12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



bringing birds of the blue and green tanager and honey creeper alliance 

 together, the mixed flocks of this alliance may still be considered pri- 

 marily feeding groups in one sense. Almost all the birds in such flocks 

 spend most of their time feeding, or (apparently) looking for food, 

 as long as they remain in the flocks. (They also spend most of their 

 time feeding and looking for food when they are not in mixed flocks. ) 

 This does not mean that birds never perform other activities in mixed 

 flocks. Any individual or group of any species in a mixed flock may 

 preen or perform other comfort activities, or engage in hostile dis- 

 putes, or start courtship, or even just pause to rest while in the flock ; 

 but the performance of such nonfeeding patterns usually tends to 

 reduce the cohesion of the flock. The birds performing such nonfeed- 

 ing patterns tend to drop out of the flock if they continue such activi- 

 ties for any appreciable length of time. The flock just moves off and 

 leaves them behind. 



The only nonfeeding activities that may greatly increase the co- 

 hesion of mixed flocks are escape and mobbing reactions. Escaping 

 and mobbing birds tend to bunch together particularly closely. Mob- 

 bing also attracts other birds in the neighborhood, and the sight of one 

 bird escaping may induce other birds to fly in the same direction. 



In the course of the present study, mixed flocks of the blue and 

 green tanager and honeycreeper association were observed most 

 thoroughly in three different areas : 



(1) On Barro Colorado Island, in and around the large clearing 

 of the biological station. This clearing is roughly rectangular, approxi- 

 mately a quarter of a mile long and 300-500 yards wide in most places, 

 and stretches up a steep hill (approximately 275 feet high) from the 

 shores of Gatun Lake. It is surrounded by fairly mature second- 

 growth forest on three sides. There are also some trees and patches 

 of shrubbery (mostly fruit trees and introduced ornamental bushes) 

 scattered irregularly within the clearing. Mixed blue and green tana- 

 ger and honeycreeper flocks are common in the trees around the edge 

 of the clearing, and sometimes visit the trees and shrubs within the 

 clearing, especially when the birds are crossing from one side of the 

 clearing to the other. 



(2) On the mainland, about 2 miles away, near the little town of 

 Frijoles. In this area observations were made along approximately a 

 half mile of railroad track and along several narrow paths leading off 

 the track. The vegetation in this area is second growth, obviously 

 younger than most of the forest on Barro Colorado Island. It is also 

 extremely diverse, ranging from low scrub and gardens around houses 





