AbstractIn opportunistic breeding species living in unpredictable environments, gonadal development and activation should be quite different from that in periodically breeding species, which live in an environment with highly predictable changes. In Australian Zebra Finch females, bred in laboratories, the development during ontogeny was investigated. Body growth, ovarian weight, and the diameter of the largest follicle were recorded in birds 10 days to some years of age. A linear follicle growth pattern from 10 to 100 days is followed by a resting phase, during which the follicles remain in a medium‐developed resting state. As soon as favorable conditions occur, a rapid logarithmic growth and yolk deposition starts, leading to ovulation within one to two, weeks. Egg laying sometimes occurs as early as day 90. The ovary as a whole reaches a medium‐developed resting state much later, but remains in this state even during long nonbreeding intervals.Compared with photoperiodically breeding species, for example Zonotrichia leucophrys gambelii, the Zebra Finch has the disadvantage of permanently keeping its gonads activated. Because of the lack of proximate factors, the Zebra Finch has to respond to the appearance of the ultimate factors as quickly as possible. Activation of fully regressed ovaries would take too much time, but starting from a medium‐developed resting state allows ovulation in less than two weeks. In addition, no detectable refractory period reduces the capacity to respond. In the young this causes precocity.These peculiarities have been selected for by the sporadic and erratic rainfalls, which cause the unpredictability of breeding periods in several parts of Australia.