[The circannual rhythm of reproductive activity in the starling (Sturnus vulgaris) under the influence of homosexual and heterosexual mates of the same species].

1. 30 European starlings, kept for 20 months under a constant 12-hour photoperiod and at constant temperature, were investigated at regular monthly intervals with regard to the state of gonadal development, bill coloration and moult. 8 male male lived in individual cages (group 1), 10 male male were housed together in a group cage (group 2), and 6 male male lived together with 6 female female in another group cage (group 3). All birds had acoustical contact with each other. In addition, the birds of groups 2 and 3 could see one another. 2. In all male male the testes regressed following the beginning of the experiment in April. Subsequently they carried out a complete postnuptial moult. In the following spring, all male male went through a full cycle of testicular size which was followed by a second postnuptial moult. The bills of all birds turned yellow as the testes grew and blackened as the testes regressed (fig. 1).--Most of the female female showed similar circannual rhythms of gonadal growth and regression, bill coloration and moult (fig. 2). 3. In 6 male male of groups 1 and 2, a small increase of testicular size was observed subsequent to the termination of the first postnuptial moult. This transient testicular activity most probably corresponds to the autumnal sexual activity known from freeliving starlings. Hence, these results suggest that both spring and autumnal sexual activity of the starling are endogenously preprogrammed. 4. The testes of most male male of group 3 developed to full size immediately after the termination of the first postnuptial moult; subsequently they remained active for at least 7 months. This observation suggests that stimuli emitted by the female female augment the endogenously preprogrammed readiness for autumnal sexual activity and prevent the subsequent testicular regression. It is concluded that exogenous factors from the social environment are capable of modifying the endogenous annual pattern of testicular function in the starling.