Experimental Corticosterone Administration Induces Fluctuating Asymmetry and Bursal Atrophy in Broiler Chickens

Corticosterone (CORT) is the primary avian stress hormone and regulates key physiological processes, but chronically elevated levels can be detrimental. This study simulated chronic stress by exogenously increasing CORT to assess effects on gait score, bone morphometry, immune indicators, and fluctuating asymmetry (FA) in broiler chickens. A total of 60 one-day-old male Ross 308 broiler chickens were allocated to six treatment groups (T0–T5; n = 10 per group). T0 received a placebo and served as the control group, while T1–T5 were gavaged daily with increasing corticosterone doses (1–5 mg/kg body weight). Treatments were administered from day 1 to day 42. CORT did not affect gait score on day 11 (p = 0.232) or day 42 (p = 0.112), but gait score was significantly correlated with body weight (day 11: p < 0.001; day 42: p = 0.004). Elevated CORT reduced bursa diameter (p < 0.001) and increased bursa follicle scores (p < 0.001), while heterophil-to-lymphocyte ratios remained unaffected (p = 0.349). Carcass and bone dimensions decreased consistently with higher doses (p < 0.001), and body weight correlated positively with all bone traits (p < 0.001). Length-to-width ratios increased with CORT dose (p < 0.001). Composite carcass FA showed dose-related variation in carcass asymmetry, with highest in T4, intermediate in T2, T3, and T5, and lowest in T0–T1 (p = 0.026). Trait-specific FA effects were detected for metatarsus length (p = 0.005) and wattle (p = 0.018), while bone FA remained unaffected (p = 0.272). Overall, exogenously increasing CORT impaired growth, suppressed immune function, and increased fluctuating asymmetry, indicating reduced developmental stability.