Effects of enrichment and heat stress on fluctuating asymmetry and feather structure of broiler chickens

• Climate change will increase heat stress challenges in broilers. • Bone asymmetry emerged as a sensitive indicator of heat stress and enrichment. • Platforms reduced skeletal asymmetry under heat stress conditions. • Feather traits showed adaptive plasticity to environmental challenges. • Multifunctional platforms can help mitigate heat stress and support welfare. Fluctuating asymmetry ( FA ) is the small, random difference between the left and right sides of paired traits and is widely used as an indicator of developmental instability and animal welfare. This study investigated whether heat stress ( HS ) and access to elevated multifunctional platforms influence FA in live birds, carcasses, bones, and feathers, as well as feather morphology and microstructure. In each round, 560 male Ross 308 broilers were reared for 43 days. The facility had two compartments: one under thermoneutral ( TN ) conditions and the other subjected to HS. Each compartment housed one pen with platforms and one barren pen. HS was applied by raising the temperature to 32 °C for 6 h daily from day 29 to 40 (except days 34–35). The experiment was conducted over three production cycles. On day 40, 20 birds per group per cycle were selected for live bilateral measurements. On day 42, the same birds were slaughtered and additional bilateral traits were recorded on carcasses, and after cleaning, on bones. First primary feathers from both wings were collected for morphometry and microstructure analyses. Composite FA did not differ for live birds (p = 0.079), carcasses (p = 0.429), or feathers (p = 0.336); however, bones showed a treatment effect (p = 0.029). Bone FA was lower in HS birds with platforms compared with TN birds in barren pens (p = 0.025), but did not differ from TN birds with platforms or HS birds in barren pens. Trait-specific effects were detected for tibiotarsus width (p = 0.026), femur length (p = 0.030) and femur width (p = 0.002). Feather morphometry showed interactions (temperature and enrichment) for rachis length (p < 0.001) and inner vane width (p = 0.001). HS birds in barren pens had the shortest rachises, whereas HS birds with platforms developed longer rachis. For microstructure, interactions were detected for barb width (p < 0.001). Barb width was greater in TN pens with platforms compared to HS pens with platforms. Overall, bone FA was significantly lower in the HS + platform group only when compared with the TN barren group. Feather morphometry and microstructure were highly plastic to treatments, reflecting adaptive responses that may also serve as welfare indicators.