59 



negative evidence afforded by the absence of certain birds in 

 particular years ; direct proof is of course practically unattainable. 

 There is also another point worth noting. Herr Gatke describes 

 migration proper as being conducted at both periods of the year 

 in one unbroken and for the most part nocturnal flight, the un- 

 broken character being more pronounced in the spring than in the 

 autuma. Why, therefore, do birds tarry at such an uninviting 

 little spot as Heligoland, when all the conditions are favourable 

 to the performance of this unbroken flight ? It may be pointed 

 out here, that while a migratory flight may be fairly called 

 unbroken which does not call at the island, if reference to Heligo- 

 land alone is intended ; still, for all that, there are many other 

 places where rests may be taken, but of which we have no records. 

 A reference to American statistics is all against this theory of an 

 unbroken flight {vide " Migration in the Mississippi Valley," 

 (Cooke and Merriam, 1888). 



Having examined into Herr Gatke's theories as to the con- 

 ditions most favourable to the performance of the so-called 

 normal manner of migration, the question naturally arises, by 

 what sense or by what means do birds, wishing to migrate, 

 make themselves acquainted with the state of the atmosphere 

 at the vast heights at which we are told they conduct their 

 journeys ? Supposing the winds at their winter quarters are, as 

 the time for departure approaches, of an unfavourable character 

 — are we to assume, therefore, that as the impulse to migrate 

 becomes irresistible, that birds make voyages of discovery up 

 to great heights in search of favourable strata of the air, or 

 are they supposed to be possessed of some sense which will 

 enable them to detect the presence of favourable air-currents 

 at great heights without this faculty being rendered inoperative 

 by the unfavourable conditions prevailing at the time near the 

 earth? His further remarks seem to favour the latter theory, 

 for on page 78 he writes : — " We can hardly admit that birds 

 seek such strata at haphazard; we ought rather to assume 

 that they are possessed of an inherent presentiment or sensi- 

 tiveness to distant but approaching phases of the weather. 

 We are supported in this view by the fact that many birds in 

 confinement manifest much uurest, by fluttering and by the 



