delay their migration, knowing that winter still holds sway in their summer dominions. Just when and where this transposal of relative position occurs is one of the problems of migration reserved for future solution. Nor is it yet settled whether the northern-bred birds remain strictly within their winter range until after their more southern congeners have passed by, or whether they begin an early migration at so slow a speed as soon to be overtaken and passed by their impetuous cousins.
Still later in the spring another transposal occurs. The northern birds pass across the southern portion of the breeding range, where the southernmost birds are already busy with their domestic duties. Spring migration seems to be therefore for some species a game of leapfrog—the southern birds first passing the northern, and the northern passing them in turn.
RELATION OF MIGRATION AND TEMPERATURE.
A popular notion exists that birds push northward to their summer homes as soon as weather conditions permit. This may be true of a few species, but certainly birds in general have no such habit. Some summer warblers that return to the Great Slave Lake region to breed, after spending the winter in Central and South America, arrive at their nesting grounds when the average daily temperature is about 47° F. According to the notion mentioned, these birds might be expected to move up the Mississippi Valley and on to their summer homes at the same time as the northward moving temperature of 47° F. But were this so, they would never leave the United States, for the average of the coldest month of the year at New Orleans is 54° F. As a matter of fact, the summer warblers of Great Slave Lake are probably too well content with the warm, humid, insect-laden air of the South to brave the arctic blasts before necessity compels. They linger in the Tropics so late that when they reach New Orleans, April 5, an average temperature of 65° F. awaits them. They now hasten; traveling north much faster than the spring does, they cover 1,000 miles in a month, and find in southern Minnesota a temperature of 55° F. In central Manitoba the average temperature they meet is 52° F., and when they arrive late in May at Great Slave Lake they have gained 5° more on the season. Thus, during the whole trip of 2,500 miles from New Orleans to Great Slave Lake, these birds are continually meeting colder weather. In fact, so fast do they migrate that in the fifteen days from May 11 to 25 they traverse a district that spring requires thirty-five days to cross. This outstripping of spring is habitual with all species that leave the United States for the winter, and also with most of the northern birds that winter in the Gulf States. Careful examination of the migration records of each species of the Mississippi Valley shows only six exceptions—Canada goose, mallard, pintail, common crow, red-winged blackbird and robin.
The robin as a species migrates north more slowly than the opening of the season; it occupies seventy-eight days for its trip of 3,000 miles from Iowa to Alaska, while spring covers the distance in sixty-eight days. But it does not follow that any individual bird moves northward at this leisurely pace. The first robins that reach a given locality in the spring are likely to remain there to nest, and the advance of the migration line must await the arrival of other birds from still farther south. Therefore, each robin undoubtedly migrates at a faster rate than the apparent movement of his species as a whole, and does not fall behind the advancing season. This is true of most, if, not all, of the other seemingly slow migrants. Late and rapid journeys of this kind offer certain advantages; fewer storms are encountered, the mortality rate is lowered, food is more plentiful along
