Sociobiology and Adaptive Significance of Interspecific Foraging Flocks in the Neotropics

The sociobiology of mixed species flocks is reviewed and used as a basis for evaluating the adaptive significance of flocking. Virtually all data currently available deal with mixed flocks of insectivores, so this review deals primarily with those flocks with only minimal reference to flocks of frugivores and granivores. Most insectivorous species that participate in mixed flocks remain paired and defend exclusive feeding territories year round. Territoriality restricts intraspecific group sizes in flocks to single pairs or small family groups. Since territorial individuals are forced to leave a flock whenever it moves beyond their respective territory boundaries, there is a high turnover rate in flock composition. Some species reduce or eliminate turnover by aligning their respective territories so that they overlap completely. This alignment enables individuals of these species to remain permanently associated in the same mixed flock (except during some phases of the breeding cycle). Territory sizes of mixed flocking species are small (< 1-10 ha), and territory holders forage throughout their entire territories every few days. Foraging ranges of mixed flock participants remain relatively constant throughout the year with slight expansions due to increased trespassing during some periods of the year. The foraging niches of mixed flocking insectivores include a wide array of behaviors. Analyses of foraging niches reveal little overlap among most species, although sympatric congeners of at least one genus of Formicariidae (Myrmotherula) appear to overlap more than expected. Hypotheses concerning the adaptive significance of flocking are categorized as those predicting that flocking enhances foraging or reduces predation pressure. Analyses of spatial use and foraging niche characteristics of flocking species support some of both hypothesis categories. In general, however, the small intraspecific group sizes and home ranges resulting from permanent territoriality, and the high diversity of foraging niches tend to minimize the applicability of forage enhancement hypotheses. Limited cases of imitative foraging and foraging facilitation from the flushing of insects (beater effect) are reported. Predation reduction, through improvement of surveillance, appears to be an important function of mixed species flocking. Support for this conclusion is still largely indirect, but increasing field evidence is accumulating. RESUMEN. Se revee la sociobiologia de bandadas mixtas (compuestas por varias especies) y se usa como una base para evaluar el significado adaptativo de andar en bandadas. Casi toda la information disponible se refiere a bandadas mixtas de insectivoros, por lo que esta revisi6n se refiere en forma minima a bandadas de frugivoros o granivoros. La mayoria de las especies insectivoras que participan de bandadas mixtas se mantienen en pareja y defienden territorios de alimentaci6n exclusivos durante todo el ano. Esta territorialidad restringe el tamano de los grupos intraespecificos en una bandada a unicamente pares o pequenos grupos familiares. Debido a que individuos territorialistas estan obligados a salir de la bandada cuando esta se translada fuera de los limites de sus territorios, hay un alto nivel de cambio en la composici6n de la bandada. Algunas especies disminuyen o eliminan el intercambio de individuos, alineando sus respective^ territorios, de manera que ellos se superpongan completamente. Esta alineaci6n le permite a los individuos de esa especie quedar asociados permanentemente en la misma bandada mixta (excepto durante algunas fases del ciclo reproductive). Las superficies de los territorios de varias especies participantes de bandadas mixtas son pequenas (< 1-10 ha) y los ciudadores de los territorios forrajean a lo largo de todos sus territorios cada pocos dias. Los rangos de forraje de miembros de bandadas mixtas son relativamente constantes a lo largo del ano con pequenas expansiones debido al aumento de las intrusiones durante ciertos periodos del ano. Los nichos de forrajeo de bandadas mixtas de insectivoros incluyen una amplia