ECOLOGICAL 73 



but perhaps they imitate their seniors. Yet this won't do either; for 

 what are we to make of the fact that many young birds leave us in 

 autumn in advance of their parents ? 



On the whole it seems that we must call bird-migration an 

 instinctive custom. Instinctive means inborn, not requiring to be 

 learned, engrained deeply in the hereditary make-up. Hive-bees 

 build a honeycomb instinctively; birds migrate instinctively. It is 

 not merely that the urge to migrate is rooted in the constitution, 

 there is an inborn capacity for obeying the impulse in an effective 

 way — some unusual sense of direction. One of the arguments in 

 support of this view is that inexperienced young birds often set out 

 alone on their long journey to an unknown goal. Another argument 

 is that the migratory custom is seen in many kinds of creatures at 

 very different levels of brains and mentality — in mammals, like 

 reindeer and seals; in reptiles, like sea-turtles and sea-snakes; in 

 fishes, like salmon and flounder; in the land-crabs also, which 

 return every year from their inland haunts to spawn in the sea. A 

 large part of the answer to our first question is in the mysterious 

 word instinctive. 



In the course of thousands of years migrating has come to be 

 part and parcel of the constitution of many birds ; therefore it must 

 have had great advantages. There must have been good reasons 

 why birds with a strong migratory instinct succeeded best, while 

 those with a weak migratory instinct were sifted out. Part of the 

 answer to this second question must be that in north temperate 

 countries it is a great advantage to avoid the cold, the storms, the 

 scarcity, and the short daylight of winter. And while the southern 

 winter quarters are well-suited for rest and recuperation, they are 

 places to get away from when summer comes, with its drought and 

 glare. Especially for nesting there is an attractiveness in the cool 

 northern haunts, where there is abundance of water, shade, daylight, 

 and midges. 



Our third question is somewhat like the second, and yet it is 

 different: What are the immediately present causes that liberate 

 the instinct to migrate? The advantage of a cool nesting-place in 

 the distant north cannot pull the trigger which sends the swallows 

 from South Africa to Britain in early summer. What are the 

 immediate causes? 



After a British eel has been feeding and growing for five or six 

 years in the pond, it becomes restless, and begins to make its way 

 down the river, whence it reaches the sea, and proceeds on its long 

 journey, perhaps as far as the Bermudas. Now we know that there 

 are changes in the eel's body and blood that pull the trigger of its 

 migratory impulse ; and the same is true for birds. There are internal 

 trigger-pullers or stimuli in both cases, though we do not as yet 

 know very much about them. 



