THE MIGRATION OF NORTH AMERICAN BIRDS ays. 
Eastern Egg Rock, Maine,’ and the other two at the Red Islands, 
Turnevik Bay, Labrador, on July 22, 1927, and July 23, 1928. The 
first was found dead in the Niger River delta, West Africa, in August 
1917, while the Labrador birds were recovered near La Rochelle, 
France, on October 1, 1927, and at Margate, near Port Shepstone, 
Natal, South Africa, on November 14, 1928. The flight shown by 
this last record is the longest known, the trip, between 8,000 and 
9,000 miles, being accomplished in less than 3 months. 
Probably no other animal in the world enjoys as many hours of 
daylight as does the arctic tern, since for these birds the sun never 
sets during their nesting season in the northern part of the range, 
while during their sojourn in the south, daylight is continuous. 
During several months of the year they have 24 hours of daylight 
and during the other months considerably more daylight than 
darkness. 
ROUTES OF MIGRATION 
While it is beyond question that certain general directions of 
flight are constantly followed by migratory birds, it is well to remember 
that the term ‘‘migration route” is to some extent a theoretical 
concept, concerned entirely with the lines of general advance or 
retreat of a species, rather than the exact course followed by individual 
birds. Even the records of banded birds usually show no more than 
the places of banding and recovery, and one must have recourse to 
intermediate records and to reasoning based on probabilities to fill 
in details of the flyway actually traversed between the two points. 
There is also infinite variety in the routes covered during migration 
by different species. In fact, the choice of migration highways is so 
wide that it seems as if the routes of no two species coincide. Dif- 
ferences in distance traveled, in time of starting, in speed of flight, 
in geographical position, in latitudes of breeding and of wintering 
erounds, and in other factors, all contribute to this great variation 
of migration routes. Nevertheless, there are certain factors that 
serve to guide the avian travelers along more or less definite lines, and 
it is possible to define general lines of migration for the majority of 
species. 
It has frequently been observed that migrating birds have a 
tendency to follow major topographic lines on the earth’s surface 
when their trend is in the general direction of the birds’ journey. 
Bird migration is generally thought of as a north-and-south move- 
ment, with the lanes of heavier concentration following the coasts, 
mountain ranges, and principal river valleys. To a considerable 
extent this is the case, particularly in North America, where the coast 
lines, mountain chains, and the larger rivers in general run north and 
south. Students of American birds thus have exceptionally good 
opportunities to study migratory movements. In cases where the 
migration is a long one, however, the notion must be abandoned 
that the birds’ flight is restricted to particular narrow routes that 
follow river valleys and the like, as many species seem to disregard 
utterly such apparently good natural flyways as river valleys. For 
example, the Arkansas River has a general east and west course for 
a great part of its length, and while it does constitute a highway for 
3 Recorded at the time of banding as a common tern, a natural error, as the downy young of common and 
arctic terns look almost exactly alike. 
