Stressed Chicken Scale: On the way to detecting chickens' discomfort

For many species, scales are used to classify distress and pain (e.g., Facial Expression / Grimace Scales). Although a significant number of vertebrates used for scientific purposes are chickens, there is no equivalent scale for birds. We developed a "Stressed Chicken Scale" (SCS) to evaluate the possibility of stress assessment in chickens based on their body posture. For this purpose, seven possible body signals for stress assessment were extracted from poultry literature: Tail and head position, eye closure, beak opening, leg and wing position, and plumage fullness. Materials and Methods: Chickens (n = 80) from laying lines and dual-purpose breeds (backyard poultry) were filmed for this study. Screenshots of the chickens in lateral view were taken after routine subcutaneous vaccination. Following a brief training session on the use of the SCS, observers (n = 20) were presented with these screenshots. Results: Participants were able to identify and interpret the body signals. They rated the body signals beak opening and tail position with substantial agreement compared to an expert standard (agreement scores using Cohen's kappa). Interobserver reliability scores ranged from κ = 0.31 (fair agreement) for eye opening to κ = 0.78 (substantial agreement) for beak opening. The number of body signals displayed by the chickens influenced the observers' overall scoring of the animals as stressed or unstressed. The presence of more than four body signals indicating stress led to an overall rating as "stressed", while two or fewer signs of stress led to an overall rating as "unstressed". Closed eyes had the greatest influence on the overall scoring as "stressed" (PPV: 90.16%). Conclusion: The results suggest that discomfort in chickens can be detected by their body posture. They also suggest that this scale could help to identify pain or distress in chickens and could be a good tool for education and training purposes.