PART II. 



FACTS AND THEORIES RESPECTING BIRD MIGRATION. 



It has been long supposed that our smaller nocturnal migrants, such as 

 Thrushes, Warblers, Vireos, and Sparrows, travel either in small flocks or 

 singly, according to circumstances or the habits of the diff'erent species. It 

 has been further assumed — very naturally, in view of the difficulty of obtain- 

 ing direct evidence — that both flocks and single birds fly at wide intervals 

 apart and more or less independently of one another. This has led to the 

 belief that birds possess a sense of locaHty and direction, either inherited or 

 wholl}' mysterious, for in no other way has it been possible to explain the 

 fact that, under favorable circumstances, young birds, apparently alone and 

 unguided, regularly find their way over thousands of miles of country 

 which they have never before seen. 



Various theories have been advanced to account for this remarkable feat, 

 but all, as far as I have seen, are inadequate and unsatisfactory; and natur- 

 ally, for they are based partly on false premises and largely on a miscon- 

 ception of the character and extent of nocturnal bird flights. The simple 

 truth is, as I am convinced by long observation, and as I hope to be able to 

 show, that while these birds really do travel both singly and in flocks, the 

 different flocks and individuals do not move independently. On the con- 

 trary, the conditions which cause one flock, family, or individual to start 

 southward are ordinarily so wide spread and generally operative, that count- 

 less flocks, families, and individuals are set in motion at nearly the same 

 time, and the members of each flock or familjs instead of flying in close 

 order, scatter about sufficiently to approach or mingle with the stragglers of 

 other flocks or families. Thus in effect they form a continuous but strag- 

 gling army, often hundreds of miles in length, and varying in breadth ac- 

 cording to the character of the country over which it is passing. 



Over a wide, level, and generally uniform region the host spreads out in 

 thin order ; following a river valley, it contracts and thickens ; and at narrow 

 passes, such as the Straits of Mackinac,* it focuses its myriads into a solid 

 stream. 



* See the Auk, Vol. II, January, 18S5, p. 64. 



