Sept., 1904 I 
THE CONDOR 
123 
delay their migration, knowing that winter still holds sway in their summer domin- 
ions. Just wdien and where this tran^posal 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 migra- 
gration at so slow a speed as soon to be overtaken and passed by their 
impetuous cousins. 
Still later in the spring another transposal occuis. The northern birds pass 
across the southern portion of the breeding range, where the southernmost birds 
are aleady busy with their domestic duties. Spring migration seems to be there- 
fore for some species a game of leapfrog — the southern birds first passing the 
northern, and the northern passing them in turn. 
RELATION OF MIGR.ATION 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 cer- 
tainly birds in general have no such habit. Some summer w'arblers 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 tempera- 
ture is about 47° F. According to the notion mentioned, these birds might be ex- 
pected 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' Orleane 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 tempera- 
ture 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 Minne- 
.sota a temperature of 55° P'. In central Manitoba the average temperature they 
meet is 52° P'., and w'hen 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 ii to 
25 they traverse a district that spring requires thirty-five days to cross. This out- 
stripping 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 fol- 
low 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; few'er 
storms are encountered, the mortality rate is lowered, food is more plentiful along 
