December 19, 1907] 



NA TURE 



161 



return from lower lalitudos often takes place when the 

 food supply there is more abundant than in the higher 

 latitude to which they are travelling, and may even remain 

 abundant. But although a deficiency of food could net 

 in such circumstances be the immediate determining cause 

 of the movement, an approaching deficiency might, never- 

 theless, be the ultimate cause, for the most appropriate 

 time for leaving a region which is to become uninhabit- 

 able would be determined for each species by natural 

 selection, and might thus appear to have no immediate 

 connection with deficiency of the food supply, although in 

 reality dependent upon it. 



It is known, however, that, as has already been stated, 

 during the whole Tertiary period there was a mild or 

 warm climate and abundant vegetation throughout what 

 are now the Arctic and sub-Arctic regions, and it was 

 under these conditions, which presuppose abundance of 

 food supply during the whole winter, even in the highest 

 latitudes, that many existing genera of birds were evolved. 

 We may take it, therefore, that at that period the 

 autumnal migration was not rendered necessary by the 

 approaching severity of the winter months. 



This being the case, the question has suggested itself 

 whether the relation of daylight to darkness may not 

 have furnished the factor of most importance in the deter- 

 mination of both the south to north and north to south 

 movements, in consequence of the necessity to most birds 

 of daylight for the procuring of food. 



In no other class of vertebrate animals is the sense of 

 sight more important than in birds, and in no other is it 

 so highly developed. Kor detecting and obtaining food 

 most birds depend entirely upon vision, with perhaps, in 

 some, assistance from audition, and, in the case of soft- 

 billed birds, from palpation, but with little or no aid from 

 the olfactory sense, which is in so many animals the most 

 important of the senses in this connection. But vision is 

 not possible in the total absence of light, nor, without 

 special retinal adaptation, in semi-darkness. Hence the 

 great majority of birds — diurnal birds — are dependent upon 

 daylight for the procuring of food; relatively few, such as 

 most owls and nightjars (crepuscular and nocturnal birds), 

 are able to obtain food only in semi-darkness (twilight, 

 moonlight, or starlight) ; a certain number, e.g. many 

 waders, appear to possess retinal adaptation both for 

 oi;dinary light and for light of low intensity ; but, so far 

 as I am aware, no birds, except those which are provided 

 with tactile bills, are able to seek food in total darkness. 



From this consideration it is obvious that the propor- 

 tion of the twenty-four-hour cycle which can be utilised 

 by birds for obtaining food becomes greatly diminished 

 during the winter months in high latitudes, and may be 

 reduced to nil within the .Arctic circle, while during the 

 summer months the amount of daylight in high latitudes 

 is proportionately increased. Many birds are voracious 

 feeders, and during the hours of daylight are almost 

 constantly engaged in the search for food. It is therefore 

 a necessity that the time during which alone they can see 

 to engage in the search shall not be unduly restricted, as 

 would be the case in high latitudes during the winter, even 

 in parts which are rarely or never frost-bound. This the 

 north to south or autumnal migration provides against. 

 During the breeding season, when the young birds also 

 have to be fed, it is important that the time which can 

 be occupied in the search for food should be prolonged, 

 and this is provided by the south to north migration in 

 the spring. Everyone who has lived in northern latitudes 

 must have been struck with the time occupied by many 

 birds during the long summer days in procuring food for 

 themselves and their young : in fact, no more striking 

 object-lesson of the utility of prolonged daylight for the 

 rearing of their offspring can well be afforded. 



The objection might be taken to the relative incidence 

 of daylight and darkness at different seasons being re- 

 garded as a factor in causing north-south migration, that 

 in the case of nocturnal birds the course of migration 

 ought to be the other way, viz. from south to north in 

 autumn and from north to south in spring (in the northern 

 hemisphere) ; but, as has been already pointed out, the 

 so-called nocturnal birds are not, as is popularly supposed, 

 birds which can see in the dark, but birds the vision of 

 which is adapted permanentlv for light of low intensity, 



NO. I9QO. VOL. 77] 



such as twilight. Migration with such birds occurs in 

 the same sense as with diurnal birds, i.e. north to south 

 in autumn and south to north in spring (in the northern 

 hemisphere). This is, in fact, what might have been 

 anticipated, seeing how greatly the summer twilights are 

 prolonged in high latitudes. 



