204 THE BIRDS OF HELIGOLAND 



theless it is hard to believe that this bird possesses so accurate a 

 memory of the stretches of land and water traversed in the preced- 

 ing autumn as would bo necessary to enable it to again map out 

 correctly the various stretches of road to be covered each day in the 

 opposite direction, and under totally different conditions. 



The statement made above, as to three hours being the time 

 required by this Crow to fly across the North Sea from this island 

 to the English coast, is based on observation; the earliest migrant 

 columns, which loft here at eight in the morning, having arrived on 

 the opposite coast at eleven, while the last companies, departing 

 here at two o'clock in the afternoon, arrived at five P.M. The dis- 

 tance is about three hundred and twenty geographical miles, giving 

 a speed of flight of one hundred and eight geographical miles per 

 hour. 



These Crows when migrating fly generally at a very inconsider- 

 able height ; this is especially the case in autumn when the air is 

 thick, and when they fly across the sea at a height of not greater than 

 ten to fifteen feet from the surface. In the spring, on the other hand, 

 they fly across the island generally at a height of from eighty to 

 one hundred feet ; this, during fine and calm weather, being some- 

 times increased to ten thousand feet or more. They are then only 

 distinguishable with great effort, appearing no larger than very fine 

 dust specks, our attention being attracted by their call-notes, more 

 particularly those of the Jackdaws and Rooks. Frequently the 

 voices of the Jackdaws, faint and yet distinct, reach us from a 

 height so great that the eye is no longer able to discern the birds. 

 I have observed that migrations, proceeding at such enormous 

 elevations, invariabty take place only on calm, sunny spring days, 

 when the sky is almost uniformly covered by a bright, clear, 

 immeasurably high stratum of cirrus clouds. At such times in 

 those elevated regions, and at heights, perhaps, greater still, a 

 powerful migration must be in progress ; for not only is it possible 

 to discover the above-named species, but one is almost invariably 

 able to hear in addition, from the far-off heights above, the faint, 

 but still clearly distinguishable cries of the Whimbrel, the Bar- 

 tailed Godwit, and other species ; frequently, also, these birds may 

 still be discerned by the eye as mere cloudlets of fine dust ; often 

 again, their far-off, but still faintly audible, cries alone betray their 

 presence. 



The Hooded Crow migrates by preference in fair weather, light 

 south-east winds, and a clear atmosphere ; during the autumn 

 months, however, the wind out at sea freshens up very often almost 

 to violence, while it is quite normal on the coast ; hence the Crows 

 not infrequently are driven into a south-easterly air-current, which 



