It has been widely suggested that inbreeding causes a loss of constitutional vigour and fertility in domestic livestock.Long-term field studies of pedigrees in avian populations have also revealed that inbreeding typically decreases the fitness of each individual, but may increase fitness under certain circumstances. The degree of inbreeding in a population depends on its geographical isolation and/or its demographic situation.Under the restricting condition of dispersal of individuals, inbreeding may be more adaptive than out-breeding. The recent introduction of DNA analysis to the field of avian ecology has facilitated research into degrees of inbreeding. It is now possible to strictly assess the relationship between inbreeding and fitness. A decrease in heterozygosity has actually corresponded with the deleterious effects of inbreeding in wild avian populations. For such studies it is essential that we obtain demographic parameters, genetic data, and the parameters of life history traits.To that end it is necessary to introduce populations, or to locate populations of which the year of foundation is known.