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? Reproduction 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 possession when other birds arrived would tell in their favour;
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