We examined changes in diet composition during the breeding period for the endangered Lesser Kestrel (Falco naumanni). Pellets were collected weekly from the last week of March until the first week of July in a colony located in southwestern Spain. Diet composition was evaluated in terms of frequency of occurrence of different prey, mean prey weight, and prey richness of each pellet. Generalized additive models were used to analyze the predictive ability of calendar week (as an index to prey availability), reproductive week (as an index to breeding demands), and nest identity (as a proxy for individual preferences/abilities) on the above dietary metrics. Primary prey species were the mole cricket (Gryllotalpa gryllotalpa) during courtship, the saddle-backed bush cricket (Ephippiger ephippiger) during incubation, and the migratory locust (Locusta migratoria) and the white-faced bush-cricket (Decticus albifrons) during the nestling phase. Small mammals were only important during some particular weeks at the beginning of the nestling period. Mean prey weight increased as the breeding season advanced, while species richness tended to decline. This was mainly due to the decreasing contribution of small prey items such as beetles to the diet and the greater incidence of large prey species. Calendar week and reproductive week had similar abilities to predict diet composition, and were better predictors than nest identity. Models fitted to particular prey species had greater explanatory power than models fitted to prey groups. Likewise, models fitted to those prey species that we considered “preferred” fitted better than those considered “refuge” prey.