Vol i's96 11 ] Allen, Gatke's 'Heligoland: 141 



hand, in spring, convey them in the opposite direction from their 

 winter quarters to their breeding haunts, — the uninterrupted con- 

 tinuity of the flight being still more marked in this latter phase of 

 the migratory phenomenon." — he says: u Observations extending 

 over many years have led me to the conclusion that, as long as 

 migration proceeds under normal conditions, this elevation 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 influences, such portions of it as 

 are brought within our notice'" (p. 46). Apparently he would 

 place the height of the migration flight as high as 15,000 to 

 30,000 feet, and brings forward evidence to show that some 

 birds attain at will a height of even 35,000 to 40,000 feet. He 

 might have brought much stronger evidence to support his con- 

 clusion than any he cites had he been more familiar with the 

 literature of the subject, for the observations made repeatedly 

 in this country with telescopes directed toward the disk of the 

 full moon during migration nights, demonstrating the fact that 

 birds reach an altitude of from one to three miles in their migra- 

 tory flights, is not mentioned. 1 In this connection he dwells upon 

 the fact that birds must be very differently constituted from man 

 or any other warm-blooded creature to be able to sustain life in 

 such rarefied air-strata and under the low temperature of such 

 elevations. He also comments at length on the ability possessed 

 by many birds to vary apparently the specific gravity of their 

 bodies, as in the case of various diving birds, and as must also be 

 the case with birds that rise to great altitudes in flight. 



The main purpose of the high altitude of the migration flight, 

 he believes, is that these high strata of the air offer, for the time 

 being, the most favorable conditions for migration, and render the 

 migrating hosts independent of the numerous meteorological dis- 

 turbances that affect the lower regions of the atmosphere, but that 

 also the rarefied air of the upper regions presents less resistance 

 to their progress. 



'See Scott and Allen, Bull. Nutt. Orn. Club, VI, 1881, pp. 97-100, 188; 

 Chapman, Auk, V, 1888, pp. 37-39- 



