Tropical Avian Phenology in Relation to Abundance and Exploitation of Food Resources

Abstract

We studied avian breeding and molting activity in relation to rainfall, temporal fluctuations in food resource abundance, and food exploitation by birds, in four arid and semiarid tropical habitats in Venezuela. Twice a month we used mist to monitor changes in breeding and molting conditions of captured birds and forced them to regurgitate to determine their diet and feeding guild membership. Food abundance was assessed by measuring the flowering and fruiting seasonality of marked plants and by evaluating arthropod abundance with four different trapping methods. Flowering activity was limited largely to the wet season. Fleshy fruits, although produced year—round, were also more abundant in the rainy period. Anthropod abundance followed the same general pattern with numbers highest in the wet season and lowest in the dry season. Birds of all feeding guilds predominantly bred and molted during the wet season, synchronously with the highest abundance of most food resources. However, the diet analysis revealed a higher occurrence of arthropods coupled with a sharp decrease in the intake of vegetable matter during birds' breeding season. Consequently, we suggest that arthropod abundance is a crucial factor governing the timing of breeding activities, even in species that normally include a high proportion of nectar and fruits in their diet. We also postulate that, in tropical habitats receiving >1500 mm of rain per year, breeding in nectarivores and frugivores in the dry season may be related to the lower reduction in arthropod numbers over the less severe drought period.