High-lighting chicken welfare: The effects of hybrid and light during incubation on physiological stress and fear behaviour in young laying hens

A promising intervention to improve chicken welfare is the presence of light during incubation. More specifically, a cycle alternating green light and darkness decreased fearfulness in broilers. The impact on laying hens, however, is unknown. We investigated effects of lighted incubation on stress sensitivity in two common layer hybrids: ISA Brown and more fearful Dekalb White. Half of the eggs from each hybrid were incubated in standard dark conditions, the other half under a green light:dark cycle of 12:12, resulting in a 2*2 design. To measure long-term HPA-axis activity, corticosterone levels in feathers at hatching and at 17 weeks old were determined. Fearfulness was measured in a voluntary approach test. We expected the light-incubated layers to show lower corticosterone levels and reduced fearfulness compared to dark-incubated layers. In addition, we expected brown layers to show reduced fearfulness compared to white layers. Finally, stronger effects of the incubation treatment were expected in white layers, given their higher stress sensitivity and a better light transmission through white eggshells compared to brown. Corticosterone in feathers did not statistically differ between treatments (N=20, GLMM, p>0.05). A descriptive analysis, however, showed lighted incubation may have had opposite effects on adult brown and white layers, respectively increasing and decreasing corticosterone levels as compared to the dark incubation. Brown layers ate the reward faster than white layers in the voluntary approach test (N=385, Cox model, p<0.001), in line with our expectation of lower fearfulness. However, the dark-incubated layers tended to eat the reward faster than light-incubated layers, appearing less fearful (p=0.06), contrary to our hypothesis. Our research shows a clear hybrid-related difference in laying hen behaviour, but not in their long-term HPAaxis activity. Upcoming further endocrine analyses will give a wider overview of the effects of light during incubation on the two laying hen hybrids.