Vol. XXII 

 1905 



Cooke, Routes of Bird Migration. 



It is a well-known fact, that it is the individuals from the eastern 

 United States, rather than from the western United States, that 

 pass to South America. When a species is so variable that the 

 eastern individuals can be distinguished from the western, it is 

 found in most cases that the individuals wintering in South Amer- 

 ica are similar to those of the eastern United States, while those 

 from the western United States winter in Mexico and Central 

 America. Since then it has already been shown that few species 

 or individuals pass through the West Indies to reach South Amer- 

 ica, it follows that the individuals that pass to South America are 

 for the most part those that have flown across the Gulf of Mexico. 



If the above reasoning is«correct, then the eleven species given 

 in the preceding list as ' wide ranging species ', can be added to 

 those that cross the Gulf of Mexico on their way to South Amer- 

 ica or Panama. The recapitulation would then stand : — 



Species that reach South America or Panama 



by way of the West Indies ....... 10 



by an unknown route ........ 7 



by way of the Gulf of Mexico ....... 49 



Total 66 



In other words, it is practically certain that 60 per cent — more 

 probably at least 75 per cent — of the species of land birds from 

 the eastern United States that winter in South America or Panama, 

 take a flight across the Gulf of Mexico on the way to their winter 

 home. 



III. Inferential Migration Routes. 



The following supposed migration routes are based largely on 

 negative evidence, that is, on the absence of proof that the species 

 use other routes. Thus, for instance, in the case of the Canadian 

 Warbler, it is a common bird of the northeastern United States 

 and breeds in the mountains south to North Carolina ; it is prac- 

 tically unknown in Honduras, Yucatan, the West Indies, Florida, 

 South Carolina, Georgia, and Alabama; but it is known in eastern 

 Mexico and Guatemala, whence it proceeds through Central 

 America to its winter home in South America. There are so 



