78 THE BIRDS OF HELIGOLAND 



phenomena of migration proceed, we can hardly admit that birds 

 seek such strata of the air as are favourable to their migration purely 

 at haphazard ; we ought rather to assume that they are possessed of 

 an inherent presentiment or sensitiveness to distant, but approach- 

 ing, phases of weather. We are supported in this view by the fact 

 that many birds in confinement manifest much unrest, by fluttering 

 and by the frequent utterance of their call-notes, on days which 

 precede nights of strong migrations. This was the case with a Snow 

 Bunting which I had kept for many years, and also with some Larks 

 kept at the lighthouse here, the latter regularly predicting by their 

 restlessness during the day the great catch that was to take place 

 on the coming night. 



The sensitiveness of birds to the first faint indications of an 

 atmospheric change must at least be equal to that of a good baro- 

 meter; at the same time, however, we must not forget that in 

 the elevated regions in which their migrations proceed, birds are 

 brought under the influence of the slightest signs of an approaching 

 change of weather long before anything of the impending change 

 is perceived on the earth's surface, where the earliest indications of 

 it are probably not felt until about twenty-four hours later. 



It can hardly be doubted that all changes of weather have their 

 origin in the higher strata of the atmosphere. Observations have 

 shown in any case that the first indications of a change of wind 

 begin to make themselves felt in the highest cirrus streaks, and 

 that the lower layers of vapour are brought under their influence 

 gradually in a downward vertical succession. Thus, frequently 

 during light easterly and south-easterly winds and fine clear weather, 

 the highest and thinnest cirrus strata may be observed for days 

 moving almost imperceptibly from west to east ; or very faint striae 

 of vapour are seen rising on the western horizon, and, as the wind 

 slightly freshens, advancing during the next twenty-four hours to 

 about the zenith. In the course of another equal period of time they . 

 slowly overspread the eastern portion of the sky likewise, and finally, 

 as the east wind rises to its utmost intensity, increase to one dense, 

 elevated vault of vapour uniformly covering the whole expanse of 

 the firmament. Below this canopy of mist, cloud-formations, which 

 already begin to assume more or less definite shapes, are next seen 

 to pass across the sky, while simultaneously with their appearance 

 a downfall of rain takes place, the west wind at the same time 

 rapidly gaining the ascendency near the surface of the ground. 

 Such, at least, has been my experience here after many years of 

 careful observations. 



At the beginning of the series of meteorological changes just 

 described, while light south-east winds prevail, migrants appear in 



