THE 

 POPULAR SCIENCE 

 MONTHLY * 



MARCH, 1914 



OX THE ORIGIN OF THE FLOCKING HABIT OF 



MIGEATOEY BIEDS 



By Professor C. C. TROWBRIDGE 



COLOMBIA UNIVERSITY 



XT is a noteworthy fact that many species of birds which take long 

 migratory journeys make these movements in large flocks. The 

 same is true to a less extent of the species which makes long daily flights 

 for food. The origin of this flocking habit is not completely explained 

 by the three ordinary theories, which neglect two most important con- 

 siderations. These theories attribute the formation of the flock to the 

 companionship which it affords, to the experience of old birds that know 

 the sign posts of the journeys and hence lead the way, and to that pro- 

 tection against enemies, which may be secured by numerical strength; 

 but two other protective features, of prime significance, are here over- 

 looked. In the first place, the large flock automatically, by its numbers, 

 maintains the proper direction of migration; secondly, and just as 

 automatically, in the case of many large birds, the flock is warned and 

 protected against its enemies by its form, or arrangement. The im- 

 portance of these two automatic methods of protection is readily brought 

 out by a consideration of the influences which tend to deviate the flock 

 from its true course, on the one hand, and by a study of the actual 

 groupings of birds in migratory flocks on the other. 



Automatic Protection of a Large Flock 



The chief causes of errors relative to direction of flight to which a 

 single migrating bird is subject, are : 



(a) Confusion with respect to the proper direction of flight. 



{o) Effect of heavy winds or thick fogs acting as a temporary confusing factor 



while a flock is migrating. 

 (c) Gradual deviation from the proper course due to unequal wing power. 



VOL. LXXXIV. — 15. 



