PLASMA-LEVELS OF SEX STEROIDS IN THE NORTH ISLAND BROWN KIWI (APTERYX-AUSTRALIS-MANTELLI) IN RELATION TO TIME OF YEAR AND STAGES OF BREEDING

North Island brown kiwi (Apteryx australis mantelli) have a seasonal pattern of egg laying with a peak in mid to late winter. This study is the first description of changes in plasma steroid concentrations during the annual reproductive cycle of this species. Blood samples were collected at intervals over a 2 1 2-year period from a population of kiwi at Tangiteroria, Northland, New Zealand. Male kiwi had annual cycles of plasma levels of testosterone and estradiol. Mean testosterone levels were low (<0.18 ng/ml) during February to April, rose in May to a broad peak lasting 4 months (maximum levels 1.90 ± 0.76 ng/ml), declined in September, and reached low levels in November to January. With respect to the breeding cycle, testosterone levels were low in the nonbreeding period, rose to high levels over 2 to 4 months before egg laying, then declined steadily toward the start of incubation. Plasma testosterone levels were very low in brooding males. Mean estradiol levels in males showed a pattern similar to testosterone levels, except that the rise to peak levels (1750 ±680 pg/ml) started in April. In female kiwi mean testosterone levels were low (<0.10 ng/ml) throughout the year. There was an annual cycle of mean estradiol levels in females which in timing and amplitude was similar to the male cycle. Estradiol levels in relation to stages of the breeding cycle in females were basal in the nonbreeding period, increased over a 3-month period before egg laying, declined in the 2 weeks before laying, and were low thereafter. Plasma progesterone concentrations did not change significantly during the year in either male or female kiwi. The North Island brown kiwi thus has annual cycles of gonadal steroids that correspond to the pattern of winter breeding. © 1992.