Endocrinological and behavioral changes associated with the onset of incubation in the duck

Plasma levels of prolactin were measured during the onset of incubation in ducks. A rapid rise in prolactin levels from a mean of 5.7 to 33.1 ng/ml occurred during the formation of the final 20% of the clutch; during which the females increased their nest-box occupancy from 9.9 to 22.2 hours per day. A pronounced decrease in plasma progesterone levels, but not estradiol, occurred in females in the days immediately preceding the onset of incubation and termination of egg laying. A similar decrease in plasma LH occurred. Plasma corticosterone levels also decreased during the transition from egg laying to incubation. In male ducks, who play no role in incubation, LH remained constant until well into incubation when it decreased. Changes in plasma levels of prolactin are discussed in relation to tactile stimulation from the clutch and nutritional stress.