Resource seasonality and the structure of mixed species bird flocks in a coastal Atlantic forest of southeastern Brazil

Multi-species bird Becks in tropical forests are maintained throughout the annual cycle despite seasonal differences in resource availability, and the reproductive schedules and ecological requirements of individual species. This study examines the relationship between seasonal variation in flock st ructure and the availability of fruits and arthropods over a 12-mo period a t a coastal Atlantic forest within the Jureia-Itatins Ecological Station, S ao Paulo, Brazil. Fruit abundance was estimated by a phenological survey of both canopy and understorey trees, whereas arthropod abundance was quantif ied monthly on the basis of a nocturnal visual census technique. The season al variation in flock structure and composition was affected by both the br eeding seasons of different core and attendant species, and the availabilit y of food resources. The number of bird species attending flocks was greate r during the dry season, declining thereafter during the breeding season. U nderstorey fruit availability exhibited a marked seasonal fluctuation with the lowest levels between the late dry and early wet season. Seasonal varia tion in canopy fruit availability, on the other hand, was far less demarcat ed than that of understorey plants. Arthropod abundance was greatest during the wettest months of the year, which apparently determined the timing of the main breeding season. Bird species richness attending flocks was, there fore, significantly correlated with the availability of understorey arthrop ods, but not with that of either understorey and canopy fruits. Arthropod a bundance thus appears to affect profoundly the reproductive schedules of th e understorey avifauna, which in turn influences the seasonal variation of flock size and composition.