140 LiBBY, Nocturnal Flight of Migrating Birds. [A^riV 



Auk, XVI, April, 1899. pp. 

 THE NOCTURNAL FLIGHT OF MIGRATING BIRDS,. 



BY O. G. LIBBY. 



It has long been a well-known fact of bird life that, during 

 the migrating season, most, if not all, of the movement north or - 

 south takes place in the night. This ensures protection from 

 enemies and opportunity for securing food during periods of rest^. 

 Under the cover of darkness, the bird passes safely and secretly 

 through the air. During the day he c-an search for necessary 

 food and by evening he is again ready to continue his flight. 



But the very conditions that shield the migrating birds from- 

 danger, also preclude any very satisfactory study of their move- 

 ments. We know, to be sure, that during the fall migrations,, 

 most of the large flocks will be found in the early morning on 

 the north side of groves or belts of timber, and in the spring they 

 are to be found on the south side. We know, too, from observa- 

 tions covering a long period of time that birds are seen in the 

 morning which were not in the neighborhood the day before. , 

 And most bird lovers-know how distinctly the calls of the migrat- 

 ing birds can be heard during the nights of middle September.. 

 Still it must be confessed that in proportion to the magnitude o£ 

 this movement in the bird world and the importance of the inter- 

 ests at stake, economical as well as biological, our actual knowl- 

 edge of the migration is exceedingly meager. 



The writer has recently made two sets of observations upoi>. 

 the nocturnal flight of birds, an account of which may prove in- 

 teresting to the general reader. The place of observation first, 

 selected was a small elevation west of the city of Madison, Wis- 

 consin, with three lakes in the immediate vicinity. The evening 

 chosen (September 14, 1896) was chilly and a raw southeast, 

 wind was blowing, though there were no clouds during most of 

 the time. A total of three thousand eight hundred bird calls 

 were recorded, an average of twelve per minute. This rate, how- 

 ever, varied greatly, sometimes running as high as two or three - 

 per second and again falling to about the same number per min- 

 ute. The largest number of calls counted for any hour was nine • 



