Tonic immobility (TI) test is used to assess the fearfulness of birds. However, its outcomes depend on many factors including the breed, species, and restraining position. This study aimed primarily to evaluate the effect of different restraining treatments on TI parameters in both chickens and ducks. Secondarily, it aimed to identify the associations between various variables measured in this study including body weight, handling time, in duction attempts, and TI response. Two separate experiments were conducted in broiler chickens and mule ducks. TI was performed on ground level in chickens and on a U-shaped cradle in ducks. Briefly, 30 broiler chickens (34 d of age) and 18 mule ducks (55 d of age) were randomly assigned to three restraining treatments: left lateral position (LLP), right lateral position (RLP), and back or dorsal position (DP). The variables measured in both experiments were body weight (BW), handling time, number of attempts needed to induce TI (NOA), and TI duration. In chicken, fewer NOA were needed to induce TI in RLP treatment compared to LLP treatment (P = 0.036), but there was no difference between RLP and DP treatment (P = 0.58) or between DP and LLP (P = 0.67). In ducks, there was a non-significant trend that fewer attempts were needed to induce TI in DP compared to other treatments (P = 0.08). Duration of TI was not affected by restraining treatment in chickens (P = 0.43) nor ducks (P = 0.61). A negative moderate correlation was observed between NOA and BW in chickens (R = - 0.45, P = 0.01), and a positive moderate correlation was observed between TI duration and BW in ducks (R = 0.46, P = 0.05). In conclusion, the RLP was the most effective method to induce TI in chickens on ground level. BW was associated positively with NOA in chickens and negatively with TI duration in ducks