A breeding colony of wood ducks (Aix sponsa) was built up over a 9-year period by erecting nesting boxes along a small slough in the Sacramento Valley. The number of breeding pairs increased faster than boxes could be added, which led to nesting interference, compound nesting, nest desertion, and generally inefficient reproduction. Effective production of young per pair was inversely proportional to density, and in the latter years the population became essentially self-limiting. Lack of territorial defense of the nest site by an established pair was primarily responsible for the inefficiency of nesting in a high density population.