A comparison of the responses to gonadotrophin-releasing hormone of adult and juvenile, and photosensitive and photorefractory European starlings, Sturnus vulgaris.

The reproductive system of juvenile European starlings appears to be similar to that of photorefractory adults, yet the increase in plasma luteinizing hormone (LH) in juveniles in response to gonadotrophin-releasing hormone (GnRH) is much less than that of photorefractory adults. To investigate this apparent anomaly, the effects of age, reproductive state, and sex on the increase in plasma LH concentration in response to im injections of GnRH were investigated. The results show that photorefractory juveniles needed 10 times the dose of GnRH to elicit the same increase in LH as photorefractory adults, and the response of photosensitive juveniles to 10 micrograms GnRH was at least 15 times as great as that of photorefractory juveniles (e.

Circannual rhythms in starlings: response of castrated birds to 12-hr days depends on photoperiodic history.

The annual cycle of reproductive function in starlings (Sturnus vulgaris) is driven by seasonal changes in daylength and consists of a 6-month period of photosensitivity, culminating in maximal gonadal development, followed by a 6-month period of photorefractoriness. If male starlings are held under a constant photoperiod of 12 hr of light per day (LD 12:12), some individuals show circannual rhythms of testicular size. Do these rhythms consist of alternating periods of photosensitivity and photorefractoriness, and does the response depend on whether daylength is increased or decreased to 12 hr? Castrated starlings were transferred from natural short days in January (LD 9.

Puberty in birds: the reproductive system of starlings does not respond to short days until birds are fully grown.

If adult starlings are kept on long days, they rapidly become refractory to long days (photorefractory). The recovery of the ability to respond to long days (photosensitivity) occurs after transfer to short days. Photosensitivity is associated with an increase in hypothalamic content of gonadotrophin-releasing hormone (GnRH), and, in castrated birds, with an increase in plasma LH.