.— Isotherm of 35° F. 

 __ Isochronal Migration Li 



Figure 4. — Migration of the Canada goose. The northward movement 

 keeps pace with the advance of spring, in this case the advance of 

 the isotherm of 35° F. agreeing with that of the birds. 



Orleans about April 5, when the average temperature is 65° F. Travel- 

 ing north much faster than does the season, they reach their breeding 

 grounds in Manitoba the latter part of May, when the average tem- 

 perature is only 47°. Encountering progressively colder weather over 

 their entire route, they cross a strip of country in the 15 days from May 

 II to 25 that spring temperatures take 35 days to cross. This "catch- 

 ing up" with spring is habitual in species that winter south of the 

 United States and in most of the northern species that winter in the 

 Gulf States. To this rule there appear to be only six exceptions — the 

 Canada goose, the mallard, the pintail, the crow, the red-winged black- 

 bird, and the robin. 



The blue goose presents a striking example of a late but very rapid 

 spring migration. Practically all members of the species winter in the 

 great coastal marshes of Louisiana, where 50,000 or more may be seen 

 grazing in the "pastures" or flying overhead in flocks of various sizes. 



22 



