114 OBSERVATIONS ON BIRDS. 



mainder. This inquiry leads to several other inqui- 

 ries connected with the statistics of birds, which it 

 would be most interesting to have answered; but 

 which, in the present state of our knowledge, it 

 would be difficult to clear up. More especially it 

 would be important to determine at what period, as 

 regards both the age of the individual, and the season 

 of the year, as well as how and in what exact manner, 

 the mortality among birds takes place. This mor- 

 tality must, in the instance of some of the smaller 

 species, be very great. The numbers reared each 

 year seem to bring no proportionate accession to the 

 general number of individuals the year following. 



Does then the mortality fall principally upon the 

 old birds or the young, and during winter or during 

 summer ? White has observed with respect to house 

 martins, that " they must undergo vast devastations 

 somehow, and somewhere ; for the birds that return 

 bear no manner of proportion to the birds that re- 

 tire."* From this one might suppose, that, in the 

 case of this species, it was during their sojourn in a 

 foreign country in winter that their numbers became 

 thinned, and that the deaths were in proportion to 

 the increase they had received here during the breed- 

 ing season; for about the same numbers seem to 

 return each year. It would follow also that it was 

 not by the destruction which fell upon the very 

 young birds, that these numbers were kept within 

 the assigned limits, but by that of the adult. I 

 think, however, it may fairly be questioned, whether, 



* Letter XVI. to Daines Barrington. 



