364 THE STRUCTURE AND LIFE OF BIRDS chap. 



not appearing- till the latter half of September. A 

 few unmated old birds precede the young ones by two 

 or three weeks. It is strange that the young and inex- 

 perienced should start, apparently, so much sooner 

 than is necessary. Possibly, however, they have it 

 strongly hinted to them by their elders that their 

 room is wanted, since the supply of food is limited 

 and the young second broods are voracious. There is a 

 very remarkable exception to the general order of 

 departure. The old Cuckoos are in the same position 

 as the unmated birds of other species. They have no 

 responsibilities to keep them in the north, and they go ? 

 leaving their young to find their way south as best 

 they can. 



These curious phenomena show how intimately 

 migration is connected with nesting. Many birds who 

 have paired stay late in order to bring up their young. 

 Yet every year it happens that some Swallows and 

 Housemartins start for the south, leaving their young 

 to die in the nest. This seems unnatural ; still, parental 

 affection keeps them in England longer than they 

 would remain but for that. We may imagine a 

 struggle within them between the love for their young 

 and the migratory impulse, and the latter at last 

 becomes overpowering and swamps the former. The 

 unmated cock birds, in whom no such struggle 

 between opposing motives goes on, have started long 

 before. Many who have no young brood to tend 

 are delayed by their moulting. It is possible that 

 the unpaired migrants, like unpaired pigeons, have 

 their moult delayed, and that their early departure 

 southward takes place before it begins. This question 



