Brave Breeds and Brains: How hybrid and the incubation environment shape laying hen coping capacities

Good early life conditions are increasingly recognized as essential for animal welfare later in life. The use of light during incubation might improve coping capacities and welfare in later life in poultry, by more closely approximating chicken natural environments, compared to the current conventional incubation in darkness. Due to the position of the embryo in the egg just before hatching, the two brain hemispheres receive different amounts of light stimulation via the eyes. This unequal stimulation results in increased lateralization of the brain, which is the specialization of brain hemispheres on certain functions. In current commercial practice, chickens are incubated in complete darkness. Previous studies showed promising results of light during incubation on broiler welfare: more lateralized chicks, a more pronounced daily behavior rhythm, earlier onset of melatonin rhythms, and lower stress reactions to various stressors after hatching. However, information about the effects of light during incubation on laying hens is scarce. Research is necessary on both white and brown egg layers, as stronger effects are expected in hybrids coming from white eggs than brown eggs. In addition, white hybrids are known to be flightier and more sensitive than brown hybrids. Light during incubation could have different effects on both types of hybrids. For this thesis, Dekalb White (DW) and ISA Brown (ISA) eggs were incubated in complete darkness (dark) or in a light-dark cycle of 12h of green light and 12h of dark throughout incubation (light), resulting in four treatment groups of 100 chickens each: DW-dark, DW-light, ISA-dark, and ISA-light. Light transmission was higher through white eggshells than through brown eggshells. Light during incubation had no effects on hatching characteristics or brain measurements, and only few effects on fear of humans and cognition tests. There were hybrid effects on some hatching characteristics, most tests about fear of humans, one brain measurement, and some cognition tests. Despite the difference of light transmission through eggshell between hybrids, there was no interaction between incubation treatment and hybrid on hatching characteristics, on fear of humans or on brain measurements, and the effects on cognition were inconsistent. Green light during incubation did not negatively affect hatching characteristics in either DW nor ISA laying hen hybrids, and did not decrease fear of humans as much as expected. In addition, light during incubation affected the brains and cognition of laying hens at different life stages post-hatching, but the effects were not consistent across tests or hybrids. As such, the light regime used in this study does not seem as promising as expected to improve laying hen welfare. In parallel, the brown hybrid was usually less fearful than the white hybrid, although there were some exceptions depending on the stressor. Future research should focus on the effects of a longer exposure to green light during incubation on laying hen welfare. In parallel, research set-ups and husbandry guidelines for commercial settings should be adapted to the specific hybrids used.