128 THE MIGRATION OF BIRDS 



and very slowly. It is probable that strong air- 

 currents at a high elevation materially assist rapid 

 and lengthened migration. Force not direction of 

 wind influences birds moving at a low elevation. 



Birds possess a certain power of orientation, a 

 homing instinct, which need not be called a sixth 

 sense. Brain and eyes assist in the development 

 of this power ; birds have an excellent memory. 

 Young birds lose their way more frequently than 

 is generally supposed ; variations in routes are 

 explained in many cases by these errors. Young 

 may or may not be guided by experienced adults ; 

 orientation is not infallible but develops with age. 



There is apparently no truth in the assertion that 

 birds travel by choice against a head wind or in 

 a beam wind ; a moderate wind behind, on which 

 they are carried, is most favourable. Leeward 

 drift through contrary winds explains many normal 

 and abnormal routes, and the occurrence of un- 

 expected species in unexpected places. The distance 

 travelled not only varies according to species but 

 in individuals of the same species ; the thesis that 

 the most northerly breeder winters farthest south 

 does not always hold good. 



Much may be learnt by the careful registration 

 of arrivals and departures of migratory birds, and 

 by the marking of birds. Ornithophaenology, the 

 science of migration study, as carried on at present 



