In birds, the preen gland produces a waxy secretion that protects the plumage from environmental conditions and ectoparasites and is, therefore, important for feather maintenance, individual quality, and health. Several studies have demonstrated that the size of the preen gland and the composition of its secretion change throughout the course of reproduction, thus indicating an increased requirement for preen gland secretion during the breeding season. However, the exact timing of the gland's size increase is unknown. To get information about this temporal pattern, we repeatedly measured the preen glands in 22 breeding pairs of adult domesticated Zebra Finches (Taeniopygia guttata) during the course of reproduction (prior to the breeding season, on the day the first egg was laid, on the day the first chick hatched, on the day the first chick fledged, and after the chicks became independent). Zebra Finches have biparental care, in which both sexes are involved in incubation and food provisioning. As expected, our data reveal that the preen gland increases in size during the breeding season and decreases afterward. Moreover, we found sex-specific differences in the temporal pattern of gland enlargement. Females reached a maximum gland size when the first chick hatched, whereas male gland size peaked earlier, when the first egg was laid.