THE SOUTHWARD FLIGHT 273 



the same route on both journeys. At least one striking case 

 has been discovered in which the spring and autumn tracks 

 lie far apart. The American golden plover flies straight 

 across the sea in autumn from Nova Scotia to the coast of 

 South America — a distance of about 2,500 miles; but it 

 returns in spring by a more circuitous route to westwards, 

 through Mexico and up the Mississippi valley. 



There seems to be an obvious reason in the failure of the 

 food-supply why birds of passage should depart southward 

 in autumn to milder climes. But it is not so easy to under- 

 stand why they should want to return in spring. It might 

 be thought that they would be well enough off where 

 they were, like the resident species of tropical forests, 

 without daring the long journey over land and sea to 

 reach some distant corner of the British islands, or some 

 haunt even further to northwards, within the Arctic circle. 

 The key to this movement is probably to be found in their 

 general habit of scattering in pairs in spring, to bring up 

 their young in privacy and with an ampler food-supply than 

 they need when there are only their own mouths to fill. 

 They would thus naturally tend to spread outwards from 

 their winter home ; and those birds would thrive best which 

 pressed further and further to northwards (or to the south, in 

 the southern hemisphere), and so gained the advantage of 

 longer daylight and a longer period each day in which they 

 could hunt for food. As they settled in a new home, 

 inherited habit would tend to attach them to it by a strong 

 bond ; and so the great double migration would grow up, at 

 the times of year when the seasons most sharply change. 

 Marvellous as the length and adventurousness of their 

 passage seems, it has after all to be remembered that birds 

 are winged creatures, constructed by nature with supreme 

 powers of locomotion ; and that they do sometimes perish 



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