THE MIGRATIOX OF IXLAN-D BIRDS. 193 



habit, l)ut one now not absolutely necessary to the bird's welfare, we 

 can see why it should be, as it frequently is, so greatly influenced by 

 surrounding circumstances and conditions. 



Taking the movement from its proper starting-point, which we as- 

 sume to be the movement from south to north, in the three sprino- 

 months, we must now look for sufficient causes to induce the under- 

 taking of such long journeys. These causes are suggested by the two 

 principal objects eflected, on their arrival at their northern destination, 

 viz., rearing of their young, and procuring suitable and sufficient food 

 for both themselves and ofispring. If migration is for these two pur- 

 poses only, then it should prove to be the case that food was not suffi- 

 ciently abundant in the south for both its resident and migratory birds. 

 This certainly could not have been the case, and we believe, thei-efore, 

 that migratory movements, at the outset, were to a very limited extent 

 only ; a few birds at a time seeking to avoid their enemies, and have 

 undisturbed possession of a locality, by pushing out from their accus- 

 tomed haunts, for, comj)aratively speaking, a few miles. The young 

 of such pioneer birds would naturally leave the neighborhood of their 

 nest, and return to their parents' usual haunt with them ; but, on the 

 return of another breeding-season, they would themselves seek a rest- 

 ing-place near where they themselves were reared, and the older birds 

 would go to the same nest or nesting-place that a year ago they occu- 

 pied. This is precisely what occurs now, year after year. Now, as 

 birds increased, century after century, the limits of this northward 

 movement would be extended, until it became in time the journey of 

 thousands of miles that it now is. 



Assuming, then, that migration arose for the dual purpose of safe 

 nidification and a certainty of sufficient food, Ave are met by the ugly 

 question, " Why do not all the southern birds come-north ? " If, when 

 the whole avi-fauna was concentrated at the south, there was any strug- 

 gle whatever for favorable feeding or breeding grounds, then, naturally, 

 the weaker would go to the wall, or, in other words, would be driven 

 beyond the limits of their accustomed habitat. These weaker birds, 

 taken together, having once formed the habit of visiting certain locali- 

 ties at stated times for given purposes, or periodically were forced to 

 do so, would vary in their methods of reaching these localities, in their 

 choice of regions wherein to remain, and the length of their annual 

 visit, just in proportion as their habits generally varied from those of 

 both other species of the same family and from species of other fami- 

 lies. For instance, to avoid a common enemy, a number of species 

 might have gradually learned to migrate at night ; while others, al- 

 though forced to migrate, had not this same enemy to contend with. 

 In this way, the habit of nocturnal migration would long ago have been 

 formed, and it would, by inheritance, be continued by their descend- 

 ants, even after the enemy had been long extinct. 



Having reached the northern summer homes, and, free from molesta- 



TOIi. TIL 13 



