652 



BIRDS. 



stork, albatross, and has been observed only when there was wind. 

 It is still imperfectly understood, but it probably depends on the 



FlG. 326. Wings coming down. From Marey. 



varying velocity of the wind at different heights. The bird probably 

 sails along the line of two currents of different velocity. 



Song of birds. Singing is a natural expression of emotional 

 intensity. It is richest at the breeding season, and is always best and 



FIG. 327. Wings completely depressed. From Marey. 



often solely developed in the males. But song in any excellence is the 

 gift of comparatively few birds, though nearly all have a voice of some 

 sort, often so characteristic that the species may be recognised by its 



