THE AUTUMN MIGRATION OF BIRDS. 223 



of birds is due to a rise of temperature in their 

 winter quarters at the approach of summer. On 

 the one hand, the rise of temperature curtails food 

 supply, as well as introduces conditions that render 

 Reproduction, so far as we can learn, either im- 

 possible or undesirable ; whilst, on the other hand, 

 the fall of temperature and its consequent ice 

 and snow cuts off the food supply more or less 

 completely and compels a southern movement. 



In autumn many of the conditions of Migra- 

 tion are exactly reversed. The birds that have 

 the longest journeys before them are the first to 

 start, leaving the high latitudes where they were 

 born, or where they have spent the summer, at the 

 earliest moment. Young Knots {Tringa canutus) 

 and young Gray Plovers {Charadnus helvetiacs) 

 begin to pass Heligoland and the British Islands 

 early in August, some even with bits of down 

 sticking to their plumage. Both these birds winter 

 far in the Southern Hemisphere, reaching Australia, 

 South Africa, and South America, with fly-lines 

 more than 10,000 miles in length! Young Sander- 

 lings [Calidris arenaria) sometimes arrive on the 

 British coasts at the end of July ; their fly-line is 

 equally lengthy, extending to South Africa, South 

 America, and the Malay Archipelago. The Bean 

 Goose [Jnsei' segetimi), on the other hand, with a 

 fly-line only extending as far south as the Mediter- 

 ranean basin, China, and Japan, breeds as far north 

 as land is known, yet migrates later, and does not 

 reach its most southern quarters until winter is close 



