THE ORGANIZATION AND PROBABLE EVOLU- 

 TION OF SOME MIXED SPECIES FLOCKS 

 OF NEOTROPICAL BIRDS 



By M. MOYNIHAN 



Resident Naturalist, Canal Zone Biological Area 

 Smithsonian Institution 



Birds often form groups that include individuals of several different 

 species. Such heterogeneous groups may be called "mixed species 

 flocks." 



There are many different kinds of mixed species flocks. Some are 

 composed of only two species, while others may include many more, 

 probably over a dozen in extreme cases. Some mixed flocks are es- 

 sentially transitory, while others are semipermanent, very long-sus- 

 tained or dissolved and re-formed at more or less regular intervals. 

 Some seem to be very loosely organized, while others are tightly 

 integrated and have a complicated social structure. 



Mixed species flocks are found almost everywhere, in almost all 

 environments; but they seem to be most common and varied and 

 probably attain the greatest structural complexity in certain regions 

 of the humid Tropics. In such regions, most of the more complex 

 flocks are largely or completely composed of passerines. 



Mixed species flocks have been noted by almost all ornithologists 

 and naturalists who have traveled in the Tropics since Bates (1863), 

 Wallace (1869), and Belt (1874). Among the more recent or longer 

 accounts of such flocks are descriptions in Chapin (1932), Davis 

 (1946), Johnson (1954), Mitchell (1957), Moynihan (i960), Rand 

 (1954), Slud (i960), Stanford (1947), Stresemann (1917), Swyn- 

 nerton (1915), and Winterbottom (1943 and 1949). Many other, 

 earlier, descriptions of mixed flocks are listed in Rand, 1954. Some 

 aspects of mixed flocks have been discussed at considerable length in 

 some or all of these publications ; but the social reactions between indi- 

 viduals of different species within mixed flocks have not been analyzed 

 in detail. 



This paper is an attempt to provide more precise information about 

 some of the behavior mechanisms responsible for the formation and 



SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. 143, NO. 7 



