MOVEMENTS OF BIRDS IN RELATION TO THE WEATHER. 387 



the date of arrival on its nesting grounds depend on the combination 

 of storm and fair weather met during the journey. 



The arrival of a migrating bird in any district south of its final 

 goal depends not so much on the local temperature of that district 

 as on its geographic relation to the place of nesting. The summer 

 warblers, for instance, which nest in Manitoba, doubtless spend the 

 winter in South America and probably start north in March, arriv- 

 ing in Manitoba the middle of May, where they find an average tem- 

 perature of about 48 F. As this is the time they begin summer 

 housekeeping, it is evident that these warblers obtain an abundance 

 of food at this temperature. Leaving South America with the ther- 

 mometer higher than 70 F., throughout the entire trip they are in 

 a temperature warmer than is required for their food supply, and 

 it is only during severe storms or unusual cold that climatic condi- 

 tions delay their northward progress. 



Thus over the whole flight way between the winter and summer 

 homes, local weather conditions have little influence on the average 

 time of the bird's arrival, except when it nears its breeding grounds. 

 Then it approaches a critical zone, where its migration is very likely 

 to be affected by the weather. The summer warbler usually finds in 

 Louisiana a temperature of 70 F.. and a drop of 10 would hardly 

 retard its progress; but if, just before it reached Manitoba, the tem- 

 perature should fall from 48 F. to 38 F., it would probably fold 

 its wings and wait. 



FALL MIGRATION. 



The data available for the study of fall migration are much less 

 in quantity, as well as less reliable, than those on spring migration. 

 It may be said, however, that almost without exception the beginnings 

 of fall migration have no relation whatever to the weather. Most 

 species migrate as soon as the young are able to care for themselves ; 

 others begin molting then and start on their southward trip when 

 their new fall suits are ready. Many species begin to go south in 

 July and most of the others early in August, long before the fall 

 storms have lessened their food supply, and, indeed, at the time when 

 food is most plentiful. 



After the tide of fall migration is in full swing, its advance is 

 varied by alternating storms and fair weather, as in spring, but with 

 exactly opposite eifects; instead of delaying migration, a fall storm 

 causes the departing hosts to hasten their movements before the chill- 

 ing northern blasts. In spring the larger part of migration occurs 

 with a rising temperature; in fall a still larger percentage occurs 

 when the temperature is falling. 



MIGRATION AND WIND. 



During spring migration the direction of the wind, seems to have 

 little if any effect on the movements of birds. Arrivals were noted 



