In some studies but not in others the average clutch size decreases with density. We propose that density-dependent fecundity occurs because, as density increases, proportionally more poor-quality sites (with small clutches) are occupied, and not because the clutch size in all territories decreases. This mechanism will only cause density dependence if the habitat is heterogeneous at the scale of the home range or territory of an individual. Density-dependent fecundity will be found in some populations but not in others because of differences in the scale of habitat heterogeneity