BIRD iMIGRATION vii 



in adaptation to the climate of the summer home. In spring and early summer climatic con- 

 ditions are decidedly variable, and yet there must be some period that has on the average 

 the best weather for the birds' arrival. In the course of ages there have been developed 

 habits of migration, under the influence of which the bird so performs its migratory move- 

 ments that on the average it arrives at the nesting site at the proper time. 



The word " average " needs to be emphasized. It is the average weather at a given 

 locality that determines the average time of the bird's arrival. In obedience to physiologic 

 promptings the bird migrates at the usual average time and proceeds northward at the 

 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 determines 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. Migration 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 con- 

 nected with sudden and extreme variations in temperature, seem to have only a slight influence 

 on migration. 



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 direction. 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 Mexican coast at nightfall and would have to 

 wait until the following morning to appease its hunger. Thus there would be 36 consecutive 

 hours without food, whereas by night migration the same journey can be performed with 

 only a 12 hours' fast. 



Migrating birds do not fly at their fastest. Their migration speed is usually from 30 to 

 40 miles an hour and rarely exceeds 50. Flights of a few hours at night, alternating with rests 

 of one or more days, make the spring advance very slow, averaging for all species not more 

 than 23 miles a day, but with great variations of daily rate among the different species. 

 The exact number of miles which a particular bird makes during one day's journey has not 

 yet been determined, and cannot be ascertained until the tagging or banding of birds by 

 means of metal rings is carried out on a far more extensive scale than has yet been possible. 



