294 THE BIOLOGY OF THE SEASONS 



stormy weather, (c) the marked shortening of the daylight 

 hours available for food-collecting, and (d) the dwindling 

 supply of insects and slugs, fruits and seeds, and the like. 

 In one or more of these external conditions we may perhaps 

 find sufficient liberating stimulus to set the migratory 

 instinct at work. 



In many ways the return Spring journey to the birthplace 

 is more difficult to understand than the Autumnal journey 

 southwards. The stimulus may be the setting-in of dry 

 heat, or uncomfortable weather of some sort, or the shrink- 

 age of the water-pools, and there is the awakening or re- 

 awakening breeding instinct. On the other hand, a large 

 proportion of the birds who undertake the northerly 

 journey in Spring are immature, and cannot be prompted 

 by any breeding impulse. Again, though one cannot 

 wonder that birds should like to get out of the heat and 

 the crowd and the multitudinous enemies of lower latitudes, 

 we cannot shut our eyes to the hazards of nesting in the 

 Far North. The young bird, as we have seen, is for days 

 after hatching very imperfectly warm-blooded, and cannot 

 be left exposed for even a short time without fatal results. 

 It is probably the constitution of the bird that makes it 

 necessary for some to go so very far north, just as the 

 eels are constitutionally impelled to go far out and far down 

 into the Atlantic. Perhaps in both cases the constitution 

 was established in relation to the conditions of the ancient 

 headquarters, the old home of the stock. Thus there 

 may be a constitutional home-sickness, though no psychical 

 one. There is no warrant for supposing that the Knot has 

 a " fond memory " for its birthplace in Northern Siberia. 

 Although Professor William K. Brooks suggested that the 

 north-bound Spring migrant who ignores so many choice 

 spots on its route, may be like " The shuddering tenant 

 of the frigid zone/' who " Boldly proclaims that happiest 



