Imprinting is a special kind of learning that occurs only during a restricted time in ontogeny called the critical period. There are two types of imprinting: filial and sexual imprinting. Konrad Lorenz studied filial imprinting on greylag goose. He concluded that birds can imprint on their parents, on members of another species, on inanimate objects or humans. Eckhard Hess concluded in his research on mallard ducks that those ducklings that imprinted 13-16 hours after hatching made a maximum score of imprinting. He also discovered that the strength of the imprinting is dependent on the effort exerted by duckling in following the imprinting object. The cross-fostering study of sexual imprinting, in wich nestling birds were raised by parents of a different species, shows that the impact of sexual imprinting is not constant across species. The cross-fostered great tits were strongly mis-imprinted on the blue tit host. That can explain the low pairing success in great tits. Sexual imprinting didn't have a major impact on pairing success of cross-fostered blue tits. Ficedula hypoleuca didn't seem to be sexually imprinted on their hosts at all. In solitarily living species like Ficedula hypoleuca, identification of a suitable mate depend less on a sexual imprinting and more in a fact that they have less social contact with other species. Further studies on the matther of imprinting are important for breeding birds in captivity in order to ensure a proper sexual imprinting for them and thereby enable a normal life when they return to the wild.