MORTALITY AMONG BIRDS. 115 



because martins return to this country in smaller 

 numbers than they quit it, it necessarily follows that 

 the overplus have all died abroad during the winter. 

 The excess of numbers at the time of migration in 

 autumn is made up of the young birds of that year. 

 Now, though it seems well ascertained, that old 

 birds, which have bred in this country, generally re- 

 turn to the same breeding-places each season, I am 

 not aware of any facts that prove that all the young 

 birds return with their parents. Is it improbable 

 that, instead of doing this, many may disperse them- 

 selves about in various directions, when the spring 

 arrives, and assist in filling up gaps, wherever, from 

 accident or other cause, the numbers in any place 

 may be deficient ? It may be in this way, partly, 

 that there is kept up, in the case of some species, 

 that equal balancing of numbers in different coun- 

 tries, to which we have already alluded in a former 

 place.* 



If it be indeed true, that, with regard to martins, 

 a vast devastation of individuals (as White supposes) 

 takes place somewhere in other countries during^ 

 winter, and if the same could be shown of our sum- 

 mer migrants generally, analogy would lead us to 

 expect that our winter migrants would receive the 

 check to their undue increase, in like manner, during 

 winter, in this country. And according to this view 

 these migrants must visit this country in autumn in 

 much larger numbers than those in which they quit 

 it in the spring. But I doubt if we have any ob- 



* See back to p. 108. 



