86 THE MIGRATION OF BIRDS. 



A southern migration is therefore best performed^ 

 not as some readers might imagine with a north 

 wind blowing directly behind the travellers, but 

 with a south-easterly or north-westerly wind blow- 

 ing obliquely across their path. A northern 

 migration is best advanced by north-easterly winds. 

 Very light head winds are often favourable tO' 

 migration ; and in rarer instances migrants pass 

 to windward with a gentle and warm breeze. 

 Birds are very careful in their choice of wind, and 

 nothing retards migration so much as contrary 

 currents, the little travellers often waiting for days 

 until a favourable breeze springs up, and they can 

 renew their flight. Sudden gales from adverse 

 quarters will frequently blow migrants hundreds of 

 miles out of their proper course with fatal results, 

 and occasionally bring them to countries which 

 they would never normally have visited. 



The influence of Temperature on migrating birds 

 has not yet been studied sufliciently well to furnish 

 much reliable data on which any very important 

 generalizations or conclusions may be based. That 

 it has important effects on Migration Flight can 

 scarcely be doubted, and it is sincerely to be hoped 

 that observers when recording migratory move- 

 ments will pay due care to this portion of the 

 subject. Rises and falls of temperature are evi- 

 dently very important impelling causes of migration, 

 especially of nomadic migration. It is even 

 possible, if we allow ourselves to roam for a moment 

 into the speculative realms of theory, that the 



