42 



BULLETIN 185, U. S. DEPARTMENT OF AGRICULTURE. 



warbler is a good example of this usual habit. Some summer warblers 

 that return to the Great Slave Lake region to breed after spending 

 the winter in Central and South America arrive at their nesting 

 grounds when the average daily temperature is about 47° F. Accord- 

 ing to the movements of the Canada goose, these summer warblers 

 might be expected to pass up the Mississippi Valley and on to their 



Fig. 19.— Isochronal migration lines of the red-eyed vireo ( Vireosylva olivacea). An example of a recently 

 extended migration route. The birds which are to nest in New England advance along the Atlantic 

 coast in approximately a straight line and at a fairly uniform speed, while those which are to nest in 

 Washington advance up the Mississippi Valley at about the same speed until eastern Nebraska is reached, 

 when they turn sharply to the northwest and more than double their speed as they journey along this 

 recently extended route to the far northwest. (See p. 37 and also fig. 18.) 



summer homes at the same time as the northward-moving tempera- 

 ture of 47° F. But if this were so they would never leave the United 

 States, for the average temperature of the coldest month of the year 

 at New Orleans is 54° F. As a matter of fact, the summer warblers 

 of Great Slave Lake are probably too well content with the warm, 

 humid, insect-laden air of the south to brave the arctic blasts before 



