MICHIGAN ACADEMY OP SCIENCE. 
19 
certain species may, with more or less regularity, travel at a given alti¬ 
tude and that this altitude may vary among birds of different families. 
With little doubt thrushes and warblers travel at a much lower level 
than do ducks and geese.”* This perhaps is a fair example of unwar¬ 
ranted inference and generalization drawn from the meagre data at 
hand. Different observers of nocturnal migration, using different instru¬ 
ments under similar conditions (that is always against the face of the 
newly risen moon) have recorded birds. migrating at heights estimated 
all tiie way from COO feet to 15,100 feet, and moving at all speeds from 
nearly stationary up to 134 miles per hour, Avith an average of sixty- 
seven miles per hour for small birds of ordinary powers of flight. 
I have not the least desire to belittle the discoveries of these pioneer 
observers, or to cast any reflection on their honesty of purpose or the 
accuracy of their records, yet I am free to say that until we have very 
many more observations in corroboration of these I cannot but doubt 
that any of our birds, large or small, at any height or under any cir¬ 
cumstances, attains a speed even approximating 100 miles an hour. In 
making this statement of course I invite criticism, but I desire neither 
moi'e nor less than the truth, and no one will welcome additional evi¬ 
dence more heartily than the writer. At a height of little more than 
three miles the density of the atmosphere is only half that at the sea- 
level, hence its resistance to the passage of a bird at that height would 
be lessened one-half. It does not follow, however, that therefore a bird 
at a height of three and one-half miles can fly at double its speed at 
the surface without increased effort. A moment’s thought will show 
hoAv preposterous is such a claim. The very tenuity of the air, which 
lessens by one-lialf the resistance to the forward motion of the bird, 
must lessen in exactly the same proportion the supporting power of the 
air and its resistance in the wing-strokes, which alone give the bird 
headway. We may dismiss as absurd the claim that birds may double 
their speed by flying in rarified air. It belongs with that class of 
popular science that told us until recently that bird’s bones were holloAv 
and filled with air, thus rendering them more buoyant and materially 
aiding in their flight. It is aauII knoAvn now that all the air contained 
in the birds’ air-sacs and bones combined does not alter its specific 
gravity sufficiently to constitute a factor of the slightest importance in 
flight. 
In this connection avc might consider some of the equally absurd 
claims made by Heinrich Gatke in 1805 in his remarkable book en¬ 
titled “Heligoland as an Ornithological Observatory.” Heligoland is a 
little island of less than a square mile’s area, located in the*North Sea 
or German Ocean, opposite the mouth of the Elbe and about fifteen 
miles from the mainland. For more than fifty years Herr Gatke lived 
on this island and studied its bird life, Avhich is of special interest 
since it is almost deserted by birds during the summer, but appears to 
be a resting point or port of call for scores of species and -countless 
thousands of individuals during their migrations. His list of these 
species, 397 in all, together Avith the dates of appearance, relative num¬ 
bers, and the meteorological conditions, is an important contribution to 
science, bul un fortunately he has included statements of an entirely 
* Popular Science Monthly, Vol. 45, 1894, p, 510. 
