THE NOCTURNAL MIGRATION OF BIRDS. 509 



solely on these casualties may be not only misleading but erro- 

 neous. In other words, the number of birds which strike a light 

 is a poor index to the number which have flown by or above it in 

 safety. 



Throughout the evening there was a more or less regular fluc- 

 tuation in the number of birds present; periods of abundance 

 were followed by periods of scarcity, and the birds passed in well- 

 defined flights, or loose companies, probably composed in the main 

 of individuals which had started together. 



The birds chirped and called incessantly. Frequently, when 

 few could be seen, hundreds were heard passing in the darkness ; 

 the air was filled with the lisping notes of warblers and the mellow 

 whistle of thrushes, and at no time during the night was there 

 perfect silence. At daybreak a few stragglers were still winging 

 their way southward, but before the sun rose the flights had 

 ceased. The only birds identified were several species of warblers 

 and thrushes, one red-eyed vireo, two golden-winged woodpeckers, 

 one catbird, one whip-poor-will, and one bobolink. The most 

 interesting and important results of the night's observations were, 

 the immediate effect of rainfall in forcing birds to migrate at a 

 lower level, the infrequency with which they struck the torch, the 

 immense number which passed beyond its rays, and the constancy 

 with which they called and chirped as they flew. 



An almost virgin field awaits the investigator who will system- 

 atically observe night-migrating birds with the aid of a tele- 

 scope. Messrs. Allen and Scott, at Princeton, and the writer, 

 assisted by Mr. John Tatlock, Jr., at Tenafly, New Jersey, and at 

 the Columbia College Observatory, have alone recorded the results 

 of observations of this nature. Their labors, however, were 

 too brief to more than show the possibilities which await more 

 extended effort. 



A comparatively low-power glass is focused upon the moon, 

 the birds appearing silhouheted upon its glowing surface as they 

 cross the line of vision. Some idea of the multitude of feathered 

 forms which people the upper regions of the air at night may be 

 formed when it is stated that during three hours' observation at 

 Tenafly no less than two hundred and sixty-four birds were seen 

 crossing the restricted field included in the angle subtended by 

 the full moon. Under proper focal conditions, birds were so 

 plainly visible that in many instances marked character of flight 

 or form rendered it possible to recognize the species. Thus 

 ducks, snipe, and sora rail were distinguished with certainty. 



The effect on the observer of this seeing of things unseen is 

 not a little curious, and may be likened to the startling disclosures 

 which a high-power microscope presents in a drop of water. 



From calculations based on an assumption that birds were not 



