CONDITIONS OF FLIGHT 235 



time for their departure approaches, and there is much 

 evidence to suggest that such restlessness is invariably 

 increased or further excited by the sight of companions 

 en route, or by the cries they utter when on flight. We 

 remark the display of a similar state of restlessness 

 among birds at liberty upon the eve of their departure, 

 the gatherings of some birds, the unusual activity of 

 others. Whether a falling temperature or a failing 

 food-supply assist in intensifying this impulse we do not 

 at present know, but there can be little doubt that the 

 departure is taken when the impulse becomes too intense 

 to be longer resisted, whatever be its initiating cause. 

 Once, however, a bird begins its migration all Instinct as 

 a guiding medium ceases ; memory and knowledge of 

 locality, in fact experience, assist it to perform that long 

 journey. Migratory birds follow routes in every case 

 that indicate the line of their extension of range or past 

 emigration. These routes have been slowly formed and 

 are continuous, either from the area where the species 

 now winters, and where in past ages it formerly bred, or 

 by the absolute overlapping of the summer and winter 

 range. The individuals that follow one route by no 

 strange chance ever follow another ; even though their 

 own may be fraught with perils and difficulties unknown 

 to the other, it is still retained ; and even though one 

 route may be much longer and more tortuous than the 

 other, it still continues to be followed. If this were not 

 so, why should some individuals of the Wheatear, for 

 instance, elect to take a wide ocean passage to the 

 south-west of Ireland, whilst other individuals cross by 

 the Strait of Dover .^ Why should the Redstarts breed- 

 ing in or passing over Devonshire, cross the English 



