J. A. ALLEN : BIRD MIGRATION. 19 



pressure. In general this direction is northward (in the 

 northern hemisphere) in spring and southward in autumn. 

 In other words, in general the migratory movements 

 of birds in spring are coincident with the alternation of 

 warm and cold waves, the former favouring and the latter 

 retarding or checking the movement. Thus the " waves " 

 or " rushes " of bird migration in spring are not only 

 necessarily from the south northward, but are coincident 

 with a warm atmospheric wave and a southerly wind. 



While these are the favourable conditions for bird 

 migration, birds move more or less under the ordinary 

 conditions of the weather proper to the season, and are 

 only held in check by the unfavourable conditions of a 

 cold wave and northerly winds. The conditions of the 

 autumnal movement are in a sense reversed, the birds 

 moving southward with, or just in advance of, a cold 

 wave and a northerly wind. 



Many recent writers, as well as those of earlier days, 

 delight to refer to bird migration as a u great mystery," 

 " a mystery of mysteries," as surrounded " with a halo 

 of mystery," etc. While there is still much to learn 

 regarding the general subject, and the faculties and mental 

 attributes of birds, it would seem that enough is known 

 to remove migration from the realm of mystery. The 

 origin and present inducement to migration seem open to 

 reasonable explanation, and some light seems also to have 

 been thrown upon the subject of how birds find their 

 way during their migratory journeys. In addition to 

 keen powers of vision, a memory for landmarks, and 

 remarkable sensitiveness to meteorologic conditions, 

 they seem also to be endowed with a sense of direction, 

 which recent experiments with Noddies and Sooty Terns 

 on the south-eastern coast of the United States* seem to 

 demonstrate as present, if as yet unexplainable. With 

 these facts in view it seems not difficult to believe that 

 while the ability of birds to find their way in migration 

 is truly wonderful, it implies little that is really 

 mysterious. 



* c/. " Bird-Lore," Vol. X., 1908, p. 134. 



