AbstractThe behavioural effects of testosterone propionate (TP), 5α‐dihydrotestosterone (DHT) and oestradiol benzoate (OB) were investigated in day‐old chicks during imprinting sessions to a duck model. TP increased the duration of peeping while inhibiting the following reaction and the twitters. DHT had more or less the same effects while OB induces the reverse behavioural changes.The behavioural effects of hormone injections agree with behavioural sex differences observed in non‐injected animals: ♂♂ peep more than ♀♀ which on the other hand produce more twitters. This could be related to sex differences in the hormonal status of the birds at hatching, as it is known that during incubation male chick embryos have higher plasma testosterone levels than ♀♀ of