Popular Science Monthly/Volume 24/December 1883/Land-Birds in Mid-Ocean
|LAND-BIRDS IN MID-OCEAN.|
THE appearance of some of the smaller varieties of migratory birds, such as sparrows, swallows, doves, etc., several hundred miles away from the nearest land is by no means an unusual occurrence on the ocean. About these little erratic visitors there are some curious and interesting facts. Their appearance is almost always one at a time, though I have known a considerable number, representing, perhaps, as many different varieties, to accumulate in the course of a day. It is usually, though not always, in stormy or unsettled weather.
The first curious fact about these birds is, that they never appear to be tired out; whereas birds are often met with near the land with their strength quite exhausted. A second curious fact about them is their preternatural tameness where there is no cat or dog on board, and the crew show no disposition to molest them, as exhibited by their apparently seeking rather than avoiding the presence of man.
Another curious fact about them is the recovery of all their native wildness and their instinctive avoidance of man's presence on approaching the land. The first time I noticed this fact was with a pair of olive-colored ring-doves, which, from their remarkable tameness and familiarity, I was led to believe had been bred in a domestic state and perhaps on shipboard. I kept them in the skylight in the cabin, where they seemed to be quite contented; but on approaching the land they became the wildest of the wild. One of them escaped and flew away. I succeeded in taking the other into port, where I gave it its liberty. Now, I am certain that these birds could not have been apprised of the approach to the land through the medium of any of their ordinary senses. This curious circumstance led me to notice more particularly the conduct of other varieties of these little wanderers upon the ocean so far from their native habitat, and I find that they nearly all exhibit to a greater or less extent the same curious characteristics. Here the observant mariner with a smattering of science may find something to cogitate upon.
Light, heat, sound, etc., are said to be effects produced upon the living organism through the medium of appropriately developed organs by as many different modes of motion, whereby the animal is brought into conscious connection with surrounding objects, the effect diminishing in a progressive ratio as the distance of the object increases. Of these special organs there are said to be five in number which are essential to the well-being of all perfectly developed animals. But may there not be other analogous modes of motion, producing analogous effects upon the living organism, whereby the animal is brought into conscious connection with surrounding objects, and by or through which it has a sense of the locality or direction of such objects as are essential to its well-being to seek or avoid? Admitting this, suppose a flock of birds have started on one of their migratory excursions, guided mainly by this sense of direction, in pursuit of some distant object. Then let us suppose that in their flight they pass obliquely, but unwittingly, into another higher stratum or current of air moving with great velocity in some other direction, but toward the ocean. The flock would necessarily become very much scattered, and in the confusion a portion of them would be carried unconsciously out to sea, beyond the range of their sense of direction having lost which, they fly at random and at ease, exerting just sufficient effort to sustain themselves in the air; while another portion of the flock, keeping within the limit of their sense of direction, will exhaust all their strength vainly endeavoring to reach their object against a violent wind.
So intimately associated with this sense of direction is their instinct of self-preservation in avoiding the presence of man, that while the one is in abeyance, the other, in the absence of anything to arouse it, remains dormant. This, I believe, is the true meaning of the preternatural tameness exhibited by the birds on the Galapagos Islands mentioned by Darwin.
From the peculiar properties of air in its relation to heat, the atmosphere has a tendency to form itself into heterogeneous strata, more or less inclined to the horizon; each stratum having a horizontal motion independent of the others—a fact the significance of which, I think, is frequently lost sight of by meteorologists, more especially by cyclonologists. That some of the higher strata of the atmosphere have an independent horizontal motion, the velocity of which is often incomparably greater than anything we experience in the lower stratum, is evident not only from the appearance in mid-ocean of birds, but of insects hundreds of miles from land, and apparently as lively as if they were in their own native haunts. I have seen grasshoppers at least a thousand miles, and dragon-flies certainly two hundred miles from land. During a recent voyage from New Zealand to New South Wales, and thence to Japan, frequently, for several days in succession, moths and butterflies were visible in the air nearly every hour in the day.
- "December 13, 1876, latitude 17° 24' north, longitude 44° 12' west. While taking the sun at noon, noticed a number of grasshoppers about the vessel. Made several unsuccessful attempts to capture one of them. The nearest point of land is the Island of Montserrat, latitude 16° 48' north, longitude 62° 12' west, distant 1,023 miles." (Extract from a private log.)
- In the vicinity of the river La Plata, violent westerly gales, called pamperos, are of frequent occurrence. One of the surest precursors of these gales is the appearance of numerous dragon-flies in the air. I have seen these insects collecting about the ship fully two hundred miles from land, off the entrance of the river, while the wind was still blowing a gale from the eastward.