50 
HOW BIRDS MIGRATE 
Our knowledge of the nocturnal migration of birds is based on 
evidence supplied by the call-notes of passing birds, on data from 
light-houses, on observations through telescopes and on field-work 
on days succeeding flights. 
It is a common experience, during the season of migration, to hear 
the notes of birds which are passing overhead. From an elevation in 
a line of flight, or where the city lights may attract birds, such notes, 
when birds are moving in numbers, are almost continuous. O. G. 
Libby (’99) states that on the night of September 14, 1896, on “a small 
elevation west of the city of Madison, Wisconsin,” a total of 3,800 
bird-calls were recorded. The average was twelve per minute, but 
the rate “varied greatly, sometimes running as high as two or three 
per second, and again falling to about the same number per minute. 
.... The great space of air above swarmed with life. Singly, or 
in groups, large and small, or more seldom in a great throng, the 
hurrying myriads passed southward.” 
Lighthouses, because of their location on the coast, on promon- 
tories or outlying islets, are often situated in the path of migrating 
birds. This fact, in connection with the fatal attraction which the rays 
of the light possess for migrating birds during stormy weather, has 
supplied an extended and definite series of records, which also emphasize 
the high mortality often prevailing in the ranks of night-migrating 
birds. Shortly after its erection, 1,400 dead birds are said to have been 
picked up at the base of the Bartholdi statue, in New York harbor, 
which had been killed by striking the statue the preceding night. 
For years light-keepers have reported to the Biological Survey 
at Washington on the birds seen about or striking the lights in their 
care and, in not a few instances, our knowledge of the migration of a 
species rests largely on this class of data. (Allen, ’80, p. 131.) 
We have also the testimony of ornithologists who have visited 
lighthouses especially for the purpose of observing the nocturnal 
journeys of birds. Brewster (l. c. p. 7), who visited Point LePreaux 
Lighthouse, in the Bay of Fundy, for this purpose, gives an impressive 
picture of observations made there on the night of September 4. 
Observations through telescopes, though limited in number, when 
one considers how easily they are made and how interesting and val- 
uable are the results to be obtained, supply probably our most satis- 
factory data on nocturnal migration. They can be made only on 
clear, moonlight nights, when the current of migration, flowing smoothly 
through the air above, is viewed under wholly normal conditions. 
A low-power telescope is focused on the moon, the glowing surface 
of which forms a background against which the birds, in passing, are 
clearly silhouetted. On September 3, 1886, at Tenafly, N. J., with the 
aid of a 6J inch equatorial glass, 262 birds were seen to cross the narrow 
angle subtended by the limits of the moon between the hours of eight 
and eleven (Chapman, ’88). Subsequent observations from the obser- 
vatory of Columbia University and at Englewood, N. J., have revealed 
