THE MIGRATION OF NORTH AMERICAN BIRDS Zo 
tropical species that in the Atlantic region reach their most northern 
breeding point on the Dry Tortugas Islands, off the southwest coast 
of Florida. They are not known to wander regularly any appreciable 
distance farther north. It was found that some were able to return 
to their nests on the Tortugas after they had been taken on board 
ship, confined in cages below deck, and carried northward distances 
varying from 400 to 800 miles before being released. Landmarks of 
all kinds were entirely lacking, and the birds certainly were liberated 
in a region in which they had had no previous experience. 
Possibly the ‘‘homing instinct”’ as shown by these terns, by the 
man-of-war birds (Fregata minor), that are trained and used as mes- 
sage carriers in the Tuamotu, Gilbert, and Marshall Islands, and by 
the homing pigeon, is not identical with the sense of perceptive 
orientation that figures in the flights of migratory birds. Neverthe- 
less, it seems closely akin and is probably caused by the same im- 
pulses, whatever they may be and however they may be received. It 
is to be remembered, however, that while homing may involve flight 
from a point that the bird has never before visited, the flight is always 
to a known point—that is, the bird’s nest—while, on the other hand, 
the first migratory flight is always from the region of the bird’s birth 
to a region it has never before visited. The spring migration might, 
of course, be more nearly considered as true ‘“‘homing.”’ 
At the present time some students lean strongly toward the possible 
existence of a ‘“‘magnetic sense” as being the important factor in the 
power of geographical orientation. No direct evidence in support of 
this has been obtained, but it is not impossible that there may exist 
some form of physiological sensibility to the phenomena of terrestrial 
magnetism. The theory as laid down (chiefly by European investi- 
gators) is highly complex, but briefly stated it 1s based on a supposed 
sensitiveness of birds to the magnetic influences that cause variations 
in the declination and dip of magnetic needles. Some experimental. 
work already done lends a little support to the theory but it is still 
far from established. 
In concluding this discussion of orientation it is pertinent to point 
out that the migratory instinct appears to be more or less transitory, 
that it is not persistent over an extended period. Migratory birds 
may be arrested en route, either by natural conditions, such as unusual 
food supplies, or forcibly by the act of man, and detained until the 
end or nearly the end of the migratory season, and then may not 
attempt to finish the journey, apparently having lost the migratory 
impulse. In the fall and early winter of 1929, abundant food and an 
open season caused an unusual number of mallard ducks to arrest 
their migration and remain in western Montana and northern Idaho. 
Later, however, a heavy snowfall with subzero temperatures suddenly 
cut off the food supply, with the result that great numbers of the 
birds starved to death, when a flight of a few hours would have 
carried them to a region of open water and abundant food. 
SEGREGATION DURING MIGRATION 
During the height of the northward movement in spring the woods 
and thickets may be suddenly filled with several species of wood 
warblers, thrushes, sparrows, flycatchers, and others, which it is 
natural to conclude have traveled together and arrived simultaneously. 
134984°—_35——_4 
