156 



KNOWLEDGE 



[August 1, 1898. 



" lu the beginning of autumn they assemble together 

 among the reeds, where, allowing themselves to sink into 

 the water, they join bill to bill, wing to wing, foot to foot." 



Many recorded cases of hibernation are quoted by 

 Mr. Dixon from various authorities, and they are on the 

 whole difficult to explain otherwise. Yet he brings forward 

 nothing as the result of his own observation, and this, be 

 it remarked, is usually the case in the literature of hiberna- 

 tion. A more recent example than any of those given by 

 Mr. Dixon is one recorded in a letter written by an observer, 

 Prof. Carlo Spegazzini, living in the Argentine Republic, 

 published in Nuture, July 4th, 1889. Writing to a friend 

 in Italy, he says : — 



" The bird known here by the name of Golondrina, and 

 which I think is Proipu- domentica, is subject to hibernation. 

 Last year, while the zinc roof of a small house was beiug 

 taken up in the month of August, just in the middle of 

 our winter, I found underneath about a hundred martins, 

 all accumulated one over the other and lethargic, but in 

 good health, so that, exposed to the sun, they awoke and 

 flew away very briskly. This year, again, having seen 

 some holes on a barrancha, a steep bank over the Plata, I 

 began to dig at them, hoping to get some bats ; but there 

 I found several hundreds of martins, of the same kind as 

 above mentioned, clustered and in a state of lethargy. Is 

 such a thing known to naturalists ? " 



"For upwards of 250 years," says Mr. Dixon, "the 

 hibernation of birds has more or less excited the curiosity 

 of man." But it was a familiar idea more than 1400 years 

 ago, as the following lines from the Eoman poet Claudian 

 (a.d. 408) show : — 



" A'l'l qaali^^ gelidis pluma labcnte pruinis 

 Arboris iiniuoritur truiico bnuualis liirimdo." 



And 400 years earlier still, Pedo Albinovamis, a friend 

 of Ovid, also alluded to the hibernation of the swallow : — 



'■ Conglaciant ur acjuic, scopulis se condit hinindo 

 Vei'berat egelidos garrula vere laeus." 



The old state of doubt as to what became of the migratory 

 birds is expressed in Pope's lines : — 



" A bird of passage, lost as soon as found ; 

 Xow in the moon perhaps, now underground." 



And Mr. Dixon reminds us, that with all our recent 

 advance in the science of migration, the winter home of 

 two of our common summer migrants, the house martin 

 and the sand martin, is still practically unknown. 



One of the most interesting results established by the 

 researches of the British Association Committee is that, 

 with rare exceptions, the young of the year migrate 

 before the old. This seems to preclude the idea held 

 by many, that the young learn the route by experience, 

 being led over it in the first case by the old. But 

 Mr. Dixon tells us, that while the main body of the 

 young do migrate before the main body of the old, yet the 

 first to take the field are certain individuals which have 

 not been able to breed, or have lost their eggs or young. 

 These old birds act as arnnt-ajurien-.i of the migratory 

 host, and gtiide the inexperienced young birds. Another 

 fact which may be thought to lessen the wonder of the 

 young finding their way is, that large numbers of them 

 are lost. The mortality during migration is enormous, 

 and according to Mr. Dixon is largely due to young birds 

 losiny their nay. Taken togethe , these two facts may be 

 accepted by many as sufficient explanation of the young 

 birds finding their way when migrating before the old ; in 

 my opinion, however, they do not go far enough. In the 

 case of the first, supposing it firmly established that a few 

 old birds do take the lead, can we suppose there would be 

 a sufficient number of them to act as guides to the young 



over large districts ? A few here and there would hardly 

 suffice. And the absence of breeding cares can hardly be 

 considered a sufficient cause for the migration of these 

 stray individuals, when we remember that the main Ixidy 

 of the old birds remain behind after such cares have 

 ceased, and even after the young have left the country. 

 Breeding cares, again, are not in themselves sufficient to 

 keep the old birds in the country, as we know from the 

 case of certain individuals breeding exceptionally late, 

 which have actually left their young in the nest in order to 

 migrate. In other words, they will stay after breeding 

 cares have ceased, and breeding cares are not sufficient to 

 keep them after a certain time. As to the second, it is 

 perhaps impossible to prove that the mortality is really 

 greater among young birds through losing their way than 

 among the old. 



The marvellous way in which birds find their way across 

 thousands of miles of land and sea in their annual migra- 

 tions has long excited the wonder of naturalists. Dr. Von 

 Middendorfl' suggested that birds might be endowed with 

 something which rendered them sensitive to the magnetism 

 of the earth, so that they were able to adjust their course 

 as a mariner does his by the magnetic compass. 



Prof. Weismann, in an essay on migration, considered 

 the faculty to be due to experience, not of the individual 

 but of the race. In other words, he believed the sharpened 

 faculties of one generation were transmitted by heredity 

 to the next ; a conclusion upon which his own recent 

 investigations on acqun-ed characters throw doubt. 



According to Mr. Dixon, the "mysterious sense of 

 direction " with which many writers have gifted migratory 

 birds is a " myth." Their power of finding their way, he 

 thinks, is dixe to their keen sight, and the fact that they 

 usually fly sufficiently high to take a bird's-eye view of a 

 great part of their route at once. And in crossing the sea, 

 in which the distance passed over is usually not more 

 than 300 miles, it is supposed that the height at which 

 they fly enables them to take in the land-marks on the 

 opposite shore. This power of sight, added to a very 

 retentive memory for land-marks, is thought to be sufficient 

 to account for their power of finding then- way. In regard 

 to the question as to the height at which migrating birds 

 usually fly, Mr. Dixon suggests the use of a captive balloon 

 from which to make observations. 



There are four classes of lines of migration, viz. : — sea 

 routes, coast routes, river routes, and mountain routes. 

 Of these, the first two are of special interest, as suggesting 

 old vanished coast lines and land connections. Birds in 

 their migrations seem usually to choose those lines which 

 present the least expanse of water to be crossed, and the 

 routes by which our summer visitors reach us and return 

 are by the Straits of Gibraltar, Corsica and Sardinia, and 

 Sicily and Italy ; and these routes correspond with the 

 old land connections between Europe and Africa, as seen, 

 for example, in Prof. J. Geikie's map of prehistoric Europe. 

 It is supposed by many that the passage of the migratory 

 hosts by these particular lines is due to the conservatism 

 of a habit practised by their remote ancestors when the 

 present water-way was land. As the land gradually sank, 

 successive generations of birds passed over the gradually 

 increasing distance of sea without experiencing much 

 lengthening of the journey. In this way, the studies of the 

 geologist and the ornithologist confirm each other. 



It is to be remarked, however, that if, as Mr. Dixon 

 believes, birds fly at a sufficient height to take in at a 

 glance ^■ast extents of country, we may well suppose that 

 individual birds at the present day may, from such a view, 

 be enabled to ehmise for thenise/ves the shortest route 

 without any aid from inherited habits. 



