HOW BIRDS MIGRATE 51 



the vast numbers of birds which throng the upper air during nights of 

 active migration. 



At Madison, Wisconsin, on the nights of September 11-13, 1897, 

 Libby (I. c.) saw 583 birds through a 6-inch glass, the largest number 

 counted during a fifteen minute period being forty-five. Additional 

 data of this nature are included in a paper by H. A. Winkenwerder 

 ('02), while Stone's ('06) observations on birds seen migrating at night 

 by the light of a great conflagration in Philadelphia have exceptional 

 interest. 



The height at which migrating birds fly has been variously esti- 

 mated. Many diurnal migrants may be seen traveling from a few 

 yards (e. g. Shearwaters) to several hundred yards (e. g. Geese) above 

 the earth, but Shore-birds evidently seek a greater altitude, and the 

 experience of Bray, referred to above, hints at a diurnal flight of which 

 we know practically nothing. 



Our knowledge of the height at which nocturnal migrants journey 

 is based on the telescopic observations already mentioned. As intimated 

 in the article itself (I.e.), the conclusions presented in my paper on 

 birds seen flying over Tenafly, were not satisfactory. More recently, 

 the problem has been attacked by F. W. Carpenter ('06) and Stebbins* 

 with results which seem far more worthy of acceptance. Their calcu- 

 lations show that while an altitude of 4,000 to 5,000 yards may occas- 

 ionally be reached, the greater number of birds observed were not 

 over 1,600 yards above the earth, while many passed considerably 

 below this elevation. The ease with which the calls of night-flying 

 birds may be heard also argues for a lower altitude than has been com- 

 monly accredited to them. 



The speed at which migrating birds fly has also been greatly over- 

 rated. Two observations with theodolites give to migrating Ducks 

 a speed of 47.8, and to migrating Geese a speed of 44.3 miles an hourf. 

 Homing Pigeons do not often exceed forty to forty-five miles an hour. 

 It is a common experience, when traveling in a train at a rate of thirty- 

 five to forty-five miles an hour, to pass birds which are flying parallel 

 to the track. I have had this occur repeatedly with such compara- 

 tively large and swift birds as the Mourning Dove. It seems probable, 

 therefore, that our smaller birds do not average more than thirty miles 

 an hour when migrating. 



Gatke's estimate of 212 miles per hour as the speed of the Golden 

 Plover when migrating, is based wholly on the assumption that fifteen 

 hours is "the longest spell during which a bird is able to remain on 

 the wing without taking sustenance of any kind." The Plover, how- 

 ever, does not fly from " Labrador to northern Brazil," as he stated, but 

 from Nova Scotia to the Lesser Antilles; as we have seen, it makes 

 special preparation for the journey, is extremely fat when it starts and 

 thin when it arrives. 



""'Popular Astronomy," XIV, 1906, pp. 65-70. 

 t Clayton, Science, 1897, pp. 26, 585. 



