increased to 40 miles a day, so that by April 15 it has reached Lake Athabasca. Spring has come with a rush on this western interior country. The result is that during the height of the migration season, from the middle of April to the middle of June, the southern end of the Mackenzie Valley in the Province of Athabasca has just about the same temperature as the Lake Superior region 700 miles farther south.
These conditions, coupled with the diagonal course of the birds across this region of fast-moving spring, necessarily exert a powerful influence on bird migration. On March 1 the earliest robins reach southern Iowa, where they find an average daily temperature of about 34° F.; a month later they appear in central Minnesota and find the same temperature, birds and spring each having gone northward at the rate of 13 miles per day. Those robins that fly from eastern Minnesota and western Wisconsin to Lake Superior and Keewatin, by increasing their speed to 25 miles per day, arrive on April 21 at latitude 52° in southern Keewatin, still closely following the temperature of 34°. But by this date the 34° F. isotherm has reached central Athabasca, and the central Minnesota robins that travel to the Mackenzie Valley and Alaska must double and quadruple their speed as they take a northwestward diagonal, if they are to keep up with the season. Though robin migration does not quite do this, yet a speed of 70 miles per day is reached by the species in this northwestward flight—more than three times the speed attained by the Keewatin birds.
THE UNKNOWN.
Interest in bird migration goes back to a remote period. Marvelous tales of the spring and fall movements of birds were spun by early observers, yet hardly less incredible are the ascertained facts. Much remains to be learned of migration; and it nay be of interest to note a few of the mysteries which still occupy attention.
The chimney swift is one of the most abundant and best-known birds of the eastern part of the United States. With troops of fledglings, catching their winged prey as they go, and lodging by night in some tall chimney, the flocks drift slowly south, joining with other bands until on the northern coast of the Gulf of Mexico they become an innumerable host. Then they disappear. Did they drop into the water and hibernate in the mud, as was believed of old, their obliteration could not be more complete. In the last week in March a joyful twittering far overhead announces their return to the Gulf coast, but the intervening five months is still the swift's secret.
The mouse-colored bank swallows are almost cosmopolitan, and enliven even the shores of the Arctic Ocean with their graceful aerial evolutions. Those that nest in Labrador allow a scant two months for building a home and raising a brood, and by the first of August are headed southward. Six weeks later they are swarming in the vicinity of Chesapeake Bay, and then they, too, pass out of the range of our knowledge. In April they appear in northern South America, moving north, but not a hint do they give of how they came there. The rest of the species, those that nest to the south or west, may be traced farther south, but they, too, fail to give any clew as to where they spend the five winter months.
The familiar cliff swallow, which swarms over the western plains and breeds from Mexico to Alaska, spends the winter in Brazil and Argentina. It would be expected to reach the United States in spring first in southern Florida and Texas, later in the Rocky Mountains, and finally on the Pacific coast. As a matter of fact, the earliest records of the bird's appearance in spring come front northern central California, where it becomes common before the first arrivals are usually
