BIRD MIGRATION. 5 



usual average speed unless prevented by adverse weather. Weather 

 conditions are not the cause of the migration of birds; but the 

 weather, by affecting the food supply, is the chief factor which deter- 

 mines the average date of arrival at the breeding grounds. After 

 the bird, in response to physiological changes, has started to migrate, 

 the weather it encounters en route influences that migration in a 

 subordinate way, retarding or accelerating the advance by only a 

 few days, and having usually only slight effect upon the date of 

 arrival at the nesting site. 



Local weather conditions on the day of arrival at any stated locality 

 are minor factors in determining the appearance of a given species 

 at that place and time. The major factors in the problem are the 

 weather conditions far to the southward, where the night's flight 

 began, and the relation which that place and time bear to the average 

 position of the bird under normal weather conditions. Many, if not 

 most, instances of arrivals of birds under adverse weather conditions 

 are probably explainable by the supposition that the flight was begun 

 under favorable auspices and that later the weather changed. Migra- 

 tion in spring usually occurs with a rising temperature and in autumn 

 with a falling temperature. In each case the changing temperature 

 seems to be a more potent factor than the absolute degree of cold. 



The direction and force of the winds, except as they are occasionally 

 intimately connected with sudden and extreme variations in tem- 

 perature, seem to have only a slight influence on migration. 



DAY AND NIGHT MIGRANTS. 



Some birds migrate by day, but most of them seek the cover of 

 darkness. Day migrants include ducks and geese (which also migrate 

 by night), hawks, swallows, the nighthawk, and the chimney swift. 

 The last two, combining business and pleasure, catch their morning 

 or evening meal during a zigzag flight that tends in the desired direc- 

 tion. The daily advance of such migrants covers only a few miles, 

 and when a large body of water is encountered they pass around 

 rather than across it. The night migrants include all the great family 

 of warblers, the thrushes, flycatchers, vireos, orioles, tanagers, shore- 

 birds, and most of the sparrows. They usually begin their flight 

 soon after dark and end it before dawn, and go farther before than 

 after midnight. 



Night migration probably results in more casualties from natural 

 causes than would occur if the birds made the same journey by day; 

 but, on the other hand, there is a decided gain in the matter of food 

 supply. For instance, a bird feeds all day on the north shore of the 

 Gulf of Mexico; if, then, it waited until the next morning to make 

 its flight across the Gulf in the daytime it would arrive on the Mex- 

 ican coast at nightfall and would have to wait until the following 



