152 : June, 



the: spring RIVAI.RY OF BIRDS. 



: SOMK VIKWS ON THE I.IMIT TO MUI.TlPI,ICATlON. , ,. 



BY c. B; MOFFAT. jr-rir C: 



(Read before the Dublin Naturalists' Field Club, loth March, 1903.)' 



In the present paper I propose to put forward an opinion 

 which I have held for a considerable time, as to what is the 

 real reason why birds — and, perhaps, the higher vertebrate 

 animals generally — do not increase in number from j^ear to 

 year. This question has, I think, always puzzled field 

 observers, the answers commonly given being of that class 

 which look well on paper, but which somehow don't carry 

 conviction so readily when we close our books and look all 

 round us for that visible and tangible evidence which ought, 

 one would think, to be forthcoming. That great tragedy, 

 the ''struggle for existence,'* as pictured for us by Darwin^ 

 requires such a death-rate among the young in their first year, 

 or before they are of age to mate, as could not be less than 90 

 per cent, in the case of most of our finches and other common 

 small birds For my part, I cannot believe that the theory of 

 Natural Selection — for which I have a great respect, and which 

 I must carefully guard myself against appearing for a moment 

 to call in question— requires this sacrifice, or anything like 

 .it,. Such a mortality— in fact, a far greater mortality — may 

 verj^ well exist among the young of cannibal fishes, or of 

 reckless multipliers like the insects. But as regards birds I 

 am altogether unable to find grounds for believing in so great 

 a death-rate, at any rate in our own land. It seems to me that, 

 despite all perils, a large proportion, amounting in some 

 species to a majority of those that leave their nest, live. 

 For a number of years I kept count of a small isolated colony 

 of House-martins, from the year in which a single pair bred 

 for the first time until they had become too numerous to be 

 counted. So long as those martins could be counted, my 

 census continued to show that the number which returned in 

 spring was approximately the number which had departed in 

 autumn. This is entirel}^ contrary to what has generally 

 been supposed the common rule ; but it satisfies me that the 

 *' perils of migration," in the case of the swallow tribe, are 



