8o8 THE POPULAR SCIENCE MONTHLY. 



the Xorth is urged as well by their delicacy of organization and con- 

 sequent susceptibility to cold, as by a failure in the food-supply. The 

 prowess of their pinion has been the astonishment and admiration of 

 all. The comings of the swallows have passed into proverb, and their 

 leave-takings have been rehearsed in folk-lore among the signs of 

 waning times. They have figured in augury ; their flight is baromet- 

 ric, for they soar on clear, warm days, and skim the surface of the 

 ground in heavy, falling weather, perhaps neither always nor entirely 

 in the wake of insects upon which they feed. These birds cross our 

 Southern border when the weather is yet cold and changeable. The 

 record for the purple martin for 1884 shows that the first four degrees 

 of latitude were passed at a rate of sixteen miles per day ; the next 

 two and one-half degrees at twelve miles ; the next four and one-half 

 degrees at sixty-three miles ; and the last two and one-half degrees at 

 but ten miles per day — making an average for the entire distance of 

 eighteen miles per day. This record shows us a species very irregular 

 in its rate of speed, and it is easily shown that this irregularity is due 

 to the vicissitudes of the weather. 



From observations such as these much has been learned regarding 

 the rate at which various other species migrate. Data on fifty-eight 

 species for 1883, for four hundred and twenty miles, show the average 

 rate to have been twenty-three miles per day. Data on not quite so 

 many species for 1884 show the average rate, for eight hundred and 

 sixty-one miles, to have been exactly the same as for 1883. Twenty- 

 five species gave an average daily rate of nineteen miles for March, 

 twenty-three miles for April, and twenty-six miles for May, thus in- 

 dicating — what I believe to be true — that the speed at which most 

 species migrate increases toward the northern limit. This was one of 

 the first important facts in migration observed and pointed out by 

 Professor "VV. W. Cooke, and subsequent observations have all tended 

 to prove the correctness of his views. 



Were migration a steady movement, with the same individuals 

 always in the lead, we might determine the exact rate of speed for 

 many different species, but the movement resembles rather a game of 

 leap-frog, and the leaders are constantly changing. Those individuals 

 which arrive first at any given place are the birds of that species 

 which will remain there to breed, while those in the rear pass on 

 farther north. " The vanguard is thus constantly arresting itself, and 

 the forward movement must await the arrival of a new corps, which 

 may be near at hand or far behind. Migration is, then, a series of 

 overlappings, and the real is evidently much greater than the ap- 

 parent speed." 



It has also been noticed that, as a rule, any given species migrates 

 earlier up the Atlantic seaboard or the Mississippi Valley than it does 

 across the more arid plains to the west ; the first arrivals appear here 

 from four to seven days earlier than in Kansas directly west of us. 



