1903- Moffat. — the Spring Rivalry of Birds. 157 



they never took the initiative in attack, were stubborn in 

 defence, and would not go. 



These two cases — within one very small area — show plainly, 

 I think, what would occur if two pairs of Blackbirds or two 

 pairs of Finches attempted to build very near to one another. 

 The cock birds would almost certainly fight, and the weaker 

 would either be killed or retreat. This practically is the 

 view which was taken a hundred and thirty years ago by 

 Gilbert White, when he wrote that in spring " such a jealousy 

 prevails between the male birds that they can hardly bear to 

 be together in the same hedge or field " ; and "it is to this 

 spirit of jealousy," added White, " thai I chiefly attribute the 

 equal dispersion of birds in the spring over the face of the 

 country." This was written before much attention had been 

 drawn to the all-important question, why does not the 

 multiplication of living creatures proceed at such a pace as to 

 overstock the earth. But I venture to say that so far as 

 birds are concerned it supplies in itself an adequate answer. 

 For, in course of time, the country — or the parts of it suitable 

 for nidification— would come to be completely parcelled out 

 between the birds, each parcel of land belonging to a par- 

 ticular pair : — I mean, as against any other pair of the same 

 kind. And, once that happy state was arrived at, the number 

 of nesting pairs each year would be exactly the same, the 

 number of nests and the average number of young birds 

 reared would be exactly the same ; and whether there was a 

 large mortality in winter, or a small mortality in winter, the 

 total number of birds in the country would remain exactly 

 the same. As long as the annual birth rate, or rather number 

 of births, is constant, and has been so for a given number of 

 years, it must be balanced by the annual death rate, and 

 further increase of the species becomes impossible. 



Suppose, for example, that ten pairs of Chaffinches have 

 nested every j^ear in one orchard, and that every suitable nest- 

 ing tree in the orchard stands on ground belonging to one or 

 other of the ten. If this has gone on for a period equal to the 

 average life of a Chaffinch, it follows that the number of 

 Chaffinches of the original stock that die every year of old age 

 would exactly equal the number hatched every year, supposing 

 that no mortality at all had taken place among young birds. 

 Thus, though we may suppose that about forty young would 



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