510 THE POPULAR SCIENCE MONTHLY. 



visible beyond a distance of five miles, we determined the greatest 

 altitude at which birds migrate to be three miles above the earth's 

 surface. Many, however, fly at a lower level ; indeed, it is not im- 

 probable that certain species may, with more or less regularity, 

 travel at a given altitude, and that this altitude may vary among 

 birds of different families. With little doubt thrushes and war- 

 blers travel at a much lower level than do ducks and geese, a cir- 

 cumstance which may account for the great abundance of the 

 first two named and the comparative absence of the last in the 

 vicinity of lighthouses. 



Such, in brief, are the sources and methods to which we owe 

 our knowledge of the nocturnal flight of birds. It will be evi- 

 dent to the most casual reader how incomplete are our data. The 

 time is still far distant when we can hope to conclusively account 

 for the many perplexing phenomena of migration, but we may be 

 pardoned if, in conclusion, we briefly review the bearing of our 

 present information. 



We need not discuss here the origin of migration or the causes 

 which now induce birds to undertake a long and perilous journey 

 twice each year. But the power and influences which guide a bird, 

 in the darkness of the night, through space, and render a definite 

 migration possible, are subjects kindred to our inquiry and worthy 

 our attention. 



Until we possess some exact knowledge of the distance to which 

 birds can see we can not estimate the aid their vision is to them 

 while migrating. We know, however, that the avian eye is far 

 more powerful than ours, and it is fair to assume that to some ex- 

 tent their journeys are directed by a sight which enables them to 

 follow mountain chains, river valleys, and coast lines, and to dis- 

 tinguish distant headlands or islands. At an altitude of two 

 miles an object would be visible ninety miles and the horizon be 

 separated by twice this distance. At no time, therefore, in their 

 journey from North to South America are birds necessarily out 

 of sight of land. But that they do venture beyond a point where 

 land is visible is shown by the regular appearance of migrants 

 in the Bermudas, six hundred miles from our coast, while Ja- 

 maica, four hundred miles north of the nearest point of South 

 America, is a point of departure for many south-bound migrants. 

 Here, with neither islet, shoal, nor reef to mark the way, it is evi- 

 dent that sight alone would prove an insufficient guide, and they 

 must rely on some other sense. Primarily, this is the inherited 

 habit which prompts birds to fly southward in the fall and to re- 

 turn in the spring. But, given the impulse of direction, there is 

 little doubt that one of the best guides to night-flying birds is the 

 sense of hearing. Birds' ears are exceedingly acute. They readi- 

 ly detect sounds which to us are inaudible. Almost invariably 



