Sept., 1904 I 
THE CONDOR 
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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 mi- 
gration. On March i 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 Kee- 
watin, still closely folllowing the temperature of 34°. But by this date the 34° P", 
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 nortwestward 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 may 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, mov- 
ing 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 from northern 
central California, where it becomes common before the first arrivals are usually 
