Great Egret (Ardea alba) and Cocoi Heron (A. cocoi) have a wide distribution in the neotropics but few data on reproductive biology for these species are available. The reproductive parameters, growth rate and diet of chicks of egret and heron in a mixed-species colony nesting were studied during the 2007 breeding season in a pampas wetland of Argentina, with 50 and 17 nests studied, respectively. Mean egret clutch size was 3.6 ± 0.5 eggs and hatching success 76%. Mean heron clutch size was 3.5 ± 0.8 eggs and hatching success 81%. The mean number of fledged egrets/egg was 0.15 and productivity 0.54 chicks per nest; for herons reproductive success and productivity were 0.17 and 0.58, respectively. In both species, the most productive clutch size was three. The first two weeks post-hatching were critical for chick survival. In egrets, the last chick to hatch had lower growth rates than the first or second hatched chicks; however in herons the first chick to hatch had higher growth rates than the second or third hatched chicks. Fish and insects were the main prey of egret chicks, while mammals and fish were the most common prey of heron chicks. Starvation was the main factor causing the relatively low breeding performance observed in both species.