Vol. XXI 

 1904 



"I Tavernek, The Origin of Migration. 3^5 



thus affected. His conclusion is apparently based upon the well 

 known and indisputable fact that birds are harder to find during 

 the breeding season than at other times. It must, however, be 

 remembered that for each pair of breeding birds observed, there is 

 somewhere about a nest full of young that are not seen at all. 

 These young are of as much economic importance in reckoning 

 population as the adults, and as such must be taken into consid- 

 eration. On the whole, I doubt very much whether the bird pop- 

 ulation m the breeding season is any less per given unit of territory 

 than at other times. 



That migration is caused by a natural dispersal of the adults 

 during the breeding season must be admitted. But this is beg- 

 ging the question. Migration is a dispersal ; and conversely, this 

 dispersal, as it manifests itself, is migration. The author fails to 

 explain the cause of the natural dispersal. The object of this 

 scattering may be seclusion, either for privacy or safety. If for 

 privacy, it seems to defeat its own ends when such birds as the 

 herons, swallows, and like gregarious nesters congregate in great 

 communities to perform their marital duties. If safety is sought, 

 it presupposes that all the safe nesting sites are monopolized by 

 other species and the migrants are crowded out. 



In our own country, we can readily see that but an infinitesimal 

 fraction of possible sites are thus occupied. How rare it is for a 

 nesting place to be used a second time by different individuals, — 

 except in the case of woodpeckers' holes, where it is obvious that 

 the supply is limited, — any field worker knows. If desirable 

 forked branches, etc., were at such a high premium, this would 

 occur frequently. If, then, the above is true in our own country, 

 how much more must it be true in the tropical stations, where, 

 though the population of both birds and their enemies is greatly 

 increased, the luxuriant vegetation affords an infinitely greater 

 number of desirable sites for nesting. Crowding in this sense 

 seems impossible. 



That individual birds cannot be driven from what they regard 

 as their proper stations, may possibly be admitted ; but that spe- 

 cies cannot (w-hen the adverse changes in surroundings take place 

 gradually enough), is absurd. As far as I am aware, there are 

 three principal ways by which geographical distribution can be 



