BRITISH WARBLERS 



capabilities too highly, and any slight inequality in power of 

 flight or endurance could give the males an advantage of but 

 a few hours only. Darwin believed that the difference in the 

 periods of their arrival could be explained thus : " Those males," 

 he writes, " which annually first migrated into any country, or 

 which in the spring were first ready to breed, or were the most 

 eager, would leave the largest number of offspring, and they 

 would tend to inherit similar instincts and constitutions." 

 But why should those males that first migrated leave a larger 

 number of offspring ? Eeproduction could not commence until 

 the females arrived, consequently the males that reached 

 sexual maturity later and travelled in company with the 

 females would have equal, if not greater, opportunities for 

 securing a mate. There is some evidence to show that 

 throughout Nature there is a tendency for the males to reach 

 sexual maturity before the females, but this does not explain 

 why they undertake the journey so much in advance ; it 

 makes it, in fact, all the more difficult to understand. For 

 we cannot but believe that a male that had reached sexual 

 maturity would be all the more reluctant on account of its 

 own passion to desert the company of the females and travel 

 on to the breeding grounds, there to await their arrival. The 

 disadvantage in their thus hurrying forward is so obvious that 

 we can only conclude that there must be some advantage 

 correspondingly great, for they, above all others, are the 

 individuals that will be called upon to withstand the rigours 

 of an inclement spring, cold winds, and even blizzards, to 

 which countless numbers frequently succumb. This desertion 

 of the females and anxiety to reach the breeding grounds, 

 which is difficult to reconcile with the belief that the first 

 step towards reproduction is the finding of a mate, becomes 

 intelligible if we accept the principle of breeding territory. 

 Those individuals that hurried forward would, on the average, 

 be more likely to attain to reproduction, for they would have 

 the choice of territory, and the fact of being already in posses- 

 sion when other birds arrived would tell in their favour ; 



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