42 THE BIEDS OF HELIGOLAND 



Some of his comrades answer, and follow in his flight. After a 

 considerable elevation has been attained the whole flock assembles, 

 and soon all, travelling eastwards, have vanished from sight. Such 

 departures take place an hour after sunset, after which there seems 

 to be a short pause. Soon after midnight, however, the migration 

 commences anew with the arrival of countless wanderers, whose 

 numbers increase with each succeeding hour of the dawning day. 



In all the johenomena of the spring migration the motive of 

 reaching a fixed goal within a strictly limited time, and for a 

 definite purpose, is clearly manifested. It follows naturally that 

 the direction of migration should be specially influenced by this 

 aim, their nesting homes being generally situated in latitudes 

 considerably above those of their winter quarters; the shortest 

 route would be one in a direct line between these two termini, i.e., 

 for the majority of species, in one directed more or less to the north. 

 Such a course is, in fact, at once adopted in spring by those species 

 whose migration in autunm had proceeded in a direction to the 

 south ; but besides these we have the case of those migrants which, 

 after at first following a main westerly course in their autumn 

 migration, terminated their passage by a deviation to the south in 

 England, France, and Spain, and in this manner also have reached 

 latitudes lying considerably to the south of their breeding haunts. 

 In the spring, however, such birds do not return to their nesting 

 quarters by retraversing in the opposite direction the circuitous 

 path of their autunm passage, but now travel in a direct line, 

 leaving points touched at during their autumn migration far to the 

 north of their new, less complicated route. In fact, if we represent 

 the line of their autumn migration by the base and perpendicular 

 of a right-angled triangle, the path of their return passage in spring 

 will be represented by the hypotenuse of such a triangle.^ This 

 will at once explain the remarkable phenomenon previously 

 referred to, that all eastern species reaching Heligoland in autumn 

 by a westerly course, but subsequently turning south, are hardly 



' As may be illustrated by the following figure :— 



^nfrot'or! 



Tr. 



