NATURE 



415 



T 



THURSDAY, SEPTEMBER 24, 1874 



THE MIGRATION OF BIRDS 

 HE "silly season" has this year been marked by 

 some discussion in the newspapers on the migra- 

 tion of birds. The various letters published have shown 

 the normal want, if not of knowledge, yet of profundity ; 

 and I fear lest the subject, which really deserves the best 

 attention from naturalists, should suffer in repute by the 

 absurdities lavished upon it. 



The discussion began, if I am not mist.aken, with a 

 theory of migration set forth by a Scandinavian poet, 

 •which treated that wonderful movement as an attempt on 

 the p.irt of birds to attain '• more light." It proceeded 

 on the hypothesis that the birds which are summer- 

 visitors to northern climes, finding that the days grow 

 shorter as summer advances, retire southwards to find 

 " more light," and that the same desire prompts their 

 return northwards in spring. To show the fallacy of this 

 hypothesis it is sufficient to observe that the southward 

 movement not only begins, but is with many species in 

 grtat partaccomplished, long before the autumnal equino.x, 

 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 equino.x, with of 

 course the same result. Whether this theory was ever 

 intended in earnest or was only a poetic fancy I do not 

 know, nor is it really worth while to inquire. It is enough 

 that it contains its own refutation. 



I have no intention of commenting upon the whole dis- 

 cussion. Few, if any, of the letters which followed contain 

 anything to the purpose either way. But one published 

 in the Times of Friday, Sept. 18, seems to require special 

 notice, since it professes to give " the latest accepted 

 theory " on the subject ; and the writer, without actually 

 saying that it is received by a very great authority, whom 

 he names, intimates that it does not meet with his dis- 

 approval. Of this " latest accepted theory " I must con- 

 fess I never before heard ; and now that it is before me, it 

 SI ems to be not only unsupported by facts, but to amount 

 to no explanation at all. After briefly touching upon. the 

 diiiiculty which the shorter-winged Birds of Passage must 

 have in effecting their voyages, the writer says : — 



" 1 believe it was only some twenty or thirty years ago 

 that anything like a practical solution of the difficulty was 

 arrived at. The birds congregating about the south 

 ci-ast are seized with a sudden impulse or mania to fly 

 upwards. This is caused by some atmospheric change 

 coinciding with a warm south wind moving in a high 

 btratum, into which the birds soar with an involuntary 

 ni'ition of their wings. This motion (involuntary like 

 mat of the heart) is continued for many hours, and the 

 buds fly blindly along until the paroxysm passes off, when 

 ihey at once begin to descend, making many a fatal drop 

 into the sea. 



"The same phenomenon occurs'in Africa and southern 

 countries, where the migratory birds congregate for a 

 northern flight about April. Experiments were tried here 

 and in Africa which tended to corroborate the above 

 lacts. Migratory birds were kept in cages along the 

 coast, and it was found that each was seized with a pro- 

 longed paroxysm coinciding with the time that the wild 

 birds disappeared. Cages were constructed with silk at 

 top and bottom to prevent the birds from killing them- 

 WiU .\.— No. 256 



selves ; and it was noticed that after the paroxysm had 

 passed away, the birds began to look about them, to 

 plume themselves, and cat and drink, apparently with a 

 notion that they had arrived at their new home." 



On reading these wonderful paragraphs, some ques- 

 tions naturally arise. How does the writer account for 

 his " birds congregating about the south coast ?" What 

 brings them there, that they may be " seized with a 

 sudden impulse or mania to fly upwards?" Who has 

 ever observed the '" atmospheric change " and coincident 

 " warm south wind moving in a high stratum ? " Do 

 these remarkable meteorological phenomena occur but 

 once in the whole season of migration, or is there a suc- 

 cession of them to suit the convenience of each migratory 

 species ? WIio, moreover, has seen the birds soar into 

 this peculiar current of air ? and who of such fortunate 

 persons knows that the motion of their wings under such 

 conditions is " involuntary like that of the heart .'' " Finally, 

 what is the cause of the " paroxysm " ? for, without know- 

 ing that, to attempt to explain the observed facts of 

 migration is an attempt to explain obscuinin per ob- 

 sciirius. 



When a satisfactory answer is given to these questions, 

 it will be time to inquire whether this " latest accepted 

 theory " of migration sets the matter in any clearer light, 

 or whether it is not as arrant nonsense as was ever foisted 

 upon an innocent public, even at the height of the " silly 

 season." The last paragraph of the writer's letter, I may 

 remark, has nothing in it of consequence. Granting that 

 the migratory impulse is instinctive, it is, like other in- 

 stinctive practices, followed as far as circumstances will 

 allow. 



Permit me now to point out to those interested in the 

 solution of this mystery of mysteries the chief matters to 

 which the attention of observers and theorisers should be 

 directed. 



I. The original Cause or Causes of Migration. — In 

 some cases scarcity of food would seem to be a sufficient 

 cause, and it is undoubtedly the most obvious one that 

 presents itself to our mind. As food grows scarce towards 

 the end of summer in the most northern limits of the 

 range of a species, the individuals affected thereby seek 

 it in other countries. Thus doing, they press upon the 

 haunt ot other individuals ; these in like manner upon 

 that of yet others, and so on, until the movement which 

 began in the far north is communicated to the individuals 

 occupying the extieme southern range of the species at 

 that season ; though, but for such an invasion, these last 

 might be content to stay some time longer in the 

 enjoyment of their existing quarters. When we con- 

 sider, however, the return movement, at the end of 

 winter, it is doubtful, I think, whether scarcity of 

 food can be assigned as its sole or suflicient cause. 

 But here we feel the want of knowledge. At pre- 

 sent we are far too little acquainted with the physical 

 peculiarities of those more equatorial regions, which 

 in winter are crowded with emigrants from the north, to 

 come to any final decision. It seems not too violent an 

 assumption to suppose that though such regions are well 

 fitted for the winter resort of the bird-population of the 

 north, they may be deficient in certain necessaries for the 

 nursery ; and it seems still less of an assumption to suppose 

 that even if such necessaries aie not wanting, yet that the 



