Birds of a Peruvian Oxbow Lake: Populations, Resources, Predation, and Social Behavior

The bird community of a small (22-ha) oxbow lake, Cocha Cashu, of the Manu River in the Amazon basin of southeastern Peru was studied during 11 field seasons, 1979-1989. Here, field observations on the population status, interactions with predators, and social systems are summarized for many of the 1 86 species that regularly occurred there. Oxbow lakes such as Cocha Cashu are characterized by narrow but very productive strips of marsh, shrubs, isolated trees, and vines along their borders. These habitats attract high populations of resident and nonterritorial birds, but also attract predators. In addition to species confined to the lake margins, Cocha Cashu attracted many forest birds to abundant flowering and fruiting trees (especially figs and Lauraceae) and isolated nest sites that provided some protection from mammalian predators. Birds respond to the constraints of limited habitat, high population density, and intense predation through various kinds of group living, including coloniality, cooperative breeding, and monoand multi-species flocking. The various kinds of sociality further influence the kinds of mating systems observed on the lake, with several species showing polygynous mating behavior. Anti-predator adaptations included mobbing of some but not all nest predators, and vigilance coupled with alarm calling against raptors that attack adult birds. Cacicus cela used different escape tactics when faced with different kinds of predators. Mobbing and group vigilance were effective at deterring most, but not all, avian predators. Interspecific aggression appeared to be most intense around cavity nests and some fruiting trees. Possible cooperative breeding was documented for two species in which this behavior has not previously been described (Ramphocelus carbo and R. nigrogularis). Relative to forest habitats, lake-margin birds showed a stronger tendency to form monospecific than multi-species flocks, perhaps because variable resource availability and high population densities of some species precluded the formation of stable, multi-species flocks. Oxbow lakes strongly affect local patterns of species richness and abundance, but appear to have few specialists that do not occur in other aquatic, wetland, or secondgrowth habitats associated with riverine systems. The combination of rich foraging and nesting resources concentrated in a small area and high predation pressure influence life histories, population dynamics, and community structure of birds of this oxbow lake. Resumen. Durante 11 estaciones de campo (1979-1989) se estudi6 a la comunidad de aves de un pequeiio recodo de no (22 ha) en Cocha Cashu en el no Manu, Peru. En este estudio se resumen las observaciones sobre el estado de la poblaci6n, interacciones con depredadores, y sistemas sociales para las 168 especies que regularmente ocurren en la zona de estudio. Los recodos tales como Cocha Cashu se caracterizan por contener lineas muy angostas, pero muy productivas de pantanos, arbustos, arboles aislados y vainas a lo largo de sus bordes; estos tipos de habitats atraen grandes poblaciones de aves residentes y no territoriales, sin embargo tambien atraen depredadores. Ademas de las especies confinadas a los margenes del lago, Cocha Cashu atrajo muchas aves de bosque hacia los arboles florales y frutales que son muy abundantes (especialmente higos y Laureaceae), y hacia sitios aislados de anidacion que proveyeron de protecci6n contra los mamiferos depredadores. La respuesta de las aves a las restricciones de la poca disponibilidad del habitat, altas densidades de poblaci6n, y altos niveles de depredaci6n consiste en distintos tipos de vida en grupo los cuales incluyen colonialidad, anidacion cooperativa, y la formaci6n de parvadas mono y multiespecificas. Los distintos tipos de vida social influyen a su vez las distintas variantes en los sistemas reproductivos que se observan en el lego, existiendo asi distintas especies que muestran un comportamiento reproductivo de tipo poligenico. Las adaptaciones contra los depredadores incluyeron el asalto de algunos, pero no todos los depredadores de nidos, y la vigilancia asociada con llamados de alarma contra las