144 THE BIRDS OF HELIGOLAND 



the birds of the former species are aware that their summer home 

 is already habitable towards the end of March, or the latter that 

 theirs will not be fitted for their reception until some four or six 

 weeks later. 



The phenomena of the autumn migration are equally astonish- 

 ing. Beginning at the end of June, and continuing till after 

 November, we observe that, throughout its course, not only do the 

 various species follow each other in a fixed order of sequence, but 

 that an order of like kind is maintained in the case of each indi- 

 vidual species by the different sexes and ages of its members, 

 without our being able to assign any cause which would compel 

 different species, or separate sexes and individuals of different ages 

 of the same species, to set out on their migrations at different 

 times. 



From very old times, mainly in consequence of the phenomena 

 which succeeded migration, it was conceived that in spring, with 

 reawakening life in Nature generally, the reproductive instinct of 

 birds also was roused afresh, and that it was this which urged them 

 to wander to their nesting-places ; while, in autumn, dearth of food 

 and cold admonished them to seek a temporary home in warmer 

 latitudes. This view has in part held its ground up to recent 

 times, for it is not so long ago that Brehm, in one of his talented 

 discourses 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. There were, indeed, 

 sufficient grounds for this conception, for every year we were 

 brought face to face with the fact that, immediately after their 

 arrival, our feathered wanderers set about building up their 

 secluded nests amid jubilant strains of song, whence, soon after, 

 we might see the young brood slip out into freedom ; and, again, 

 were they not seen to depart with equal certainty as soon as the 

 gales of autumn began to sweep the stubbles bare and to whirl 

 before them the brown foliage of the woods ? 



These explanations however do not suffice to explain all the 

 phenomena of migration; thus, 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, at least three 

 or four weeks after the latter. Inasmuch as this last portion of the 

 spring migration, consisting of individuals not yet capable of repro- 

 ducing their species, probably constitutes a third of the whole host 



