A study of the diet of nestlings and the overlap of trophic and spatial niches of sympatric Scarlet Ibis (Eudocimus ruber) and Yellow-crowned Night Heron (Nyctanassa violacea) in a northern Brazilian mangrove swamp was made from 1993 to 1997. A collection of regurgitates from nestlings was taken and stratified samples were taken at foraging areas. Both species fed mainly on saltwater ocypodid crabs, the night heron being more stenophagous than the ibis. About 90% of the prey of the night heron were Mangrove Land Crabs (Ucides cordatus) in the range 11-100 g, while fiddler crabs (Ucaspp.) were 64% of the diet of ibises, being mostly in the range 1.1-10 g. The niche of the ibis was broader than that of the night heron, both in trophic and spatial dimensions. The distribution in foraging areas of the Yellow-crowned Night Heron showed a significant, positive spatial correlation with its main prey. The land crab formed a significantly greater proportion in the bird’s diet than its relative abundance in the field. Scarlet Ibis distribution showed no significant spatial correlation with its prey. The niche overlap of the two birds was only moderate, and they seemed to use adjacent regions of the ecological space. As Mangrove Land Crabs and fiddler crabs are similar, the main factor of prey segregation between the two bird species seemed to be prey size. In relation to other geographical areas, both Yellow-crowned Night Heron and Scarlet Ibis occupied more restricted trophic niches, which may be related to the high availability of prey in northern Brazilian mangroves. Prey availability is more stable in saltwater ecosystems than in freshwater ones, so the high levels of niche overlap should not be attributed to seasonally relaxed competition. Competition does not seem to play a significant role in defining guild structure. It is not clear if the Scarlet Ibis in Brazil suffers physiological constraints from dietary salt, as has been reported from North America.