Vo | 8 ^ VI J Libby, Nocturnal Flight of Migrating Birds. 1 43 



were taking place later in the night. This latter conclusion is 

 borne out by the larger number of calls heard toward morning, 

 which may be explained as arising from the effort to reassemble 

 the scattered members of the migrating companies. As a general 

 conclusion to be drawn from the whole observation, it would seem 

 that the great mass of migrants thrusts itself rapidly forward for 

 the first two or three hours in one main direction and that sepa- 

 rate flocks maintained this movement many hours later. And 

 that after the first advance was completed, the remainder of the 

 night was spent in more miscellaneous movements, having for 

 their purpose, partly at least, the collecting of the widely separated 

 fragments of the different groups, and the selecting of suitable 

 feeding grounds. 



This fugitive glimpse into a new phase of bird life reveals 

 many things besides the two chief points already noted. When 

 one recalls the relatively small size of the moon's surface com- 

 pared to the length of its path from east to west, within the range 

 of vision, some idea of the whole number of birds passing this 

 line may be obtained. Prof. A. S. Flint of the Washburn 

 Observatory estimated that about nine thousand per hour passed 

 during the entire period of observation, or a total of one hundred 

 and sixty-eight thousand. And when the length of this line is 

 compared to the breadth of the whole country over which birds 

 move, the total number of migrating birds for a given area may 

 be roughly estimated. This states in numerical fashion the 

 meaning of the semi-annual migration of our birds. It falls as 

 far short of expressing what the movement really is as does a 

 census report of revealing the daily life of a city like New York 

 or Chicago. 



The movement of the birds across the field of vision irre- 

 sistibly suggested the rapid, undulatory motion of animalculae 

 under high magnifying power. The time of passage varied from 

 one-tenth to one-half a second. In most cases the movement of 

 the wings was plainly visible, though occasionally a bird passed 

 across like a flash. One bird hung for several seconds on the 

 edge of the field of vision, poising itself by rapid motions of the 

 wings. Several times a bird was seen to change its direction of 

 flight completely, usually going off at right angles. Very rarely 



