72 LIFE : OUTLINES OF GENERAL BIOLOGY 



bulk of migratory flight goes on below 3,o(K) feet, whether by day 

 or by night. It will Ix* understood that the altitudes referred to are 

 estimated as above i^roiindAeve] There is no doubt that swallows 

 cross the cols of the Alps at a height of 10,000 feet and more, or 

 that others cross the passes of the Himalayas at heights of 18,000 

 feet at least, above st'rt -level. 



In recalling the main facts of bird-migration, we must conclude 

 bv referring to the contrast often marked between the northerly 

 spring flight to the nesting-places and the southerly autumnal 

 flight to the winter quarters. The vernal flight is often more impe- 

 tuous and continuous, and sometimes its route is more direct than 

 that followed in autumn. In many species the males arrive earlier 

 than the females, and may select a nesting "territory" before their 

 future mates arrive, as is well known in the case of warblers. The 

 last to arrive in the summer quarters may be immature birds that 

 will not pair that season. The autumn movements are often on a 

 grander scale, for the ranks have been increased by the young birds 

 of the year. There are often preliminary congregations and false 

 starts; the young birds may go off first; but the adult cuckoos hurry 

 away a month or more ahead of the young ones, whom they have 

 never known. 



PROBLEMS OF BIRD-MIGRATION.— There is no doubt that 

 the facts of bird-migration are becoming clearer every year, but they 

 raise many deep questions that we cannot as yet satisfactorily 

 answer. The whole subject bristles with brain-stretching problems. 



Our first question is in regard to the nature or true inwardness 

 of this yearly journeying of so many birds between summer and 

 winter (juarters, between nesting-places and resting-places. If we 

 visited a northern country and found that most of the inhabitants 

 went in winter to the South of France, but spent the summer in 

 the Scottish Highlands, we might say: "How very intelligent, to 

 arrange to have two summers in the year". May we say the same 

 for the birds that "change their season in the night and wail their 

 way from cloud to cloud down the long wind"? Hut it is impossible 

 tf) take this generous view, for though birds are sometimes able to 

 put two and two together, and profit by experience, we must 

 rememlHT that for many generations our summer visitors have 

 known no winter in their year. They cannot by intelligence provide 

 against the entirely unknown. Whatever may be the answer to our 

 fjuestion. it is not in the word intelligence. Yet we must be careful 

 not to think tliat birds are unintclliiicnt in their migration or in any 

 f»ther activity. 



Hut it may Ix- suggestefl that the success of the migratory flight 

 dep<'nds on a tradition kept up from generation to generation. We 

 must admit that the migrants do not meditate about their flight. 



