90 THE BIRDS OF HELIGOLAND 



as suddenly as they appear, so that hardly one of them is dis- 

 coverable on the following morning. It is not known whether they 

 proceed further west, though this is probably the case. It is certain, 

 at any rate, that they do not remain here, otherwise one would 

 undoubtedly find them lying about dead after the heavy rain of 

 a thunderstorm. 



We must, in conclusion, mention some other phenomena of 

 migration of rarer occurrence, and only repeated at intervals of 

 many years. These also are undoubtedly occasioned by meteor- 

 ological influences, although it is not always possible to prove this 

 to be actually the case. To this category specially belongs the 

 sudden occurrence en masse of a species in parts of the world 

 far remote from its home, and in which under ordinary conditions 

 it is hardly ever known to visit. To such cases belongs the 

 irruption of Pallas' Sandgrouse (Syrrhaptes paradoxus), an Asiatic 

 species, over the whole of Europe, where it appeared in thousands 

 in 1863, and again in tens of thousands in 1888. The causes of 

 such phenomenal migrations may be assigned, with a fair degree 

 of certainty, to similar meteorological events of exceptional char- 

 acter; in the instance last cited, possibly to a sudden very ex- 

 tensive snowfall, which, having covered all the nests of these early 

 breeding birds, induced them to leave their nesting homes in such 

 astonishing numbers. The cold spring months of 1888 would seem 

 to lend some support to this supposition. 



Next in order must be considered the sporadic and simultaneous 

 appearance in numbers of different species originating from Eastern 

 Asia, which, though not repeated at regular intervals, always takes 

 place only under certain well-defined conditions of weather. A 

 phenomenon of this kind on a most extensive scale presented itself 

 in the autumn of 1847. The most noticeable feature of the migra- 

 tion of that season was the enormous and unprecedented quantity of 

 Mealy Redpoles (Fringilla linaria) which visited the island, mixed 

 with which, to the extent of about one-third, was the Eastern form, 

 F. exilipes, a species which is somewhat smaller than F. linaria, 

 besides having the shortest beak of this group of Red-breasted 

 Finches, as well as a pure white unspotted rump. Redpoles are 

 altogether of extremely rare occurrence here. A few specimens at 

 most are taken at any one time, and even these cannot be relied on 

 every year. In the year in question, however, which was also very 

 productive of other Eastern visitors, these birds appeared from the 

 middle of October to the middle of November in hundreds almost 

 daily ; while on the 4th and 5th of November these numbers in- 

 creased to such enormous proportions that the whole island was 

 literally covered with them. In fact, as Glaus Aeuckens, who at 



