14 



BULLETIN 185, TJ. S. DEPARTMENT OF AfiRICULTURE. 



this same direction across the Gulf to eastern Mexico. The birds of 

 the central Mississippi Valley go southward to and over the GuK. 

 The birds between the Missouri and the edge of the plains and those 



Fig. 5. — Migration of the black-poll warbler {Dendroica striata). This is a night migrant and flies directly 

 across the West Indies in contradistinction to the cliff swallow (fig. 6), which is a day migrant and 

 flies around the Gulf of Mexico (see p. 19). The hea^■y solid isochronal lines show the places at which 

 birds arrive at the same time. As birds move northward, these lines are farther apart, showing 

 that the birds are movLag faster; from April 30 to May 10, the average speed is about 30 miles a day, 

 while from May 25 to May 30 it is more than 200 miles a day. (See p. 43.) 



of Canada east of the Rocky Mountains move southeastward and 

 south until they join the others in their passage of the Gulf. In 

 other words, the great majority of North American birds bound for a 



