The effects of restricting chickens and ducks to cages on their haematology (as compared to that of free-roaming animals) were investigated in this study. Significant differences were observed in several haematological parameters and the study showed that conclusions that could be drawn concerning inter- and intraspecies differences depended on how the birds were housed and how susceptible they were to stress. It is suggested that experimental design concerning bird haematology should take these factors into account.