36 The Migrations of Birds 



obtained in that area. "It will follow that those birds which do not leave the 

 breeding area at the proper season will suffer, and ultimately become extinct; 

 which will also be the fate of those which do not leave the feeding area at the 

 proper time." His further argument is ingenious, and, it must be added, extremely 

 plausible. He says: "Now, if we suppose that the two areas were (for some re- 

 mote ancestor of the existing species) coincident, but by geological and climatic 

 changes gradually diverted from each other, we can easily understand how the 

 habit of incipient and partial migration at the proper seasons would at last become 

 hereditary, and so fixed as to be what we term an instinct." 



It will probably be found, however, if anything like a satisfactory explanation 

 can be arrived at, that this habit or instinct has arisen in more than one way, 

 but we may appropriately turn from a consideration of theories to a review of 

 certain observed facts of migration. 



It is now abundantly established that migration is mostly carried on at night, 

 and further mainly during clear nights. Only a comparatively few species, 

 such as Ducks, Cranes, certain large Hawks, Swallows, Swifts, and Nighthawks, 

 migrate during the daytime, and these, it will be observed, are either rapacious 

 birds or mainly those that enjoy such power of rapid flight as to be relatively 

 safe from capture. All the vast horde of Warblers, Sparrows, Finches, Fly- 

 catchers, Thrushes, and Woodpeckers, as well as many waders and swimmers, 

 migrate at night. On clear, still nights during the migrations birds may often 

 be heard calling to each other high overhead, and, as will be described later, 

 may be actually seen by powerful telescopes. Woods and hedgerows that were 

 untenanted one day may become fairly alive with birds at daylight the next 

 morning, showing that they have arrived during the night. They remain to 

 feed and rest during the day, and, if the weather be favorable, may practically 

 all disappear the next night. That they only venture on these journeys during 

 clear nights is shown by the fact that on such nights very few birds are killed by 

 lighthouses, monuments, or other obstructions, whereas on cloudy or rainy 

 nights, especially such as opened clear and later become overcast, thousands of 

 birds become confused and dash themselves against these obstructions. Thus 

 over 1500 birds have been found dead at the base of the Bartholdi Statue in New 

 York harbor in a single morning, and 230 birds of one species Black-poll 

 Warblers were killed in a single night (September 30, 1883) by the Fire Island 

 light. The Washington monument, although not illuminated at night, causes the 

 death of hundreds of birds annually. 



The height above the earth at which migrating birds travel has been made 

 the subject of some interesting observations, the first of which appear to have been 

 by Mr. W. E. D. Scott, on the night of October 19, 1880, at Princeton, New Jersey. 

 In company with a number of visitors he was being shown through the astronomi- 

 cal observatory at that place, and after looking at a number of objects through 

 the 9|-inch equatorial, they were shown the moon, then a few days past its full 

 phase. His attention was at once arrested by numbers of small birds that could 

 be more or less plainly seen passing across the field of observation. Most of the 

 kinds seen were the smaller land birds, among which were plainly recognized 



