Permanent Canopy and Understory Flocks in Amazonia: Species Composition and Population Density

Two kinds of permanent, multi-species flocks inhabit the lowland forest of Amazonian Peru, canopy and understory flocks. Each flock type has a core composed of 5 to 10 different species of largely insectivorous birds, and each species is represented by a single bird, a mated pair, or a family group. Besides these core species, the flocks are joined by individuals of an additional 80 species with varying degrees of regularity. Since 1976, color-marking and long-term following of hundreds of individuals of flock species has demonstrated that the individuals of the core species generally spend their entire lives in one flock. Species that have higher population densities than the core species enter and leave flocks as flocks pass through their territories, whereas species with lower population densities than the core species alternate between several flocks on a daily or hourly basis. Based on 1 500 h and 250 h of following individually-recognizable understory flocks and canopy flocks, respectively, I present territory maps for the 25 understory and seven canopy flocks occurring on a 1.8 km2 study plot. I also present population density estimates for 78 flock species and detail their use of the 32 different study flocks. The population density estimates for some understory flock species are several times lower than Karr's (1971) estimates for the same species in Panama. Density compensation is hypothesized to explain this difference.