Many organisms advance their seasonal reproduction in response to global warming. In birds, which regress their gonads to a nonfunctional state each winter, these shifts are ultimately constrained by the time required for gonadal development in spring. Gonadal development is photoperiodically controlled and shows limited phenotypic plasticity in relation to environmental factors, such as temperature. Heritable variation in the time required for full gonadal maturation to be completed, based on both onset and speed of development and resulting in seasonally different gonad sizes among individuals, is thus a crucial prerequisite for an adaptive advancement of seasonal reproduction in response to changing temperatures. We measured seasonal gonadal development in climate-controlled aviaries for 144 great tit (Parus major) pairs, which consisted of siblings obtained as whole broods from the wild. We show that the extent of ovarian follicle development (follicle size) in early spring is highly heritable (h2 = 0.73) in females, but found no heritability of the extent of testis development in males. However, heritability in females decreased as spring advanced, caused by an increase in environmental variance and a decrease in additive genetic variation. This low heritability of the variation in a physiological mechanism underlying reproductive timing at the time of selection may hamper genetic adaptation to climate change, a key insight as this great tit population is currently under directional selection for advanced egg-laying.