III. ALTITUDE OF THE MIGRATION 



FLIGHT 



The altitude at which the various species of birds fly during their 

 migrations is another aspect of the jDhenomenon which has in a 

 special degree attracted the notice of observers. Observations 

 extending over many j'ears have led me to the conclusion that, as 

 long as migration proceeds under its normal conditions, this eleva- 

 tion is, in the case of by far the larger number, so great as to be 

 completely beyond the powers of human observation; while we 

 must regard as disturbances and irregularities of the migration 

 movement proper, due to meteorological mfluences, such portions of 

 it as are brought within our notice. Here I ought to remmd the 

 reader, that when 1 speak of migration proper, I mean those large, 

 extensive movements which, on the one hand, in autumn, conduct 

 our migrants from their breedmg homes to or very near to their 

 winter quarters in one uninterrupted, and for the most part, 

 nocturnal flight ; and on the other hand, in spring, convey them in 

 the opposite direction from their winter quarters to their breeding 

 haunts — the uninterrupted continuity of the flight being still more 

 marked in this latter phase of the migratory phenomenon. 



Quite of another kind from this main movement are those 

 shorter peregrinations, at low elevations in the air, which many 

 birds perform in greater or smaller companies during the day, 

 chiefly in autumn. By wanderings such as these, which cease as 

 the day declines, the birds pass from field to field and wood to 

 wood, picking up food on theu- way. Companies of this kind are 

 probably more or less mixed as regards the species composing 

 them, and may be looked upon as consisting partly of members of 

 the main body of migrants which are taking a temporary rest 

 during their journey, or of birds from more or less neighbouring 

 districts which are about to join the main stream of wanderers, all 

 such individuals, though influenced by particular meteorological 

 conditions, being nevertheless unable to resist the migratory 

 impulse working from within. Short day-trips of this kind, how- 



