Plasma concentrations of luteinizing hormone (LH), prolactin and testosterone, and pituitary content of LH and prolactin, were measured in free-living starlings, Sturnus vulgaris, from hatching until 12 weeks of age.
Plasma LH concentrations were elevated in both sexes until four days after hatching, then they decreased. Throughout the period, plasma LH levels were low compared to those in breeding adults but were comparable to levels in post-breeding photorefractory adults. Pituitary LH content increased until 12 days after hatching, but this increase was due to physical growth during this period. Plasma prolactin concentration and pituitary prolactin content increased dramatically during the nestling period. The increase in pituitary prolactin content was in excess of that accounted for by increasing size. Plasma prolactin remained high during the immediate post-fledging period, but had started to decrease by 12 weeks after hatching. Plasma testosterone concentrations were lower than those in breeding adults, but generally higher than in post-breeding photorefractory adults. The gonads of both sexes remained regressed.
These results suggest that the reproductive system of nestling and juvenile starlings is in a similar state to that of post-breeding photorefractory adult starlings. The comparatively high levels of testosterone may reflect involvement in sexual differentiation.