18 CIRCULAR 363, U. S. DEPARTMENT OF AGRICULTURE 
Orleans about April 5, when the average temperature is 65° F. Tra- 
veling north much faster than does the season, they reach their 
breeding grounds in Manitoba the latter part of May, when the 
average temperature is only 47°. Encountering progressively colder 
weather over their entire route, they cross a strip of country in the 
15 days from May 11 to 25 that spring takes 35 days to cross. This 
“catching 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 excep- 
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35° MARC oe 
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—— /SOCHRONAL MIGRATION LINES 
B4508M 
FIGURE &.—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. 
crow (Corvus brachyrhynchos), the red-winged blackbird (Agelaius 
phoeniceus), and the robin. 
The blue goose (Chen caerulescens) (fig. 10) 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 Louis- 
jana, where 50,000 or more may be seen grazing in the “pastures’”’ 
or flying overhead in flocks of various sizes. Their breeding grounds 
are chiefly on Baffin Island and on Southampton Island in the northern 
part of Hudson Bay, in a region where conditions of severe cold pre- 
vail except for a few weeks each year. The birds seem to realize that 
even though the season in their winter quarters is advancing rapidly, 
their nesting grounds are still covered with a heavy blanket of ice 
and snow. Accordingly they remain in the coastal marshes until the 
last of March or the first of April, when the local birds are already 
