In most birds living at subtropical and temperate latitudes, seasonal changes in photoperiod play a key role in determining the timing and duration of the breeding season. Most photoperiodic birds breed on long days but some, such as the emu, are short day breeders. A unifying hypothesis has been developed to account for long or short day breeding in birds (Blache et al. 2001). Central to this hypothesis is that long or short day avian breeding is the product of an interaction between two independently controlled, asymmetrical cycles of photoperiodically dependent prolactin and luteinizing hormone (LH) secretion. Co-incidence between increased prolactin and LH secretion results in a suppression of LH secretion. Prolactin acts at the level of the anterior pituitary gland to suppress LH β gene expression, and at the hypothalamic level to depress gonadotrophin releasing hormone (GnRH) gene expression. Studies on mammalian GnRH cell lines suggest that prolactin may act directly on GnRH neurones through the prolactin receptor. Support for this possibility in birds comes from neuroanatomical studies showing the presence of prolactin receptors in the vicinity of GnRH neurones. Photoperiodically dependent prolactin release may also control reproductive function at the level of the gonads, which are also a site of prolactin receptor gene expression. Prolactin may have several functions at the gonadal level, including the inhibition of LH-dependent steroidogenesis.