Posthatch age and developmental age as a baseline for determination of the sensitive period for imprinting.

Two experiments comparing developmental age and posthatch age were performed. In the first experiment 143 Peking ducklings of different posthatch ages were imprinted, and their following responses examined in a simultaneous choice test. A sensitive period was evident on the basis of posthatch age. In the second experiment 99 ducklings were separated in three clusters of different posthatch ages, which did not vary within the clusters relative to their probability to be imprinted. The ducklings of each cluster were imprinted at various developmental ages, and the results show the involvement of developmental age. Thus, both experiments considered together favor the concept of an interaction of developmental and posthatch age. Imprinting and the determination of the sensitive period are dependent on prehatch and posthatch learning, so both developmental age and posthatch age are involved in setting the limit of the sensitive period.