The first study to imply a role of SWS for memory consolidation in birds came from Gabriel Horn's lab, close to the end of his life. Horn dedicated part of his career to the search for the engram, i.e., the memory trace induced by learning. He used filial imprinting as a most robust memory formed in birds during a critical period at an early stage of life, approximately during the first 3 to 4 days in the life of chicks. Filial imprinting refers to a strong social recognition and bonding response learnt after short exposure to an object, which enables the chick to selectively follow this object. Naturally the object of attachment is a member of its own species, mostly the mother. However, in experimental conditions, this bonding can be done to humans or even objects like a moving red box, as Horn used in his experiments (Fig. 4A, Horn, 2004)