We studied stomachs contents of 50 specimens of four common cuckoo species (Cuculidae) from southern Brazil: Guira guira (Gmelin, 1788) (n = 21), Coccyzus melacoryphus (Vieillot, 1817) (n = 8), Crotophaga ani (Linnaeus, 1758) (n = 11), and Piaya cayana (Linnaeus, 1766) (n = 10). We measured stomach volume and prey were identified and quantified based in their dimensions. The size of the prey was associated with bill width and body mass. Diet items were analyzed and identified to the lowest possible taxonomic level, based on literature, scientific collection data, and specialists. The diet of the four species was composed by 100% animal species, with invertebrate dominance. Only in G. guira stomachs the presence of vertebrates was observed more than once (amphibian, reptile, and bird). C. ani, which had similar diet, ingested a large number of Arachnida and Orthoptera. Piaya cayana ate mostly Hemiptera. The predominance of caterpillars (Lepidoptera), suggested in the literature was not observed in the diet of C. melacoryphus. There was a statistically significant difference in bill width and body mass among the bird species, and there was no correlation between bill width and the proportion of medium and large-sized prey.