MOVEMENTS OF BIRDS IN RELATION TO THE WEATHER. 381 



mountains to the shrubbery of the Coastal Plain in order to obtain 

 food. The storm continued to the north of Washington for more 

 than a week, while in the South fair weather tempted additional 

 thousands to continue their migratory flight until they reached the 

 inhospitable zone. The result was a " tidal wave " of birds never 

 before or since equaled in the neighborhood of Washington. 



This is a striking illustration of the fact that bird migration occurs 

 in what may fitly be called " waves." Moreover, the relation of the 

 weather to bird waves is that of cause and effect. Disconnected bird 

 parties traveling northward are arrested by a cold snap, and suc- 

 ceeding swarms are similarly delayed until a great migrant host is 

 waiting to continue its progress northward as a pronounced "bird 

 wave." 



It is not to be understood, however, that bird waves and tempera- 

 ture waves are always synchronous. Indeed, if a chart showing the 

 weather waves at a given place for a series of years is compared with 

 a similar chart showing the bird waves, the lack of uniformity is 

 such as to suggest that there can be no relation between migration 

 movements and the weather. Thus one season at Lanesboro, Minn., 

 during the first week in May, the temperature was above normal and 

 a great bird wave flooded the woods with songsters. The next year 

 the same species came five days earlier, apparently because of a warm 

 wave that carried the mean temperature far above the average. But 

 the following year one of the most pronounced bird waves of the 

 season occurred on the last day of April, when, although the tem- 

 perature was far below normal, the birds arrived almost a week ahead 

 of their usual dates; and notwithstanding the continuance of cold 

 weather for the next three weeks, the birds kept coming, two-thirds 

 of them earlier than usual. The largest bird wave ever recorded at 

 Lanesboro, Minn., was on May 9, 1891, when after several days of 

 cold the temperature rose suddenly some 20 above normal. Yet in 

 another year the largest number of arrivals was on May 7, the last 

 of three days of increasing cold, when the temperature was fully 20 

 below normal. 



Evidently it is impossible to foretell what bird movements will 

 accompany any given set of weather conditions. It may seem that a 

 certain storm has held back the travelers, but another year, under 

 apparently identical conditions, the birds may continue northward in 

 spite of a storm. Birds fail to arrive when circumstances seem pro- 

 pitious and again come in myriads when conditions seem adverse. 



4 

 EXPLANATION OF MIGRATION UNDER APPARENTLY UNFAVORABLE 



CONDITIONS. 



The probable explanation of these wide departures from the inti- 

 mate connection that has been supposed to exist between bird migra- 



