l62 



NA TURF. 



[December 19, 1907 



of species which show no norlh-south tendency in migra- 

 tion and whicli are confined to the temperate zones there 

 is sutllcicnt opportunity, even in shortened days, of obtain- 

 ing such food as they require for subsistence in the region 

 towhich they have betaken themselves for winter quarters. 

 Many of these east-west migrants are either gramini- 

 vorous or live on insects and grubs whicli they seek in the 

 ground or on trees. In the summer their food is most 

 abundant in the great grain-producing or forest-clad 

 central regions of the Asio-European continent, while in 

 the winter they are compelled to seek their subsistence in 

 a less severe climate. 



Another kind of migration is that which may bo pro- 

 duced by local conditions of food and pressure of bird 

 population. If in any particular zone food of appropriate 

 character is obtainable at all times of the year in sufficient 

 abundance, the necessity of migration to a higher or lower 

 latitude is no longer necessary for a specific number of 

 individuals, and their migration, and that of their 

 ■descendants, will accordingly tend to limit itself to that 

 zone, within which such migration as does occur will be 

 more or less local. 



Newton ' suggested that the arrival of a large batch 

 of migrants in a particular area or zone which is 

 already occupied by birds of the same species may compel 

 the individuals of that species which are in possession to 

 move on in any direction where food is readily obtainable. 

 It is perhaps more probable that later migrants into a 

 zone already occupied by birds of the same species or 

 liabits mav, on finding others already in possession, them- 

 selves push on into other regions. In this case the later 

 migrants of species which vary in the extent of their 

 migration would tend also to become the more extensive 

 migrants, and would by natural selection transmit this 

 tendency to their descendants. This conforms to the 

 statement that those individuals of a species which migrate 

 to the higher latitudes in the range of distribution of the 

 species start their spring migration later than the in- 

 •dividuals which migrate to less high latitudes.' 



The theory that bird migration in the Holarctic area 

 was originally determined by the encroachment and sub- 

 sequent retreat of the ice-sheet over the temperate zone 

 during the Glacial epoch (or epochs) is one which appears 

 impossible to accept. .Such a theory in its bare form 

 involves the assumption that the habit of migration which 

 so extensively pervades the avine class of vertebrates has 

 been acquired during comparatively recent geological 

 times, for which there is neither evidence nor probability. 

 When we consider how extensively diffused is the tendency 

 to migration of some kind amongst birds, it appears 

 reasonable to assume that the habit was acquired at a 

 comparatively early period of their evolutionary history. 

 It may even be that the advantage gained by a more and 

 more extensive movement of the kind was the predeter- 

 mining cause, in the hands of natural selection, of the 

 complete evolution of the avine type of vertebrate. 



1 have been able to find in the literature only two refer- 

 ences dealing directly with the subject of the influence of 

 light on bird migration. Seebohm (" The Geographical 

 Distribution of the Family Charadriidae," London, ]888, 

 p. 34) writes as follows ; — " The first migrations of the 

 ancestors of the Charadriida: were probably not in search 

 of -ifarmth, for the climate of the Polar Basin was in 

 those remote ages mild enough : nor in search of food, 

 which was probably abundant all the year round ; but in 

 search of light during the two or three months when the 

 sun never rose above the horizon. The habit of migra- 

 tion thus formed became deeply rooted in the species, in 

 accordance with the law of heredity : and doubtless 

 acquired additional force when the terrors of a glacial 

 epoch exterminated the conservative party amongst the 

 Charadriidse (if any of them were foolish enough to neglect 

 to adapt themselves to the changed circumstances), and 

 compelled the survivors to extend their migrations far 



' " Bictionary of Birds," art. Migration. 



2 This appears to be the case with the s» allow fsee W. Eaele Clarke, 

 Bntisli Association Report, 190. , p. 10), the individuals which are losummer 

 in Scandinavia passine through this country after our own swallows have 

 arrived. Similarly, the return of the Scandinavipn swallows also appears to 

 be somewhat later than ours (middle of Sentember, p. 12). (.See also the 

 same authors " Digest," British Associalion Report, 1896, p. 17). 



