V0 'i8^ VI ] Libby, Nocturnal Flight of Migrating Birds. 14I 



hundred and thirty-six, between two and three o'clock, though 

 nearly that number were noted for two other hours. Nor were 

 the calls at all confined to the few hours during which they were 

 recorded. They began much earlier in the evening and when the 

 observations ceased, at a little after three, they were heard stead- 

 ily on long after that hour, with the regularity of the ticking of a 

 clock. Manifestly it is quite impossible to estimate the number 

 of birds represented by these calls. The equation contains so 

 many unknown quantities that no satisfactory mathematical solu- 

 tion is to be expected with our present knowledge of the subject. 

 But it may be very safely assumed that the number of calls must 

 be multiplied many times to express even approximately the size 

 of the flocks that were heard to pass during the course of the 

 observation. 



Nothing but an actual experience of a similar kind can at all 

 adequately convey the impression produced by such observations. 

 The air seemed at times fairly alive with invisible birds as the 

 calls rang out, now sharply and near at hand, and now faintly 

 and far away. Repeatedly it seemed as if some of the nearer 

 ones must be visible, so vividly was their presence felt as they 

 passed overhead. All varieties of bird calls came sounding out 

 of the darkness that evening. The harsh squawk of a water bird 

 would be followed by the musical chink of the Bobolink. Almost 

 human many of them seemed, too, and it was not difficult to 

 imagine that they expressed a whole range of emotions from 

 anxiety and fear up to good-fellowship and joy. The fine shrill 

 notes of the smaller Sparrows or Warblers were heard only close 

 at hand but the louder ones came from all along the line, east 

 and west. More than once an entire flock, distinct by the unity 

 of their calls, came into range and passed out of hearing, keeping 

 up their regular formation with the precision of a swiftly moving 

 but orderly body of horsemen. The great space of air above 

 swarmed with life. Singly or in groups, large and small, or more 

 seldom in a great throng the hurrying myriads pressed south- 

 ward. It was a marvel and a mystery enacted under the cover 

 of night, and of which only fugitive tidings reached the listeners 

 below. 



The next station chosen was the Washburn Observatory, over- 



