V. METEOROLOGICAL CONDITIONS 

 WHICH INFLUENCE MIGRATION 



Though the meteorolog-ical influences Avhich affect the migrations 

 of birds are at the present time still very imperfect!}- understood, it 

 will not be out of place here once more to collect and examine in 

 detail the various references made thereto in previous chapters, 

 even if such a recapitulation may only serve to attract a more 

 general attention to this important subject. ^ 



We have already laid stress on the fact that such portions of 

 the migration phenomenon as become apparent during its perio- 

 dical recurrences, are brought within the range of our observing 

 faculties almost exclusively by meteorological conditions which are 

 exercising a disturbing influence upon the normal progress of the 

 migratory movement. When this latter proceeds on its regidar 

 normal course, it lies far beyond the limits of either our visual or 

 auditory capacities, and it is only by the development of meteoro- 

 logical disturbances in those inexplorable regions that it is brought 

 within the range of our perceptive faculties. 



The force and direction of the wiiid are however by no means the 

 only factors which exercise a determining influence on migration. 

 The various forms and phases in which this movement manifests 

 itself are afl'ected in a determinate manner by another meteoro- 

 logical condition, viz. : The greater or less amount of moisture 

 present in the atmosphere, and the particular form which this 

 moisture assumes ; whether it is distributed throughout the air as 

 a vapour of uniform density, or condensed into fog or mist, or takes 

 the shape of clouds either loose and feathery of the cirrus form, 

 or dense and rounded of the woolpack type ; or again, in a clear, 

 cold air, ajjpcars as dew or hoar-frost, or under other conditions, in 

 the form of a thundercloud heavily charged with electricity. 



In general proof of this may be cited the simple fact that 

 whereas birds appear in great number Avhen the wind is in a 

 particular direction, they are scai'cely seen at all when it is in some 

 other quarter. The latter, for instance, is the case during south- 



