MICHIGAN ACADEMY OF SCIENCE. 
25 
has been fully explained recently by Mr. Austin H. Clark,* 
on the theory (well supported by observation) that these and 
many other birds while migrating prefer to fly with what 
sailors call a “beam-wind,” that is they fly at right angles to the course 
of the wind, which naturally causes them to make more or less lee-way 
according to the strength of the wind, but in no way interferes with 
forward progress—in fact greatly helps the bird by giving it a steady 
support. According to this theory birds coming north in the spring 
would move northeastward when the wind was from the northwest, and 
northwestward if the wind were from the northeast, while they would 
make a direct northward trip only on an east or a west wind. Since 
the winds blow alternately from different directions, any deviation to 
east or west would tend to be corrected in any extended migration, and 
in this way on the average the bird would have as good a chance of 
reaching his goal as if he attempted to proceed in a direct line, in spite 
of head winds or deflecting winds of any kind. Naturally a migrant 
would alight, if possible, rather than be blown backward on his course, 
and naturally also if the wind threatened to carry him out to sea he 
would alight or coast alongshore as. the case might be. Precisely this 
condition of affairs seems to arise frequently in the case of hawk migra¬ 
tion along the southern - New England coast, where the birds, flying 
at right angles to the prevailing southwesterly winds are gradually 
collected along the sea-shore and follow the coast southward to New 
Jersey and the Carolinas. 
It is also certain that not enough attention has been given to the 
influence of the rising currents of warm air which are utilized con¬ 
stantly by soaring birds and doubtless by most migrants. Considera¬ 
tion of these subjects, however, would involve a discussion of the 
mechanics of flight, for which this is not the time. 
In conclusion let me urge everyone who aspires to contribute his part 
to the fund of science that he be scrupulously careful in observation 
and record of every fact, and that he remember always that a theory 
is of use just so long as it serves as a point on which facts can be 
securely hung; when the point becomes too small or the hook points the 
wrong way it is time to find another place for the facts, while the theory 
may well be consigned to the waste basket. 
Agricultural College, Mich., March, 1906. 
* Aulc, XXII, 1905, pp. 134-140. 
4 
