METEOROLOGICAL CONDITIONS 83 



drove them to seek safety in flight ; and that this flight took 

 place, as one might have expected, in the normal direction of the 

 autumn migration of their species. 



Besides the birds above enumerated, there frequently appear, on 

 occasions of this kind, other visitors whose numbers are specially 

 increased during severe cold, viz., the Common Buzzard, the Rock 

 Pipit, the Sanderling, the Purple Sandpiper, less numerously, the 

 Knot, and, what is singular, at times also, one or two Hen Harriers. 

 An exceptional winter migration of this kind took place on the 

 14th of February 1876, during a heavy snowfall, which continued 

 incessantly throughout the day ; and on another occasion at the 

 beginning of 1881. The weather in the latter case had been mild 

 until the middle of January; but from the 16th to the 22d, the 

 thermometer sank to 10 C. (14 F.), an unusually severe degree 

 of cold for Heligoland, on account of the warmth, radiated by the 

 sea, which surrounds this island on all sides. 



On the 17th, a mass-migration of Larks and Fringillidse took 

 place, accompanied, in this instance, by Shore Larks and Snow 

 Buntings, as well as extraordinarily large numbers of Rock Pipits, 

 and the before-named northern species of Tringce. Goosanders, a 

 few examples of the Smew, Swans, and northern Sea Ducks, also 

 made their appearance, giving ample proof that winter had set in 

 with great vigour in some region or other. 



We must now mention a third exceptional phenomenon of 

 migration, somewhat similar in character, which is likewise evoked 

 by the occurrence of a sudden spell of wintry weather. In this 

 case, birds already in full pursuit of their spring migration are 

 compelled by frost and snow to recede completely from the journey 

 to their nesting homes. This phenomenon is much more sur- 

 prising than the migratory movements previously discussed, which 

 though exceptional, nevertheless invariably proceed in the nor- 

 mal direction of the migration. One has, in consequence, much 

 more rarely an opportunity to observe a real backward movement 

 of the kind referred to. In fact, during the whole of my experience, 

 I have Avitnessed only one instance of this kind, but that on a 

 really grand scale. This happened in March 1879 the weather 

 in the course of the first week of the month had been raw and 

 cold, although the temperature remained all the time above freezing- 

 point. During the second week a powerful migration took place. 

 Blackbirds, and even Song Thrushes, were fairly abundant. The 

 Pied Wagtail also was already seen in strikingly large numbers ; the 

 same was the case with the Linnet and Twite. From the llth to the 

 14th, the wind was a stormy north-west one, accompanied by snow 

 and hail, and the temperature sank to several degrees below freezing- 



