ALTITUDE OF THE MIGRATION FLIGHT 55 



vertical elevation at which they appear and disappear invariably 

 represents the limit to which human vision is able to penetrate. 

 On the other hand, the number of species whose normal migration 

 flight does not exceed a few hundred feet above the earth's surface, 

 is hardly worth mentioning; and even of these, some, like the Rooks 

 and Curlews already referred to, will, under certain conditions, pass 

 across Heligoland at heights as considerable as 10,000 to 15,000 

 feet. 



I have seen Sparrowhawks arriving here during the autumn 

 migration, which, as they became visible vertically overhead, 

 appeared no larger than small specks of dust, and must, according 

 to a fairly reliable estimate, have been at a height of no less than 

 10,000 feet. My scale of comparison in this computation is the 

 distance of the extreme southern point of Sandy Island, amounting 

 to 8000 feet. Among the large droves of Hooded Crows which fly 

 across this point during the migration period it is possible to dis- 

 tinguish from the cliff, with the utmost ease, every single bird; 

 whence it seems quite out of the question that the height at 

 which the above-named Hawks became visible was in any way 

 estimated too highly. 



The arrival of these Sparrowhawks took place on a bright 

 autumn afternoon ; the sky was uniformly covered by the high, 

 white, streaky kind of clouds, specially favourable for observations 

 of this nature. For the space of about an hour the birds kept 

 coming in sight at the height before mentioned, singly and in 

 groups of three or four, wheeling about in circles as they descended 

 to lower altitudes. 



In the case of other species, this descent from heights at which 

 the birds are not visible proceeds in a different manner. Ringdoves 

 and Woodcock often precipitate themselves with the rushing noise 

 of a rocket, but with far greater velocity, in an almost perpendicular 

 line, or describe a zigzag single or double in their descent. The 

 bird itself may not yet be visible ; but if, attracted by the rushing 

 sound, one looks in the direction whence it proceeds, one notices 

 a small irrecognisable dot, which however, almost at the same 

 moment, shoots past one in the form of a bird. Doves break their 

 descent when they are still a considerable distance from the ground; 

 but Snipe rush down to within two or three feet from the surface, 

 and continue in a roving course at a very low elevation above it. 

 Occasionally, too, they sweep with undiminished velocity right down 

 to the rubble at the base of the cliff, where all of a sudden they are 

 found sitting as quietly as though they had never been on the move. 

 In each instance of this kind one is surprised that the bird had not 

 dashed itself to pieces against the ground. Song Thrushes, also, 



