The evolution of the number of individuals of the animal populations works like in a countable model. This means that it works according to some exits and entries, that is to say, the mortality and the incorporation of new individuals. Likewise, the latter fact depends on the productivity.

If we have a closed population of eagles, that is to say, that does not receive outer individuals and those of the population do not leave, the evolution of the number of adult pairs that occupy the territory will depend on the fact that young eagles that are born (productivity) and come to the adult age, are sufficient to compensate the death of the adults (adult mortality). In case of Bonelli´s Eagle the young eagles take approximately three years to be able to incorporate the adult population and during this period they suffer a high mortality (pre-adult mortality).