354 THE BIRDS OF HELIGOLAND 



of unusual occurrence even late into April. The autumn migration 

 takes place during October and November. 



The Skylark belongs to those birds, the altitude of whose migra- 

 tory flight hardly ever passes beyond the utmost limits of human 

 vision ; for they never, not even during fine and sunny spring days, 

 rise to elevations at which they appear no larger than scarcely 

 perceptible particles of dust, as is so often the case at that time 

 with Jackdaws and Rooks, but the separate birds of a flock can 

 invariably be distinguished to perfection by a keen and practised 

 eye. In the autumn migration, on the other hand, especially in 

 dull and heavy weather, they often fly so low over the sea that 

 they have to adapt their flight to the undulations of the slowly 

 rolling waves. During uniformly dark, damp autumn nights the 

 height of their migration-flight seems to amount to about 200 feet, 

 for all the birds seen here by the lantern of the lighthouse arrive 

 flying in the same plane as the latter, as is also the case with all 

 the other various migrants which arrive on these dark nights. As 

 soon, however, as this uniformly dense darkness begins to get 

 at all scattered, with the appearance of one single star, or of 

 the faint line of light which announces the speedy rise of the moon, 

 all these Larks, like all other migrants present at the same time, 

 rise at once to elevations from which not a single one of their call- 

 notes is any longer audible. If, a few hours later, the firmament 

 once more becomes shrouded in uniform darkness, the stream of 

 migrants forthwith again rushes past at the former lower altitude. 

 As we have already expressed in the chapter on the Altitude of the 

 Migration Flight, these phenomena illustrate in a striking manner 

 how much the height at which birds fly in their migratory journeys 

 is dependent on the conditions of the atmosphere prevailing at 

 the time ; and an apparently slight change in these conditions 

 either brings the wanderers within our view, or completely with- 

 draws them from the observation of our senses. Accordingly, 

 weeks and weeks sometimes pass without the birds due at the 

 particular time being seen ; it is then generally assumed that they 

 have been kept back by bad weather. Should, however, the 

 weather suddenly improve at the end of the spring migration of 

 some particular species, all that will be seen of the latter is a rear- 

 guard composed of females and younger birds ; whence it follows 

 that the males which initiate the migration have, during the pre- 

 valence of unfavourable conditions in the lower regions of the 

 atmosphere, made use of strata offering more favourable conditions, 

 and have passed on their way far above the range of our observing 

 faculties. 



The migratory phenomena of these Larks, as they are displayed 



