( 18 ) 



There is auotlier jibase of the subject that deserves notice. In witnessing 

 migration in jirogress, esj)ecially over the sea, one can iiardly fail to remark that 

 certain " fly-lines " are followed. This phenomenon has sometimes led observers 

 to conclude that birds cross the sea from certain points only. This is probabl)' an 

 erroneous view, and we may expect to find that the departure takes jilace from 

 any point within the section of the coast corresponding to the migration route. 

 AVben an indiviilnal or a party takes the initiative, the force of example causes 

 the lead to be followed by other individuals or parties, and in this fashion a 

 " flv-line " is established. 



HOW DO BIRDS FIND THEIR WAY? 



When we turn to the (jnestion as to how birds direct and maintain their tlight 

 in the right direction, we are confronted with a problem of the most perplexing 

 kind, and one which is much complicate! by reason of the great diversity to be 

 observed in the movements of most s])ecies, and in the conditions under which 

 Ihe movements take place. Want of sustenance and temperature changes are 

 doubtless sufficient to lead birds to wander, but these causes operating alone are 

 just as likely to lead them to wander in the wrong direction. They need to be 

 associated with some other and more important factor before orderly progres.sion 

 in a definite direction becomes possible. 



Many writers have supposed that guidance is afibrded by the i)romiuent 

 topographical features of a country, such as rivers and mountain systems. Let 

 us take the case of a common and widely distributed migratory species in our 

 own countrj', say, the Swallow, and consider how the individuals may, by the 

 aid of physical features, reach the sonth coast. During their flights in search of 

 food, individual Swallows, no matter what part of the couutry they occupy, are 

 pretty certain to explore a sufiiciently wide radius to make themselves acquainted 

 with numerous waterways. If a waterway is followed in the direction of the 

 stream, sooner or later the sea-board is reached. This, then, would be an easy 

 way of reaching the coast ; but there remains the difficulty of reaching the south 

 coast, and this difficulty is not at ail lessened in the case of those birds which 

 reach the east and west coasts, since a faculty that would enable them to follow 

 these coasts in a southerly direction would suffice to enable them to attain the 

 desired end by directing their flight from the first in a southerly direction. In 

 the case of great masses of land, the guidance afforded by following rivers or 

 mountain ranges would, as often as not, lead birds right out of their course. 



Any one who examines the evideuce that has been adduced in support of 

 the theory that birds are guided by the i)rominent physical aspects of the laud 

 they traverse will, I think, not fail to become convinced of the inadei|uacy of 

 such guidance. 



Some authorities of note have thought that the guidance may be due to a 

 "sense of direction," and in support of the theory have referred to the exercise 

 of such a faculty by human beings, especially savages, and by wild and domesti- 

 cated animals. It is to be observed, iiowever, tiiat this fjiculty is, to a very 

 large extent, correlated with experience ; and it seems doubtful, to say the least, 

 whether it can ever be " wholly indej)endent of intellectual forces," as is averred 

 by Trofessor Newton (oj/. cit. ]). oO!', footnote) ; for, if this were true, young 

 Buvages and young "homing" Pigeons wnuld find their way as easily as adults. 



