In terms of reproduction, fecundity means the biological ability
to have a particular quantity offspring. Fertility means the actual
use of fecundity to produce the offspring. Both terms can be used
either of an individual or a population.
In everyday speech, most people use the term "fertility" to
cover both meanings. But when needing to distinguish the ability to
have offspring from the actual use of the ability, "fertility"
should be used for reproductive ability, and "fecundity" for actual
number or rate of offspring.