Flocks of Brown and Black New Guinean Bird: A Bicolored Mixed-species Foraging Association

Abstract

A type of itinerant mixed-species foraging association of New Guinean birds, the brown and black flock, is distinctive in that member species converge on two alternative colours rather than on one, and in that the flock retains its character through many geographic and altitudinal changes of species composition. To complement Bell's intensive study of a single lowland flock, I survey the flocks throughout their geographic and altitudinal range. The flocks are composed of 35 species belonging to 7 families and are led hierarchically: by the Rufous Babbler Pomatostomus kidorei if it is present, otherwise by the Rusty Pitohui Pitohui fermgineus if it is present, otherwise by one of four other Pitohui species. The followers include either of two drongo species, 15 bird-of-paradise species, seven cuckoo-shrike species, and five other species. Flock-joining species are all medium to large forest passerines coloured predominantly brown or black. In nine species the male is black, the female brown. Most member species are endemic to the New Guinean region, eat both fruit and insects, and can forage from the understorey to lower canopy. Different flock members may derive different benefits from joining the flocks, including access to food flushed by other species, opportunities for kleptoparasitism, protection from predators, overwhelming of temtorial defenders, and increased foraging efficiency. The possible benefits of convergence in colour include enhancing the confusion of predators, maintaining spacing of allospecific individuals, and promoting flock cohesion through economy of signal. I hypothesize that a flock forms when a noisy social species ('leader' species) provides a critical level of vocal and visual signals that attract other, ecologically and visually similar species ('follower' species).