Vol. XXI 

 1904 



1 Taverner, The Origin of Migration. 7)?i?s 



the spring migration, but reversed. The shortage in food, how- 

 ever, is not caused, except indirectly, when the first migrants 

 encroach upon tliose below them, by the increase of population, 

 but by the direct failure of the food supply. It is perfectly 

 evident that certain species must return south again, or stay and 

 surely starve. The total population, however, of any area, cannot 

 permanently remain greater than the number that can be sustained 

 through the season of least food supply. During the height of 

 the breeding season, there are many more birds than can be 

 carried through the winter in the restricted southern stations, and 

 if they are to return there again, the excess must be got rid of. 

 Many of them are killed off at a very tender age — probably the 

 great majority of them fail to survive the fledgling stage. Many 

 more, young and inexperienced, must perish when first they leave 

 the protecting influence of the parent's care. Others are bat- 

 tered about by the storms and destroyed by the perils incident to 

 the fall migration. The few surplus that remain are subjected to 

 a stricter and stricter process of selection as they reach more con- 

 gested areas ; and, in the end, the total population fits into its 

 place in the winter quarters, to the extreme limit of the sup- 

 porting powers of the land. 



These migrations, in their earliest stages, must then have 

 originated in a conscious seeking for food — not special food, but 

 any food that would support them. Accidental wanderings taught 

 them where to find it, and experience suggested their return there 

 on the first approach of a stringency in the food supplies. In 

 course of time, the movement became habitual, and generations 

 of repetition rendered it instinctive. Instinct, having the same 

 relation to judgment as automatic machinery has to ordinary 

 mechanism, would be favored through natural selection ; and as 

 the birds acquired the peculiar powers necessary, migrations 

 assumed all the varied phenomena that they exhibit to-day. 



