82 THE MIGRATION OF BIRDS. 



only visible place for which to steer. An inherent 

 knowledge teaches them that light leads to safety. 

 Indeed, it is by no means improbable that reflected 

 light acts as a great and important guide to migratory 

 birds. Now, the most important routes of migration 

 are near or over the great water- w^ays of the world 

 — down river valleys and coast lines ; or along the 

 direction of mountain ranges, whose summits are 

 usually more or less covered with snow in autumn 

 and spring. Water and snow reflect light to a 

 very great extent, and it is easy to conceive how 

 a bird could gain some considerable knowledge of 

 its general course by following the gleaming expanse 

 below. The tide of migration flows high on moon- 

 light nights ; starlight is also favourable to its 

 progress. Migration is discontinued during an 

 overcast sky, when the moon and stars are shut 

 out from the earth by a fog or cloud-veil, as is 

 abundantly proved by the promptness of migrants 

 to visit the lighthouses when the earth is for the 

 time hidden in gloom, and the readiness with which 

 the pilgrimage is resumed as soon as the heavens 

 are clear again. If the fog, however, is of a local 

 character only, migration is often continued above 

 it, because the earth area which it conceals is not 

 suflriciently large to cause birds to lose their bear- 

 ings. Thus the seas, and lakes, and rivers, and 

 snow-capped mountains, glistening in the cold white 

 light of a brilliant moon, or gleaming gray yet full 

 of reflected lustre from a star-lit sky, serve as so 

 many flashing scintillating guides or steady glow- 



