The literature indicates that predation of seabirds by ciconiiform birds is limited to terns and is generally of little impact. We monitored predation of the eggs and chicks of Cape Cormorants (Phalacrocorax capensis) by Sacred Ibises (Threskiornis aethiopicus), and Gray Herons (Ardea cinerea) for 46 days over three breeding seasons at Penguin Island, Lambert’s Bay, South Africa. The ibises were observed to take at least 152 cormorant eggs and chicks and 37 heron chicks. Extrapolation of the results to the full season of availability indicates that these ciconiiform predators may take as many as 960 cormorant eggs or chicks in a season. If so, they would be the third most serious cause of mortality among the 4,800 pairs of Cape Cormorants at Penguin Island. The observed combined effect of ciconiiform predation was greater than that of the local, predatory, Khoisan Gulls (Larus (dominicanus) vetula). We document the seasonality and diel timing of predation, behavior of the predators, and the cormorants’ responses. To place our observations in context we review the global literature concerning the incidence of ciconiiform predation upon seabirds, which we consider to be under-reported, and factors affecting it. Finally we consider the effects of ciconiiform predation on seabirds at Penguin Island relative to a broader, five-year, study of the causes of seabird mortality there, and make recommendations for conservation management.