Increase of the corticosterone level in ducklings during the sensitive period of the following response.

Increased fear may be one of the factors limiting the sensitive imprinting period. Thus, we analyzed corticosterone levels in 121 Peking ducklings during and after this sensitive period using the method of competitive protein binding. The data pointed to a corticosterone increase within a period of 12-28 hr after hatching as well as to the presence of a circadian rythm. The low coefficient of determination of post-hatch age and corticosterone level (8%) pleads for a multi-dimensional system. However, the corticosterone increase may be the basis for an increase in fear and a decrease in the likelihood of imprinting.