Brave breeds and brains under the spotlight: how do genetics and lighted incubation impact young laying hens stress responsivity?

A promising intervention to improve chicken welfare is the presence of light during incubation. More specifically, a cycle alternating green light and darkness phases decreased fearfulness in broilers. The impact on laying hens, however, is not known. We thus investigated the effects of lighted incubation on stress sensitivity in two common layer hybrids: ISA Brown and Dekalb White. The latter is known to be flightier than the former. Half of the eggs of each hybrid were incubated in standard dark conditions and the other half in a green light:dark cycle of 12:12 throughout the incubation, resulting in a 2*2 design. Because the developmental stage has a major impact on the ability to cope with stressors at adulthood, our observations focused on the rearing phase. To measure fearfulness, the group behaviour response to a novel object was monitored during live observations. To measure long-term HPA-axis activity, individual corticosterone levels in feathers at 17 weeks old were determined with an ELISA technique. We expected the light-incubated chicks to show lower fearfulness and HPA-axis activity than dark-incubated chicks. In addition, we expected brown chicks to show lower fear responses than white chicks. Finally, stronger effects of the incubation treatment were expected in white chicks – given their higher stress sensitivity and a better light transmission through white eggshells compared to brown.
There was no significant difference in fearfulness during the novel object test between the incubation treatments or the hybrids (N=120, parametric survival regression model, p ≥ 0.4). Further investigation through video recordings is required to reach a sufficient statistical power and draw reliable conclusions. The preliminary analysis of corticosterone did not show any significant effect of the hybrid or the incubation treatment (generalised linear model, N = 20, p ≥ 0.18). However, a descriptive analysis shows lighted incubation may have decreased corticosterone levels in the primary feathers n°2 of both the hybrids, which would confirm our hypothesis of lighted incubation to lower HPA activity. Processing of additional feathers is, however, required to draw any conclusion. Our research shows no effect of lighted incubation on laying hen fearfulness and HPA-axis activity. Hybrid differences were not found here neither, though they were found in behaviour tests not included in this report. Later in the project, a video tracking technology coupled with ArUco markers for identification will enable us to extract individual data from the novel object test, and assess the link between behaviour and physiology.