The number of birds breeding in a given area (breeding density) is affected by several abiotic and biotic factors. Availability of suitable nesting sites plays a major role in determining the size of the local breeding population of birds, particularly in those species, like the common kestrel, that do not build their own nests. Kestrels do actually use old nests of corvids or holes in buildings to breed. By provisioning kestrels with artificial nest boxes, it is possible to increase the number of breeding individuals and, possibly, the population size. However, a number of factors need careful consideration to evaluate a priori the characteristics of nest boxes and locations to install them and to assess a posteriori the effects of the nest box provisioning on the reproductive ecology and population dynamics of kestrels.