NO. 7 FLOCKS OF NEOTROPICAL BIRDS MOYNIHAN 9 



individuals of different species just tend to stick together whenever 

 they happen to run across one another in the course of their ordinary- 

 activities. 



Mixed blue and green tanager and honeycreeper flocks can be 

 formed at any time of the day between sunrise and sunset; but they 

 are probably more common and larger, on the average, during early 

 morning, when all the species of the alliance are most active, than 

 during later periods of the day. 



The social spacing of individual birds in mixed blue and green 

 tanager and honeycreeper flocks is quite as variable as the composition 

 of such flocks. Sometimes some or all of the members of a flock are 

 very close together, within a few inches of one another. Oftener they 

 are more scattered, most of them being separated from their nearest 

 neighbors by distances of several feet or yards. Sometimes they are 

 so widely scattered that their membership in the same flock is revealed 

 only by their tendency to move in the same direction at approximately 

 the same time. 



The more highly integrated mixed blue and green tanager and 

 honeycreeper flocks (which are not necessarily the flocks in which 

 the members are nearest to one another in space) seem to be confined 

 within certain definite ranges. In some cases, at least, the borders of 

 these ranges seem to be directly determined by features of the vegeta- 

 tion or topography, rather than social contacts between adjacent flocks. 



Individual birds, pairs, and larger family groups of some species 

 may defend territories while they are part of mixed flocks (see 

 below) ; but there is no joint defense of the flock range as a whole. 



Most of the species of the blue and green tanager and honeycreeper 

 alliance seem to have rather generalized feeding habits. Most of the 

 tanagers, and probably the saltators, eat a great deal of fruit; the 

 honeycreepers eat fruit and nectar; and all the species of the alliance 

 eat many insects. The feeding habits of all these species seem to 

 overlap widely. It is very common to see at least three or four species 

 feeding on the same food at the same time in the same place. This 

 occurs most frequently when some favored fruit or flower is particu- 

 larly abundant, and when swarms of insects emerge after hatching. 



It might be supposed, therefore, that the association of many of 

 these species in mixed flocks is purely coincidental. It is conceivable 

 that such flocks might be formed and maintained simply because many 

 species are attracted to, and remain near, the same food sources. 



There are various indications, however, that at least some (and 

 perhaps the majority) of the associations between species of the blue 



