NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN 13 



to moderately thick forest. There are a few cultivated fields (most 

 of which were bare or planted with low crops at the times of observa- 

 tion) scattered in the midst of the forest and scrub, and a few yards 

 of grass along both sides of the railroad track. The ground beside the 

 railroad track is level, but the rest of the area is quite hilly. Mixed 

 blue and green tanager and honeycreeper flocks occur in almost all the 

 scrubby and forested parts of this area. (A peculiar feature of this 

 area is a series of steel pylons, carrying electric cables, at regular 

 intervals along the railroad track. Most of these pylons are separated 

 from the nearest trees by an appreciable distance. The distribution 

 of some birds is sometimes greatly affected by the presence of these 

 pylons.) 



(3) About 10 miles from the other two, near the town of Gamboa. 

 The vegetation in this area is much the same as in the Frijoles area. 

 Most of the observations in this area were made along approximately 

 a mile of concrete road. Mixed species flocks are common along the 

 edges of the forest and high scrub on both sides of the road. 



The nature of the mixed flocks in these areas may be illustrated in 

 more detail by the accompanying tables. 



(It should be emphasized that the data in these tables are merely 

 examples. The analyses of flocks and the behavior of individual 

 species in the following pages are based upon many other types of 

 counts and other observations, in addition to the data summarized in 

 these particular tables.) 



The data summarized in the first group of tables (i to 19) were 

 obtained by observation of birds in and around the clearing on Barro 

 Colorado Island between October 26 and December 20, 1959. This 

 time of the year is the nonbreeding season for the members of the 

 blue and green tanager and honeycreeper alliance. 



Table 1 is an attempt to indicate the frequencies with which various 

 species may occur in mixed flocks in this area at this time of the year. 

 It is a count of the number of times individuals of the 12 most fre- 

 quently observed species of tanagers, honeycreepers, and warblers 

 were seen in mixed flocks and not in mixed flocks during the whole 

 period of observations. This count was made as follows : 



An observer walked around the clearing and counted every finch, 

 tanager, honeycreeper, and warbler seen. Any of these birds seen in 

 a tree or bush in which there were also one or more other birds of 

 some other species (any other species, including nonpasserines) at 

 the same time were counted as being in a mixed flock. The only excep- 

 tions were some birds in very large trees. In such trees a bird was 



