The relationships between age, fear responses, and walking ability of broiler chickens

Several validated fear tests rely on animals' ability to approach or avoid a fear-provoking stimulus. However, as broiler chickens often suffer from walking impairments, inaccurate conclusions concerning their fear levels may be drawn. In the present study, the relationships between age, fear responses, and walking ability was studied in 12 fast-growing broiler chicken flocks on two conventional farms. Welfare assessments were performed when the birds were 21, 27, and 33 days old. Fear levels were assessed through two fear tests: The Stationary Person Test (SPT) and the Novel Object Test (NOT). Walking ability was assessed using the 6-point Bristol scale for gait scores. It was found that walking ability decreased with age (P < 0.001). Overall, the interaction between age and walking ability affected the outcome of both the SPT (SPT: P = 0.004) and the NOT (NOT: P = 0.016). The outcome of the SPT at the different ages indicated that fear levels decreased with increasing gait scores at 21 and 33 days of age, whereas the fear levels increased with increasing gait scores at 27 days of age. In comparison, the outcome of the NOT suggested that the fear response increased with increasing gait scores for ages 21 and 27. However, at age 33 days, increasing gait scores caused only a slight increase in the fear response. In conclusion, these findings suggest that the Stationary Person Test and the Novel Object Test may be influenced by poor walking ability. The results from this study suggest that walking impairment should be taken into consideration when performing the fear tests in question. The study further highlights the need to investigate other conditions, such as space limitations, that may affect the applicability of fear tests.