NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN 123 



tween them might otherwise be most intense, than during the non- 

 breeding season. 



(One aspect of the social relationships between these two species 

 in Panama may possibly be typical of the social relationships between 

 many other overlapping species of equally similar habits. Palm tana- 

 gers and blue tanagers associate with one another very frequently in 

 Panama; but this is probably more often due to the fact that they 

 both tend to join and follow plain-colored tanagers than to their 

 tendencies to join and follow one another. Their associations, in 

 other words, are more often indirect than direct.) 



It seems likely, therefore, that the first highly specialized bonds to 

 develop during the evolution of most highly integrated mixed flocks 

 of passerine birds were bonds between species that were not very 

 closely related, phylogenetically (i.e., species of different genera, or 

 even, in many cases, different families). 



All or almost all highly integrated flocks of passerine birds usually 

 or always include individuals of certain particular species that show a 

 high degree of intraspecific gregariousness and play a nuclear role 

 in the mixed flocks. Numerous examples may be cited. Among 

 the flocks of tanagers, honeycreepers, and finches in Panama, there 

 are the plain-colored tanagers in mixed blue and green tanager and 

 honeycreeper flocks, the brown-capped bush-tanagers in mixed mon- 

 tane bush flocks, the variable seedeaters in mixed flocks of small 

 finches, and the yellow-rumped tanagers in Ramphocelus flocks. 

 Among other types of flocks there are white-flanked ant- wrens 

 (Myrmotherula axillaris) in mixed flocks of insectivorous birds in the 

 lowland forests of central Panama (Johnson, 1954, and R. H. 

 Barth Jr., in litt.) ; green-headed tanagers (Tangara seledon) in 

 mixed flocks of tanagers and honeycreepers in southern Brazil 

 (Mitchell, 1957) ; various species of Acanthiza, gray fantails {Rhipi- 

 dura flab ellif era), orange-winged sittellas (Neositta chrysoptera) , 

 and probably some other species, in mixed flocks of insectivorous 

 birds in Australia (Gannon, 1934, and Hindwood, 1937) ; white- 

 headed vangas (Artamella viridis) in some mixed flocks of forest 

 birds in Madagascar (Rand, 1936) ; and black-capped chickadees 

 (Parus atricapillus) in mixed flocks in North American woods in 

 winter (Odum, 1942). 



More often than not there is only one species that shows a high 

 degree of intraspecific gregariousness included in any given type of 

 mixed flock in any given area. This would suggest that the usual 

 course of evolution of many highly integrated mixed flocks of pas- 



