vin CASES OF ADAPTATION 151 



He also tells us that 1 5,000 sky-larks have been caught 

 on Heligoland in one night ; and all agree that the count- 

 less myriads that are seen passing over Heligoland are but 

 a minute fraction of those that really pass, high up and 

 quite out of sight. This is shown by the fact, that if, on a 

 dark night, it suddenly clears and the moon comes out, the 

 swarms of birds immediately cease. Another fact is, that, on 

 what the islanders call "good nights," the birds that come 

 to rest seem to drop down suddenly out of the sky. One 

 other fact is mentioned by Mr. Seebohm. It is that every 

 year the regular migration season is preceded by a week or 

 two during which a few stragglers appear ; and these are 

 all old birds and many of them slightly crippled, or partially 

 moulted, or without some of their toes, or only half a tail, or 

 some other defect. These are supposed to be mostly un- 

 mated birds or those whose young have been destroyed. 

 It is also supposed that, during favourable weather (for the 

 birds), migration goes on continuously during the season of 

 about six weeks, though for the most part invisible at 

 Heligoland, but often audible when quite invisible. 



Now, the fact of the young birds only appearing on 

 Heligoland for the first week or so of the season of each 

 species is easily explicable. Remembering that the autumnal 

 migration includes most of the parent birds and such of 

 their broods as have survived, it is probable that the latter 

 will form at least half or, more often, two-thirds of each 

 migrating flock. But the young birds, not having yet ac- 

 quired the full strength of the adults, and having had little, 

 if any, experience in long and continuous flights, a con- 

 siderable proportion of them on the occasion of their first 

 long flight over the sea, on seeing the lighthouse and 

 knowing already that lights imply land and food -crops 

 below them, and being also much fatigued, will simply 

 drop down to rest just as they are described as doing. The 

 old birds and the stronger young ones, however, pass high 

 overhead, till they reach the north coast of Holland, or, 

 in some cases, pass over to our eastern coasts. We must 

 also remember that the longer the birds are in making the 

 journey overland, the more young birds are lost by the 

 attacks of birds of prey and other enemies. Hence the 



