Nest-site characteristics of two hornbill species, the Red-knobbed Hornbill Aceros cassidix and the Sulawesi Dwarf Hornbill Penelopides exarhatus, are presented and compared at two sites in north Sulawesi, Indonesia. Seventy-four Red-knobbed Hornbill and 12 Sulawesi Dwarf Hornbill nests were located. Sulawesi Dwarf Hornbill nests were at lower densities and more evenly spaced than Red-knobbed Hornbill nests. Although selective, neither species appeared to be nest-site limited, except perhaps for the Red-knobbed Hornbill in hill forest. Principal components analysis indicated that Red-knobbed Hornbills were ‘catholic’ in their preferences, although nests were generally in tall, mature specimens of large girth, located high and in the upper half of the tree. The Sulawesi Dwarf Hornbill nested exclusively in live trees and nests were on average located half way up in tall canopy trees with large girths. Within certain parameters, both species were flexible with respect to their nest cavity size and shape requirements; Red-knobbed Hornbills choose sites with larger entrances and internal dimensions. Discriminant function analysis produced parsimonious models for each site, with high rates of correct classification, based on structural, floristic and breeding success parameters. Lack of transference of predictive powers of the models between sites demonstrates the apparent variability in hornbill nesting behaviour and indicates the need for caution when extrapolating models to other sites. Possible factors affecting the nesting density and distribution of each hornbill species are discussed.