it were not for a regular annual immigration from overseas. This journey, so it seems, has nothing whatever to do with that strangest of all motives in all types of living things, the continuation of the race. None of these immigrants or their children are able to bridge the winter interval between one year and the next. The family of every immigrant is doomed to extinction after one generation in the new place of abode. The continuity of the race is actually handicapped by the migrating instinct. Nor can the strange habit be hereditary.
Birds have perhaps slowly developed this tendency to migrate. It is at least curious that they migrate either over land or over shallow seas. They pour over the North Sea, for example, which is shallow, because not so long ago it was land — or so the geologists say, and they deal in millions of years with as light a thought as the rest of us talk of months. To-day’s birds, they say, carry on a habit gradually acquired when the sea was land. If this is not so, how do we account for the fact that birds in myriads cross the comparatively wide North Sea, but very seldom venture over the very deep but narrow strait that separates Madagascar from Africa? The young birds inherit a desire to follow a habit established by their ancestors over æons of time when this sea was dry land. This cannot be said of the immigrant butterflies, for their offspring are never given the chance of such a journey; the return journey is never made: so it was argued only a few months ago, but, thanks to the new campaign, evidence begins to appear that some of the species, at any rate, go as well as come — that the insect migrants are more like the bird migrants than was thought.
In and about Britain the most suggestive of the evidence has been supplied by observers on the sea. All round the coast the men in the lightships and lighthouses have been mobilized as observers and asked to forward to the research centres their exact observations. They have seen many curiosities that had previously escaped notice.
Butterflies were regarded as playthings of the wind. Larger in the sail and lighter in ballast than the tenderest of racing yachts, they could do nothing but fly in the direction that the wind commanded. So large and susceptible are their wings that their habitual flight even on the calmest day is a chassé, a wavering course up and down and sideways, as though the sail were too strong for the rudder. How could such light, ethereal fliers set forth on a direct and purposeful flight in defiance of the currents of the air? Their flight is now known to be much more under control than seemed likely. Flocks of butterflies have been seen tacking their way against strong winds over the sea as both immigrants and emigrants. The island of Britain sends out emissaries as well as receives them, and the journeys in both directions are not the result of a blind obedience to external force.
In some respects the flight of butterflies is better than the flight of birds. They have a marvelous power of rising almost vertically. The huge light-blue butterflies of the Brazilian forests — or, for that matter, the purple emperor butterflies of France and Britain — rise and disappear over tall trees with an ease and at a speed that are scarcely credible. Many of the larger and more splendid butterflies collected in tropical or subtropical forests in South America can only be secured by the agency of a gun. They are shot down as a bird is shot, but the pellets are, of course, smaller, and are usually described as dust shot, though the description is not very accurate. Many a collector goes out into the forest with a two-barreled gun containing cartridges of heavy shot in one barrel for the destruction of any dangerous snake that may appear on the way, the other loaded with this dust shot for bringing down these towering butterflies.
Nor do butterflies suffer in like measure one disability of the bird. Most birds weaken if they do not feed frequently. In some zoölogical gardens of the North, many birds in the past have died of starvation because they could only feed in the light and could not endure the fourteen hours’ fast necessitated by the long nights. It was not till artificial daylight was supplied, and their feeding time thus extended, that their health was ensured. Some butterflies feed grossly, on rotten fruit or even on animal matter, though most of them sip daintily the very essence of the flower only at such hours as the sun is shining; but all of them, whether they feed grossly or daintily, seem to be able to live for an indefinite time with little or no food. This power of abstinence would doubtless be a great aid to migration.
Birds tire, some of them quite soon, for their weight of body is very great; and such birds as ducks and pigeons are able to maintain flight only by the development of extremely powerful muscle, used almost to the full at all stages of the flight. They often reach a pitch of utter exhaustion, and after the passage, even of so short a distance as the English Channel, can be picked up by hand, too weary to attempt to use either legs or wings.
