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Annual variations in moult, body mass, and the plasma concentrations of luteinizing hormone, prolactin and the gonadal steroids, oestradiol and testosterone, were measured in captive White storks (Ciconia ciconia). Data were collected at approximately monthly intervals for 13 months on storks hatched over five successive years, the youngest ones being in their first, the oldest in their fifth, year of life.

Moulting of wing feathers occurred during the summer months, April through August. Young birds began to moult at least one month earlier, and replaced more wing feathers each year, than did three- to five-year-old birds. Body mass exhibited remarkably consistent annual cycles with minima of2–9-3-5 kg, during the summer months, and maxima of3–8-4-5 kg, during the winter months. Young males were heavier at all times of the year than young females, although this difference was gradually reduced to non-significant levels by four years of age. Luteinizing hormone, prolactin and the gonadal steroids, oestradiol and testosterone, exhibited annual cyclic changes, usually of a bi-modal nature. No major differences in hormone profiles, however, were found between any of the age groups. The phenomenon of deferred maturity in the stork does not parallel that of puberty found in many mammals.