Seasonality, Resource Availability, and Community Diversity in Tropical Bird Communities

Seasonal changes in the species composition and abundance of birds in seven habitats in central Panama are investigated to determine the extent to which equitable seasonal patterns in tropical regions are important in the evolution of species-rich tropical avifaunas. A brief review of the literature documents the distinct seasonality of rainfall and seasonal variation in the abundance of food resources (especially insects and fruits) exploited by the avifauna. Seasonal variation in avian community structure decreases with increasing vegetation complexity. This is apparently due to the increased buffering of the physical environment by the more complex vegetation. However, it is not sufficient to use a single index to examine whole avian communities. The impact. of seasonality varies among subsets of the avifauna. Insectivore species diversity and abundances generally vary more seasonally than do frugivore diversity and abundances in structurally mature habitats. Within shrub and forest habitats the degree of seasonal variation in avian community composition changes with stratum. The buffered segments of the vegetation profile (e.g., low stratum) change less than the less buffered high stratum. In addition, the more buffered strata generally maintain a more diverse fauna. Seasonal changes in avifaunal structure are examined for several temperate habitats and contrasted with those of tropical areas. Patterns of variation in insectivore diversity for shrub and forest habitats in tropical and temperate areas are consistent with the hypothesis that species diversity increases with seasonal stability of microhabitats selected for foraging and is correlated with the stability of resources in those microhabitats. I propose the following working hypothesis: species diversities of subsets of avifaunas will be increasingly variable in each of the following sequences: (1) microhabitats--bark stratum; ground stratum; low, medium, and high foliage strata; and open spaces within and above the vegetation; (2) food resources--omnivores, frugivores, insectivores; (3) vegetation types--sequences of decreasing vegetation complexity. The frequently suggested correlation between community diversity and resource seasonality is supported and correlated with seasonal irregularities in physical environment (especially rainfall in the tropics and temperature in temperate areas).