قراءة كتاب The Migration of Birds
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physical features present as attractions to birds in search of suitable nesting places and food supplies.
The study of Migration involves reference to the work of ornithologists of the past and present, the mass of contradictory literature already referred to, and we are repeatedly faced with the difficulty that some particular theory about the vexed questions of the cause or origin of migration, the height and speed at which birds travel, whether they do or do not follow routes, how they find their way, in what order they migrate, how and why they do or do not avoid dangers, or any similar problem, which seems to give finality so far as certain cases are concerned, is met by an absolute negation in other instances. The truth seems clear; more than one factor has influence on most birds, and different species in different places are influenced by different factors. Elliott Coues' sweeping statement, though I strongly disagree with the article in which it occurs, expresses much that is true. "Isepipteses and magnetic meridians, coast-lines and river channels, food-supply and sex-impulses, hunger and love, homing instincts and inherited or acquired memory, thermometer, barometer and hygrometer, may all be factors in the problem, good as far as they function; but none of them, and not all such together, can satisfy the whole equation."
Some of the theses may be laws or rules, but there are no rules without exceptions, and these exceptions may become local rules. Laws regulating migration in one area, whether it be the great continent of America, the British Islands or the islet of Heligoland, may have little application in other parts of the world: local evidence alone can never solve the great problems.
CHAPTER II
CAUSE AND ORIGIN OF MIGRATION
The question—What makes Birds Migrate? or what causes them to remove from one zone to another at certain seasons, has been answered, no doubt to the satisfaction of the respondents, in many varied ways. Closely connected with the question of immediate impulse is the deeper, and less easy to prove problem as to how migration originated.
It has been dogmatically asserted repeatedly that birds invariably breed in the most northerly part of their range, and winter in the most southerly. Winter, when speaking of Holarctic birds, only applies to the season in the northern hemisphere; the birds which pass south of the equator winter in summer. Whilst accepting this as a rule, two reservations must be made. First, that it only applies to birds of the northern hemisphere, and secondly that it is a rule with exceptions. It seems probable that the breeding area of some of the birds which reach the British Islands in autumn by the so-called east and west route is in more southerly latitudes than our islands, and certainly it seems evident that the temperature of the winter refuge has more effect upon the birds than its geographical position. Perhaps the statement that a bird always nests in the coldest part of its range is more universally correct. Even this may not be invariably the habit, but in acknowledging it as a rule we must clearly understand that this cold district is resorted to at the period of the year when its temperature is at its highest. There are certain birds which breed in Australia and winter in Oceanic islands where the temperature is cooler than in their breeding area.
When considering the migration of birds which summer in the extreme north or breed in the extreme south—alas, but little is known about the migratory habits of many southern breeders—it is comparatively simple to offer an explanation; in the long winter months this home, so desirable in the short weeks of daylight, is dark, ice-bound, and foodless; it is wholly unsuited to the requirements of birds, which, in spite of many assertions to the contrary, have never been proved to hibernate, the only way in which animals can survive for any lengthened period when food supply is entirely cut off.
Birds are structurally provided with the means of escaping from the disastrous effects of adverse circumstances; the power of flight, though not the only way in which animals can migrate, is at the root of the migration of birds. The advantages of the power of flight, to which also it owes its development, include the ability to avoid active and passive enemies, and to remove from one feeding ground to another undeterred by the barriers which restrict the terrestrial animal. A natural sequence of this ability to take advantage of aerial locomotion is the habit of wandering in search of food, more or less noticeable in all birds. The habit of wandering led to the discovery of feeding grounds and suitable nesting places; where these nesting places, probably at first, only removed a short distance from the parents' nesting site, were suitable, dispersal and an extension of the distributional area or range of the species followed; but where the feeding area was unsuited or not so well suited to the needs of the species, hereditary attachment to the original home and memory of the direction of this home, or even in some cases accidental wandering back to the more suitable locality, would originate a migration. Coupled with this are two important factors which would tend to make the habit periodical and regular both as regards time and locality. The memory of the bird, call it instinctive memory if we like, would limit the wanderings in search of food to a certain number of places where food was most abundantly found, and the passage between feeding area and breeding area become regular journeys, at the seasons of the year when an increasing number of young birds in the breeding area drove the overgrown population to seek food further from the base, and again when the sexual impulses urged the birds to seek secure nesting sites. The other factor is the weeding-out influence of mistaken effort, the natural selection which leads to the survival of the fittest. The young wanderer which reached unsuitable lands must either wander further or perish. Judging by the juvenile mortality amongst young birds the failures would be many, and only the successful competitors would return to leave progeny.
Great stress has been laid on the attachment of birds to certain nesting sites, an undoubted fact, and it has been argued that because, in some cases, for hundreds of years certain sites have been occupied by the same species, it is evident that after the death of parents the young will return to and occupy the home. This has even been put forward as evidence that birds do not wander in search of fresh nesting sites. The argument is not sound. It is improbable that in most cases both parents perish in the same year. Birds of prey, and many of the cited instances of long tenancy refer to raptorial birds, have a wonderful power of finding a mate, male or female, to complete the hatching and rearing of the young, when one of a pair has been destroyed. The survivor of any pair might have the home attachment and by bringing a fresh mate create an attachment which would be passed on from mate to mate indefinitely. Again it must not be overlooked that certain sites present advantages to particular species which must be evident to all in search of those advantages; it by no means follows that the occupiers of a nesting site are in any way related, except specifically, to those which occupied it in previous years.
The answer to the argument that birds do not seek fresh nesting places and thus extend their distributional area, is evident when we consider those species which, at the present time, are extending their range. Within the last few years, for