THE BIRDS OF HELIGOLAND 441 



occurred in smaller numbers ; but on the 20th a flight of twenty 

 individuals was seen in addition to several solitary examples. 



Moreover, on nearly all the days enumerated above, large and 

 small flocks were observed by fishermen from the island passing 

 over the sea ; and occasionally a dead bird, still in a fresh condition, 

 was seen drifting on the water. Some of our iishermen who had 

 been for some days, about this time, at the island of Neuwerk within 

 the estuary of the Elbe, reported, with expressions of astonishment, 

 that they had seen thousands of these birds roving about in that 

 locality. 



What the further development of this movement will be re- 

 mains to be seen ; in any case, the number of birds which has 

 occurred up to the present is already more than ten times as large 

 as that observed during the whole period of the irruption of 1863. 

 There is little doubt that the birds will again take to breeding in 

 places which they find suitable ; but I do not share in the hope, 

 as indeed I did not in 1863, that the species will become naturalised 

 hi Europe. Species from very distant regions do not establish 

 themselves in new areas by sudden and powerful immigrations of 

 this character, but advance their range slowly and steadily, like, 

 for instance, the Shore Lark, and undoubtedly also Pallas' Grey 

 Shrike (Lanius major = borealis), which probably occupied a century 

 in advancing their breeding range from the east of Asia to Scan- 

 dinavia. The fact of birds having been bred in a certain locality is 

 not alone sufficient to induce them to return thither after an absence 

 of eight months. In proof of this may be mentioned the attempt 

 made in Great Britain to establish Nightingales in the more northern 

 division of the empire ; to this end a considerable number of eggs 

 were brought from near London to Caithness in Scotland, and 

 placed in the nests of Redbreasts. These birds duly hatched the 

 strange eggs thus foisted on them, and also reared the young ; the 

 latter remained until September when they left with the main 

 body of autumn migrants, and never returned to the place of their 

 birth (Newton, in Yarrell's British Birds). 



The sudden appearance en masse of the Sand Grouse in central 

 and western Europe is so remarkable a phenomenon that one is 

 forced against one's will to speculate as to its possible causes. One 

 feels inclined to assume that meteorological conditions, which so 

 largely dominate the normal migration-flight, may also exercise 

 a determining influence in exceptional cases of this kind. Thus, 

 for example, if a sudden snowfall late in April during a cold spring, 

 like the present one of 1888, were to occur in central and northern 

 Mongolia, where these birds breed in large numbers, it might, by 

 overwhelming their nests and eggs, so disturb large numbers of 



