I A Chapman on the Origin of Bird Migration \ ]an 



are all affected at nearly the same time by an impulse which 

 urges them to hasten to a certain place. 



This impulse is periodic and is common to all birds. There is 

 a regular nesting season in the tropics, just as there is a regular 

 nesting season in the Arctic regions. It is evident, therefore, 

 that external conditions have not created this impulse, though it 

 is possible that in many instances they may have governed its 

 periodicity. On the contrary, its causes are internal. In the 

 case of the sea-birds, for example, dissection will show an 

 enlargement of the sexual organs and it is this physiological 

 change which warns the birds that the season of reproduction is 

 at hand. 



The organs of male birds apparently begin to enlarge before 

 those of the females, and it is not improbable that this may account 

 for the earlier migration of the males of many species. Further- 

 more, individuals found south of the breeding range of the 

 species during the nesting season are generally barren birds, and 

 their presence may be due to an absence of conditions which 

 would impel them to migrate to the nesting grounds. 



Now returning for a moment to the period of glaciation, it is 

 not improbable that the period of reproduction may have been 

 coincident with the return of the warmer part of the year and, in 

 addition to the desire for seclusion and the pressure exerted by 

 the crowded conditions of existence which then prevailed, was 

 potent in inducing birds to seek breeding grounds in the north 

 during the summer. 



I do not presume to attempt to trace the varied influences of 

 changing climate which, acting with the factors I have mentioned, 

 have brought about the conditions of the avifauna of to-day, with 

 its resident species and transient visitants, but will speak briefly 

 of the two classes of our strictly migratory birds. 



These are, first, those which breed continuously from our 

 southern borders to the northern limit of their range ; second, 

 those in which an area of varying extent exists between the 

 southern limit of their breeding range and our southern boun- 

 daries. 



Examples of the first class are Tyrannus dominicensis, 

 Vireo calidris, De/idroica virgorsii, and Compsothlyfiis ameri- 

 cana. I believe the presence of these birds to be due to normal 



