THE COMPOSITION AND BEHAVIOUR OF SOME MIXED‐SPECIES BIRD FLOCKS IN SARAWAK

SummaryMixed‐species flocks of birds feeding on insects were observed mainly in forest in Sarawak at a time of year when insect availability is known to be near its annual minimum. The approximate individual and specific composition of most flocks were noted. Twenty‐six species occurring in 25% of flocks were regarded as occasional members, 15 species occurring in 40% of flocks being classed as regular members.Recording species' feeding behaviour was the main priority and few observations of inter‐ and intra‐specific interactions within the flock, horizontal distribution or vertical stratification were made, although the last proved to be of potential significance. From the analysis of feeding behaviour two groups of species were distinguished. The larger group contained those usually having an exclusive common feeding pattern and showing very little overlap with the other such species. Fewer species used a wide range of feeding methods, all of which were shared with other species, but the least overlap was with other members of the same category.The observed composition of these flocks, in terms of regular members, might be interpreted as ensuring a low level of inter‐specific competition and it is suggested that the less specialized foragers may occur in the flocks by utilizing the ‘gaps’ between, and absences of, specialists.The possible advantages of membership of mixed‐species flocks are briefly considered and the likelihood of the selective advantage in any situation being the result of a balance of factors emphasized. In the apparent absence of regular potential predators the existence of these flocks is interpreted primarily as an adaptation for augmenting available insect food, particularly perhaps at critical times of year, by flushing insects as a result of the foraging activities of flock members. It is suggested that the varied responses of insects on being disturbed coupled with the different and fairly specialized feeding techniques of the birds could ensure benefit for all members of the flock.