Temporal variation in food availability may generate changes in the feeding strategies and trophic preferences of animals. We studied the temporal variation in feeding behavior, diet composition, and prey availability of the Black-faced Ibis (Theristicus melanopis) in pastures of southern Chile between May 1999 and January 2000. Soil core samples showed that hypogeous invertebrates such as, earthworms (Lumbricus spp.), black cutworm larvae (Agrotis spp.), black pasture caterpillars (Dalaca spp.), and southern green chafers (Hylamorpha elegans) were the most abundant prey in the pastures and that their populations experienced pronounced seasonal variations. Insect larvae were the main prey of the Black-faced Ibis and were consumed in greater proportion than expected by chance. The diet composition (based on feces) and trophic preferences coincided with variations in prey availability. During periods of reduced prey abundance Black-faced Ibises foraged on more different types of prey and their consumption rate diminished. At the same time, the abundance of Black-faced Ibises decreased in the study area. These variations were most evident in late spring and summer, when prey abundance in the pastures was lower than in the other seasons. Our results suggest that the life cycle of the prey is the main causal factor that influences the feeding ecology of the Black-faced Ibis in pastures of southern Chile.