36 



BULLETIN 185, U. S. DEPAETMENT OF AGEICULTUEE. 



was freed from the overlying ice cap, the tendency would be for the 

 migration route at its northern end to turn and be extended east- 

 ward to enter the new and as yet uncrowded districts. The route 

 at this stage would be a haK circle, and a tendency would soon 

 develop to shorten some of the curve through Texas by a short flight 

 over the western end of the Gulf of Mexico. This short flight would 



Fig. 16. — Isochronal migration lines of the black-and-white warbler {MniotUta varia). An example of a 

 slow and uniform migration. Isochronal lines indicating the advance of tlie van during each 10-day 

 period of spring show an average speed of about 20 miles a day in March in Florida and about 25 miles 

 a day for the whole trip to southeastern Canada. (See p. 25.) The opposite (a rapid migration) is shown 

 by the gray-cheel^ed thrush (fig. 17). 



gradually be lengthened and its points of arrival and departure 

 at the mainland carried eastward until eventually the curve would 

 be replaced by a straight flight across the Gulf. 



Some migration routes have been so recently developed that they 

 still plainly show their origin. The red-eyed vireo, a striking example, 

 is a ^woodland bird and as such is essentially an inhabitant of the 



