r6o 



NA TURE 



[December 19, 1907 



great divergence of opinion, and as to which scarcely any 

 two naturalists who have studied the subject are in complete 

 agreement. 



The part of the problem that I propose here to consider 

 can be thus stated : — Is there any physiological reason to 

 account for the fact that for that class of birds which we 

 may call the north-south migrants northern latitudes have 

 determined themselves as the most suitable for summer 

 quarters and breeding grounds, and southern latitudes for 

 winter quarters? ' 



The physiological reason for this choice of quarters 

 which most naturally suggests itself concerns the 

 presence or absence of food, or its relative abundance and 

 the means of procuring it. From the Arctic circle, where 

 during winter the whole of nature, sea and land alike, is 

 in the grip of an intensity of cold of which we can form 

 little conception, and which few animals can withstand, 

 birds, at any rate, must move southward, or they would 

 inevitably perish of cold and starvation. It thus appears 

 easy to account for one aspect of the north-south migration 

 problem by referring it to the necessity of avoiding de- 

 struction by starvation ; but even for this aspect of the 

 question the answer is not quite so simple as at first sight 

 appears. For it fails to account for the distances which 

 such migrations often take, since a passage into the north 

 temperate region alone would sufifice to obviate this diffi- 

 culty ; yet this region is, for the most part, passed in the 

 migration of many Arctic birds, which may not stop until 

 a tropical or even a southern hemisphere region is attained ; 

 and not only so, but the north-south migrants of the north 

 temperate zone themselves share in the migration, passing 

 away for the winter to a southerly clime. Many of these 

 cannot be said to be driven south by the lack of food, for 

 at the time the migration occurs food is usually still 

 abundant, and there is plenty of food during the whole 

 winter in many of the countries which are passed to sup- 

 port, not only the permanent avine inhabitants, but in 

 some regions myriads of east-west migrants besides. 



Moreover, there is evidence that during the Tertiary 

 period the climate of the Arctic circle was entirely different 

 from that which now exists — warm and mild, and abound- 

 ing with vegetable and animal life — and there was then 

 no necessity for north-south migration on the score of 

 want of food materials. Yet it is impossible not to suppose 

 but that migrations occurred then as now, since the habit 

 of migration is so ineradicably engrained in the nature 

 of the bird that it is difficult to believe that it was not 

 evolved along with the development of the organs of 

 flight. 



If we now turn to the other aspect of the north-south 

 migration problem and consider the causation of the move- 

 ment from south to north, we see that the explanation re 

 food supply, which seems easy to formulate for the north 

 to south movement, at once breaks down, for the tropical 

 and temperate regions are at any rate not less abundantly 

 provided with food during summer than the regions of the 

 fir north to which the majority of these migrants wend 

 their way. The difficulty is a serious one. The e.xplan- 

 ation which was used to account for the north to south 

 movement is not available for the opposite movement ; 

 some other explanation must be found. Here the weakness 

 of the original explanation tnanifests itself, for it would 

 be natural to suppose that the reversal of an effect would 

 be the result of the reversal of the cause which produced 

 the effect, and this is not the case in the present instance. 



What, then, are alleged to be the reasons for the south 

 to north migration in the spring? One of these supposed 

 reasons is both given and at the same time refuted by 

 Gatke (" Birds of Heligoland," English translation, p. 144) 

 in the following passage : — " From very old times, mainly 

 in consequence of the phenomena which succeeded migra- 

 tion, it was conceived that in spring, with re-awakening 

 life in Nature generally, the reproductive instinct of birds 

 r'lso was roused afre.sh. and that it \vas this which urged 

 them' to wander to their nesting places ; while in autumn, 

 dearth of food and cold admonished them to make a 

 temporary home in warmer latitudes. This view has, in 

 cart, held its ground up to recent times, for it is not so 

 long ago that Rrchm, in one of his talented discourses 

 1 For convenience of Hescription the migrants are here assumed to helong 

 to the northern hemisphere. 



