72 THE BIRDS OF HELIGOLAND 



During the spring migration, in the absence of some specially 

 exceptional cause, none of the countless wanderers prolong their 

 stay here beyond the few hours previously mentioned ; all of them 

 speed onwards to their breeding homes in restless haste. Some, 

 like the various Yellow Wagtails, do not even tarry till the close 

 of the day on the dawn of which they had arrived, but resume 

 their journey at about the middle of the forenoon. From the 

 phenomena of migration as displayed on this island, we can, how- 

 ever, form no estimate as to how long or how far northward 

 the wanderers continue their restless voyage ; what we know 

 for certain is that they push unceasingly forward as long as they 

 are not hindered by meteorological influences, and that none would, 

 unless compelled, break their journey for any length of time before 

 reaching their nesting quarters. On the other hand, Seebohm's 

 interesting observations at the mouth of the Petchora and Jenesei 

 rivers go to show that all species breeding in very high northern 

 latitudes may frequently be detained in the course of their spring 

 migration shortly before its termination. On the disappearance of 

 winter, and the break-up of the immense icefields formed upon these 

 rivers, countless swarms of both land and water birds are seen to 

 fill the air in the most motley confusion. Just as after the com- 

 pletion of the first great stage in the autumn migration many of 

 the wanderers slacken their speed until cold and frost once more 

 impel them onward, so in this case the persistence of wintry con- 

 ditions has the opposite effect by causing the birds to relax the 

 speed of their advance, or by bringing them to a standstill for a brief 

 time, shortly before reaching their breeding homes. As long, 

 however, as neither the one nor other of these influences comes 

 into play, the Hooded Crow, the Bluethroat, and the Virginian 

 Plover pursue their migration at the velocities which have been 

 established in their cases respectively. In the case of the last of 

 these birds this velocity is indeed so enormous that we are obliged 

 to assume the assistance of other factors besides the mechanical 

 instruments of motion with which the bird is equipped. 



In treating of the height of the migration flight, we have con- 

 sidered in detail that birds, as distinct from other warm-blooded 

 creatures, are provided with a respiratory mechanism enabling 

 them to remain for any desirable length of time in regions of the 

 atmosphere so rare in density and poor in oxygen as must neces- 

 sarily result from elevations extending to 40,000 feet ; and we have 

 further seen that they are, in addition, provided with a very 

 extensive system of air-sacs which they are able to fill or empty 

 at pleasure. These peculiarities, either alone or in co-operation, 

 appear of no ostensible use to the bird during its ordinary daily 



