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We studied the reproductive ecology of the Ringed Warbling-Finch Poospiza torquata and the Cinnamon Warbling-Finch P. ornata during four breeding seasons in the Reserve of (N) over tilde acu (n) over tilde an, western Argentina. We described the breeding biology of both species, examined temporal variation in reproductive parameters under contrasting weather conditions, and compared reproductive traits of these sympatric Poospiza species to evaluate if differences in food supply and nest predation between species are responsible for striking differences in reproductive ecology. Nests and nest sites were similar in both species. Timing of breeding varied among years in both species, especially in the Cinnamon Warbling-Finch which only bred in substantial numbers during the wet 1997-98 season. Laying dates also differed between species in 1997-98. Clutch size and egg volume did not differ among years for the Ringed Warbling-Finch. Within the breeding season, clutch size remained constant in both species, and Ringed Warbling-Finch egg volume was positively correlated with laying date. In 1997-98, clutch size was larger in the Cinnamon Warbling-Finch, but relative egg mass of the Ringed Warbling-Finch was greater than that of the Cinnamon Warbling-Finch. Nestling growth patterns were similar for both species. Both species suffered similar and very high rates of nest predation, and daily survival rate of nests did not differ among years or within the breeding season. Our results suggest that interspecific differences in reproductive traits, in particular the unreliable breeding of the Cinnamon Warbling-Finch at (N) over tilde acu (n) over tilde an, may be an evolutionary response to different energetic (food) limitations suffered by these congeneric bird species.