NO. 1990, VOL. 77I 



on this inexhaustible theme, maintained that the two great 

 factors in the world's action. Hunger and Love, also 

 dominated the migratory movements of birds. . . . These 

 explanations, however, do not suffice ... it cannot be the 

 reproductive instinct which prompts birds to set out on 

 their spring migration, for many species do not breed in 

 the first, second, or even third year of their life, and yet 

 migrate to their homes just like those of their congeners 

 who are endowed with the capacity of breeding ; nor are 

 they induced to travel by the example of their parents, for 

 they start on their journey alone, and independently." 

 Gatke concludes as follows : — " In regard to this question 

 as to the immediate cause of the departure of birds in their 

 migration ... we are confronted with a riddle which has 

 hitherto defied every attempt at a solution, and which 

 indeed we may hardly expect will ever be likely to receive 

 a final explanation." 



We may take it, then, that the hypothesis that the 

 commencing recrudescence of the generative functions in 

 spring is the determining agent for the migration from 

 south to north does not furnish an adequate explanation 

 of the phenomenon, even if it were certain, as is by no 

 means the case, that such recrudescence begins before the 

 commencement of the movement. It seems obvious that 

 there must be something in the higher latitudes which is 

 favourable to breeding or to the rearing of offspring. Are 

 we to suppose this favourable factor to be relative cold- 

 ness? Prima facie this seems improbable. Other animals, 

 including non-migrant tropical birds, breed freely in the 

 hottest regions of the earth's surface, and warmth is 

 favourable for incubation. Many of the east-west migrants 

 have their breeding grounds in the interior of the great 

 Asio-European continent, which is in the summer much 

 warmer than its western shores. I have been unable to 

 come across any fact which would lead one to suppose 

 that mere diminution of temperature assists breeding. 

 There are, it is true, some fishes and possibly a few other 

 animals that produce their eggs and young in the winter, 

 but in by far the majority it is the warmer season of the 

 year which is occupied with the propagation of the race. 

 We are therefore forced to conclude that the south to 

 north migration is not brought about because of the 

 necessity or advantage of a colder climate for breeding 

 and nesting. Is there, then, any other means of explain- 

 ing why it is advantageous for certain birds to pass the 

 summer, and especially to breed, in high latitudes, which 

 will equally account for the fact that low^er latitudes pre- 

 sent corresponding advantages during the winter season? 

 It is an answer to this question that I will now attempt to 

 give. 



Let us begin by admitting that bird-migration must 

 have been brought about by the necessity for procuring a 

 sufficient supply of food. The importance of this at all 

 periods is self-evident, but it becomes accentuated in the 

 breeding season, when not only the needs of the parent 

 birds, but also those of their voracious offspring, have to 

 be met. It appears to have been assumed by most 

 writers that for the north-south migrants the higher lati- 

 tude or summer region of distribution, to which they 

 resort for the breeding season, represents their original 

 home or habitat; to which it is only natural they should 

 desire to return when the desire for breeding conies upon 

 them, and that the niia:ration to lower latitudes is brought 

 about by climatic conditions, such as frost and cold, which 

 render the procuring of food a matter of difficulty or 

 impossibility during winter. There are, however, as has 

 already been pointed out, difficulties in accepting the 

 climatic conditions and accompanying deprivation of food 

 supplv .as affording the onlv or even the chief explanation 

 of migration, and more especially of the acquisition by 

 birds of the north-south migratory habit. Thus it fails, 

 as we have seen, to explain the south to north migration 

 in the spring, and would be .an inadequate reason for 

 much of the autumnal migration which occurs from the 

 northern temperate zone, such as that of those migratory 

 marine birds the food of which is abundant in the northern 

 seas throughout the winter. It at first sight appears also 

 to fail to account for the fact that with many snecies of 

 birds autumnal migration occurs before the advent of 

 severe weather; and at a time When the food supply in 

 the higher latitude is as abundant as ever, and that their 



