Assessing good physical health and resilience as a foundation for positive welfare in chickens

This review proposes candidate animal‑based indicators of good physical health and resilience in chickens (Gallus gallus domesticus) as a foundation for assessing positive animal welfare, complementing existing approaches to animal welfare assessment focused on use of iceberg indicators to detect severe health problems. We outline potential anatomical indicators involving the comb and wattles, eyes, beak, plumage, skin, footpads, claws, and overall body for rapid on-farm screening that could be automated for ease of application (e.g. using computer vision). We also identify health- and resilience-related anatomical and physiological indicators that could provide deeper, context‑dependent insights but require controlled testing conditions and/or laboratory analysis. For each indicator category, we summarize biological significance, influencing factors, and measurement methods under commercial and research settings. We classify candidate indicators according to their focus (health vs resilience) and response directionality on a scale from tolerable to optimal (whereby optimal values are highest for unidirectional measures such as plumage condition and intermediate for bidirectional measures such as claw length). We also rate potential ease of data collection (invasive, catching required, or remote sampling), on-farm applicability, and level of promise as a guide for indicator selection and prioritization for validation. Following validation and establishment of an appropriate scoring range from tolerable to optimal for each indicator depending on age, breed type, and reproductive status, we propose the use of continuous visual analogue scales or algorithms for scoring, followed by aggregation of indicator scores to obtain an overall rating of each bird's health and resilience. This narrative review thus provides a biologically grounded roadmap for developing proactive assessment tools that support thriving in poultry, as a foundation upon which affective and cognitive components of positive animal welfare can also be added.