MUTUALISTIC AND COMMENSAL ORGANIZATION OF AVIAN MIXED-SPECIES FORAGING FLOCKS IN A FOREST OF WESTERN MADAGASCAR

I examined the feeding rates and foraging niches of seven species regu larly participating in heterospecific avian flocks in a deciduous dry forest of western Madagascar in two different social situations, viz. when the species was foraging alone or with conspecifics, and in heter ospecific flocks, respectively. The species responded differently to m ixed-flocking. A change in feeding sites and/or techniques was found i n six species, five of which increased their Feeding rates when foragi ng in heterospecific flocks. In mixed flocks, they tended to use simil ar substrates. That such interspecific convergence enhanced feeding ra te might be explained by social learning, kleptoparasitism or a beatin g effect. Benefits gained through these effects were greatest in two s pecies consistently acting as followers, Crested Drongo Dicrurus forfi catus and Paradise Flycatcher Tersiphone mutata. Although as leaders, Newtonia Newtonia brunneicauda and Long-billed Greenbul Phyllastrephus madagascariensis often attracted other species, they also increased t heir Feeding rates through some mutualistic effects. Another leader, R ufous Vanga Shetba, rufa was the only species that neither changed for aging niche nor feeding efficiency in different social situations. Its participation in heterospecific associations probably resulted from o ther species exploiting the vanga's ability to detect predators. There fore, the organization of multispecies flocks in Madagascar was mainly based on mutualism and commensalism which increased feeding efficienc y.