Page:Popular Science Monthly Volume 30.djvu/832
THE POPULAR SCIENCE MONTHLY.
the North is urged as well by their delicacy of organization and consequent 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 barometric, 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 indicating—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 W. 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 apparent 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.