Abstract: Fault bars are structural deformities formed during feather growth due to disrupted keratin synthesis, leading to weakened or fractured barbules, loss of vane integrity, and consequently impaired flight capacity and survival adaptability in birds. Studies on wild birds have shown that the occurrence of feather fault bars may be associated with multiple factors, such as nutritional deficiencies, environmental stressors, disease infections, and genetic predispositions. Although the exact etiology remains unclear, it has been widely accepted that physiological or psychological stressors during feather growth can induce fault bars formation. In modern farming systems, poultry are frequently exposed to multiple stressors in the high-density environment, compromising their welfare status. However, evaluating stress level in poultry remains a persistent challenge in practical welfare assessments. Recently, fault bars have gained attention as a non-invasive, practical, and promising biomarker for poultry welfare evaluation. This review synthesizes the influencing factors and formation mechanisms of fault bars, as well as their current applications in poultry welfare assessment, aiming to provide insights for improving poultry welfare evaluation systems.

Key words: feather fault bars, stress, welfare assessment, poultry