It is difficult to imagine either a hungry or a weary butterfly. The species differ immensely, even more than birds, in their power of flight. Indeed, the females of some moths are wingless, and their migrations, of a few yards at most, are dependent on their legs alone; others have a wing area, in relation to both the bulk and the weight of the rest of them, that far exceeds what Vergil calls ‘the oarage of the wings’ of any other creature, whether bat or beetle or fly. The speed is not high, but it is very much greater than is generally supposed. It used to be thought that migrating birds traveled at a speed comparable to the aeroplane’s. We now know that the majority go less fast than an express train. Thirty miles an hour is a good speed for some of them, and a strongwinged butterfly such as the monarch would not be left far in the rear.
III
Recent discoveries in details of bird migration have been made largely through the agency of aluminum rings, each lettered and numbered, that are attached to the legs. Some of the birds so ringed are caught in traps of the size of large aviaries. The first was built in Heligoland, which may be called the headquarters of the study of migration; and in spite of its date Gädke’s marvelous book on migrating birds remains a classic. There are few other places in the world where quite so many species in such fantastic numbers rest for a while on their way across the sea. Another cage built on a similar model on the island of Skokholm on the west coast of Britain is producing a succession of minor discoveries. Such cages are rare, but in most countries nestlings are ringed; and as the study of birds grows more and more popular, more and more recoveries of these ringed birds are reported all over Europe and North America, and even in Asia and Africa.
To ring an insect would indeed be to ‘break a butterfly upon a wheel.’ The achievement is next door to the impossible. Nevertheless butterflies can be marked. A hint has been taken from modern students of bees. A number of queer little discoveries in the economy of the hive and its social organization have been made of late by marking each bee as it emerged from the cell. The mark is made by dropping a speck of colored cellulose on the right spot (just above the thorax); and this both lasts a long time, as the time of insects is reckoned, and is singularly conspicuous. In an observation hive you can quite clearly follow the activities of a chosen individual or a particular hatch. You can see how the young bee is trained progressively to a succession of tasks, beginning as housemaid, proceeding to nursemaid and then to general caterer, and perhaps finally to the feeder of a queen. Butterflies and moths are being similarly marked, and out of doors in the garden you can see these marks at a fair distance.
It is not expected that a moth ‘cellulosed’ in Detroit or Aberdeen will be discovered in Labrador or Griqualand, to give two examples from the recent history of a wild goose and a swallow; but some rather unexpected results have been observed, though the experiment has had only one season’s trial. One discovery is that the butterflies we see to-day in our garden, perhaps on the same Michaelmas daisy or Buddleia flower, are not for the most part the butterflies we saw yesterday. They are successors. The butterflies pass on.
What these movements mean we do not know. How are they prompted? What is their purpose? These are questions not yet answered, but we know a great many more facts than we did; and perhaps the most notable is this: that in some cases, at any rate, the migration is a two-way migration, a to-and-fro movement.
It used to be held, for example, that butterflies and moths (such as the red admiral, the painted lady, the larger hawk moths, and the silver Y-moth) flew over the seas into Britain but never returned to the Continent. Lately flights over the seas in a southerly and easterly direction, as well as in a northerly and a westerly, have been fully authenticated, and the likeness between the bird and the butterfly brought a step further.
A number of animals have a certain prompting to trek. One of the most startling examples is in quadrupeds. Those ratlike creatures, the Scandinavian lemmings, will gather in huge companies and march forward in a given direction with such blind determination that nothing will stop them. They have gone straight on without flinching into river, lake, or even sea, and drowned themselves in thousands. Swarms of mice have been seen going forward in much the same way in Australia; and it has been presumed that the cause was hunger, due to an excessive multiplication of the species.
Hunger is a potent compeller. It doubtless affects both bird and quadruped; but it is not a full explanation. It does not explain the massing in great companies and congregations; and it does not apply to all sorts of migrations. It quite certainly is not the stimulating cause of these long flights of butterflies.
‘All things go out into mystery’ (omnia exeunt in mysterium), not least the winged being that sets forth over sea and land to a bourne as yet unknown. Do not the young birds of some species precede their parents and fly straight and quite undirected by night as well as by day to the home district from which their parents came? The mystery is made yet more mysterious, for it seems less rational, by the new knowledge on the way of a butterfly in the air; but we are all mystics enough to feel that a happy compulsion is put on us to plumb the depths and heights, however remote the chance of approaching either the nadir or the zenith of the mystery that defers us.