BIRD MIGRATION. 5 



usual average speed unless prevented bj^ 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 bii'd, in response to physiological changes, has started to migrate, 

 the weather it encounters en route influences that migration m 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 locahty 

 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 sprmg usuafly occurs with a rising temperature and in autunm 

 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 shght 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 roiles, 

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

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

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

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

 soon after dark and end it bcfoic diiwu, nnd go farther before than 

 after mi(hiight. 



Niglit rnigru(ion |)i-obal)ly r<'sul(s iu more casualties fiom naiui-al 

 causes tiian would of;ciir if the birds made tlie samc^ jouriK^y l)y day; 

 but, on the other liiiud, I here is ii, decided gain in the matter of food 

 suf)ply. For instiincc, a bird feeds all (biy on i\u\ nortlv slior(^ of the 

 Gulf of Mexico; if, llien, il waited unlil the next morning to make 

 its flight across tlio (iidf in I lie dnytime it, would arrive on the Mex- 

 ican coast at nij/lilfid! :ind \v(»uld \\uv(\ to wnil. until the following 



