Plasma gonadal steroid levels in wild starlings (Sturnus vulgaris) during the annual cycle and in relation to the stages of breeding.

Blood samples were collected from male and female free-living starlings in every month during the year and at all stages of the breeding cycle. The samples from females were assayed for oestradiol-17 beta, oestrone, and progesterone, and the samples from males were assayed for testosterone and 5 alpha-dihydrotestosterone. The testicular weights were recorded. All hormones other than oestrone showed a pronounced unimodal cycle. In females, oestradiol was highest in April, corresponding to the period of greatest nest-building activity. It decreased during the later stages of the breeding cycle and maintained low levels until the autumn when levels began to increase. Progesterone was highest in laying birds and reached a peak in May. During the rest of the year it remained at a lower but fairly constant level. Oestrone was normally only detectable in laying birds. In males, testicular weight and plasma concentrations of testosterone and 5 alpha-dihydrotestosterone were all highest during April. Plasma testosterone and 5 alpha-dihydrotestosterone were highest in nest-builders. They decreased during the later stages of breeding, remained low during the summer and increased slightly in the autumn. Testicular weight was high in nest-builders, but peaked during laying, by which time testosterone and 5 alpha-dihydrotestosterone had decreased significantly. Testicular weight decreased during incubation and feeding the young and remained low until the following year.