NO. 1990, VOL. yy] 



•and wide, until the shores of nearly all the rest of the 

 world were visited on passage, or included in the winicr 

 range of some species of the family." 



.Seebohm evidently realised that, however warm the 

 circumpolar area and however well stocked with food, it 

 would be impossible for birds to subsist there all the vetir 

 round on account of the absence of daylight duiing the 

 ■A'inter months. It is the more strange that he should 

 have failed to perceive the obvious corollary that these 

 birds inight seek such high latitudes during the summer 

 months in the breeding season on account of the advantage 

 offered for the procuring of food by the prolonged day- 

 light. 



Some fourteen years prior to the publication of 

 Seebohm's work there appeared in the Academy (1874, 

 vol. vi., p. 262), under the head of " Notes and News," 

 the following paragraph,' which, however, bears no in- 

 dication of the source whence the information it contains 

 was derived : — 



" The aged poet Runeberg, the greatest scald that 

 Sweden has ever had, has been in extremely weak health 

 for many years past. It appears that as he has lain on 

 his sick bed, at Helsingfors in Finland, he has occupied 

 himself by close observation of the habits of birds, and 

 specially with regard to the causes of migration, and he 

 has at last put forward a singularly beautiful theory on 

 the latter point. He believes, in fact, that it is the long- 

 ing after light, and that alone, that draws the birds south- 

 wards. When the days shorten in the north, the bird? 

 go south, but as soon as ever the long northern nights 

 (sic) set in, with all their luminous and long-drawn hours, 

 the wanderers return to their old haunts. It is generally 

 supposed that they move southward to get more abundant 

 food : but why, asks Runeberg, do they leave their rich 

 hunting-grounds to return to the north? The central 

 regions of Europe are in every way more desirable than 

 the wastes of Scandinavia. Only one thing is richer 

 there, and that is light. The same instinct that makes 

 plants firmly rooted in the ground strain towards the 

 light, spreading upwards in search of it, works in the 

 birds, who, on their free wings, fly after and follow it. 

 This very suggestive and poetical notion is further carried 

 out by reference to various analogies in natural history, 

 and the final sentence is quite epigrammatic : ' The bird 

 of passage is of noble birth ; he bears a motto, and his 

 motto is Lux mea dux.' " 



The idea which is given voice to in the above para- 

 graph bears a certain resemblance to that which I have 

 endeavoured to set forth in this paper, but on close con- 

 sideration it will be seen that the reseinblance is purely 

 superficial. What I have tried to urge is, not that these 

 north-south migrants seek light qua light; but that the 

 sense bv which alone thev are for the most part able to 

 obtain food necessitates their passage to regions where 

 at one or another time of year there will be sufficient day- 

 light to procure it. This is a special part of the general 

 problem of food supply, itself an all-important agency in 

 natural selection, which last there can be no doubt has 

 been instrumented in determining the habit of migration. 

 The theory attributed to Runeberg, in so far as it seeks 

 to explain north-south migration by the endeavour of the 

 bird to follow light alone without reference to the ultimate 

 reason for such movement, in no way explains why birds 

 rather than other animals should require light, and may 

 well have merited the criticism to which it was at the 

 time subjected by Prof. Newton (Nature, .September 24, 

 1874, p. 415), who pointed out that since " the southern 

 movement not only begins but is with many species in 

 great part accomplished long before the autumnal equinox, 

 when consequently the birds are journeying to increasingly 

 shorter days ; and in like manner their northward move- 

 ment is set on foot before the vernal equinox," the theory 

 (that it is light alone that is the attraction) " contains 

 its own refutation." 



The object of this paper has been to endeavour to give 

 a reasonable explanation of the north-south tendency, 

 which is the most prominent feature of bird-migration. 

 No attempt is m,ade to explain all phenomena of migra- 

 tion. Obviously there are some migrations which cannot 

 be explained on the assumptioB that the object of move- 

 ^ The paragraph was r opied in the Times for September 18, 1874. 



