The black stork Ciconia nigra is a threatened tree-nesting species in Europe. The relatively poorly studied and isolated Iberian population is unusual in Europe in that the majority of known pairs are cliff-nesting. This permits the analysis of productivity differences in relation to nesting substrate in different subpopulations, comparing not only the mean number of fledged chicks/nest between cliff-nesting and tree-nesting pairs but also considering whether the nests are located in restricted- or open-access areas. The results reveal that nesting substrate does not determine differences in productivity. Only those cliff-nesting pairs that breed in open-access areas have significantly lower productivity than pairs that breed elsewhere. Therefore, whether or not black storks nest on cliffs and on trees should not have a relevant effect per se on the productivity of the species but such an effect may arise in relation to the degree of human access to the cliff-nesting sites. Further in-depth research is needed about the exact causes of lower productivity in pairs that nest on cliffs located in open-access areas.