76 THE BIEDS OF HELIGOLAND 



and in no case more pointedly that that of the Northern Blue- 

 throat, a bird which has already been brought into evidence on 

 several other occasions. 



In the chapter on the height of the migratory flight, we have- 

 in briefly mentioning the night capture of Larks on this island 

 pointed out how largely the greater or less elevation, at which 

 the migratory movement proceeds, is dependent on apparently 

 slight changes in the condition of the atmosphere at the time being. 

 Therefore, although under normal conditions the migrations of 

 most species proceed at a height of at least 20,000 feet, they ought 

 not, if they happen to become impossible at this elevation, at once 

 to be regarded as interrupted, inasmuch as the birds, if compelled 

 to abandon the highest limits of their migration-path, descend only 

 so far until they reach a stratum of air in which they fall in with a 

 current in the desirable direction and of the requisite strength, and 

 it is only when they fail to meet with such a current that they 

 finally alight on the earth. 



Now this little island, where the whole vault of heaven con- 

 stantly lies open to the view, furnishes very striking proofs of the 

 variety of air currents prevailing simultaneously in different gradu- 

 ally ascending strata of the atmosphere. Thus we find that loose 

 mist-like cloud-formations at a height of scarcely a thousand feet 

 frequently deviate considerably in their course from the direction 

 of the wind, which sweeps along the surface of the water, while it is 

 by no means rare to find two further cloud-strata passing in dissimilar 

 directions, interposed between the lower patches of vapour just 

 referred to, and the cirrus streaks at immeasurable heights above 

 them, so that very frequently the highest clouds move in a com- 

 pletely opposite direction to the air currents which sweep along 

 the earth's surface. 



Birds naturally choose for their migrations those strata of the 

 atmosphere which offer the most favourable conditions to their 

 progress. It is however a fact of peculiar interest, that during 

 both migration-periods of the year, all species, without exception, 

 approach in largest numbers to the earth's surface when very light 

 south-easterly winds, accompanied by clear warm weather, happen 

 to prevail for any length of time in the lower regions of the atmo- 

 sphere. If autumn brings a long spell of weather of this kind, we 

 may not only reckon on the appearance of large numbers of all our 

 common visitors during September and October, but may also look 

 forward with certainty to the frequent occurrence of species very 

 rare in Europe, and originating from the far East, such as Yellow- 

 browed Warbler (Sylvia superciliosa), Siberian Chiffchaff (8. 

 tristis), and other Siberian Warblers, Richard's Pipit, Rustic 