Again, it might be objected that the circumstance of 

 many birds leaving the higher or lower latitudes before 

 the autumnal or vernal equinox militates against the 

 assumption that the autumnal migration is determined 

 by a relative deficiency of light in higher latitudes 

 during the winter months, and that the vernal migration 

 is determined by the longer daylight which obtains in 

 those latitudes during the summer months. This objection 

 is, however, obviously met in the same manner as with 

 the analogous objection raised to the " food-supply " 

 theory pure and simple, viz. that the most appropriate 

 time for the actual commencement of migration will have 

 been determined for each species by the process of natural 

 selection. 



Further, the assumption that the relation of light to 

 darkness rather than severity of climatic conditions has 

 been the determining factor in producing the north-south 

 migrations would better explain the singular constancy 

 in the times of year at which these migrations occur. 

 For not onlv are the times of migration in many cases 

 independent 'of the actual climatic conditions which are 

 supposed to be the determining cause of the movement, 

 but the climatic conditions themselves vary considerably 

 from vear to vear in their inception and progress. On the 

 other 'hand, the incidence of the proportion of light to 

 darkness is a constant factor, and might even be conceived 

 to be operative in exciting the migratory instinct into 

 activity in the same manner as it is here assumed to 

 have been the original determining cause of north-south 

 migration. That there are other stimuli seems probable 

 from the circumstance that some birds have their winter 

 quarters in the equatorial region, where the proportion 

 of day to night does not vary throughout the year. There 

 are, however, very regular seasonal changes in that 

 region, which are accompanied by marked differences in 

 amount of daylight, and for those migrants which_ winter 

 there these seasonal changes may serve as the initiating 

 stimulus to northerly migration. That it is a result of 

 developmental changes in the sexual organs is improb- 

 able, since sexually immature individuals are also subject 

 to the migratory tendency ; nor is there any evidence that 

 such changes begin prior to migration. In any case, the 

 regularity with which migration occurs indicates that the 

 exciting cause must be regular. There is no yearly 

 change, outside the equatorial zone, that occurs so 

 regularly in point of time as the change in the duration 

 of daylight. On this ground this may well be considered 

 a possible determining factor in migration, and it has the 

 advantage over other suggested factors that it applies 

 to the northerly as well as to the southerly movement. 



Besides the north-south migrations with which we are 

 more immediately concerned, there are also the great 

 east-west vernal and autumnal movements which are so 

 prominent a feature in the eastern parts of these islands, 

 and also migrations of a more local character, both of 

 which merit some allusion in connection with the general 

 question of migration. 



As regards the east-west movements, which are, in 

 fact, for many species a part of the general north-south 

 migrations," it has been supposed that these divarications 

 from the main north-south stream have become evolved 

 either as the result of changes in the earth's surface, which 

 have produced a modification of the general north-south 

 trend,- or that they are the expression of the course of 

 expansion of the breeding range of the species as it 

 approaches its northern limit.' The physiological reason 

 for the east-west movement must ultimately be sought, _ as 

 in the north-south movement, in facility for the obtaining 

 of food, and it may fairly be assumed that in the case 



1 For the evidence of this see Gatke, " Heligoland an OrnithologicaT 

 Observatory," pp. 39-45. Also the British Association reports on bird 

 mieration, especially the " Digest," by W. Eagle Clarke, in report of Liver- 

 pool meetine, 1896. 



- Evans, Cambridge Natural Historv, " Birds," p. 18. 



'■> Dixcn, "The Migration of Birds," iS97,p. 35, also p. 40. 



