Influence of light-exposure during late incubation and early feeding on the performance of layer chicks in a multitasking test and step detour test.

During on-farm hatching, chicken embryos experience continuous light during late incubation, and have immediate access to feed and water post-hatch. While on-farm hatching compared to hatchery-hatching improves several welfare indicators in broilers, the combined effects of light exposure and early feeding on behavioral and cognitive development in broilers and layers remains unclear. On-farm hatching systems are expected to expand into the laying hen sector due to the advances regarding in ovo-sexing and vaccination technologies. We investigated effects of continuous light exposure from 18 until 21 days of incubation and early feeding on multitasking abilities and visuomotor lateralized behavior in layer chicks. Using a two-by-two factorial design, 1,280 hatching eggs were assigned to one of four different groups of factor combinations with early feed and water access (FW) and continuous light exposure during late-stage incubation (L): FW+/L+, FW-/L+, FW+/L- or FW-/L-. In total, 128 focal chicks were subjected to behavioral testing. A multitasking test assessed vigilance towards an overhead predator while foraging at 10 to 13 days of age. L+ chicks identified the predator faster than those incubated in darkness (p < 0.0001) and resumed pecking faster after detection (p = 0.006), suggesting better multitasking abilities in both FW+/L+ and FW-/L+ chicks. These findings suggest enhanced behavioral flexibility and resilience in L+ chicks, which may improve adaptation to daily stressors and overall welfare during rearing. A step detour test assessed footedness when crossing a step and detour direction (i.e., laterality proxies) at five and seven weeks of age. L+ chicks exhibited weaker footedness compared to L- chicks. FW+ chicks showed more right-side foot and left-side detouring preferences, indicating greater left-hemispheric engagement and a more optimistic response to testing. Detour preferences also shifted with age, possibly reflecting motivation to approach the social stimulus. Differences in side-preferences may result from light-induced modulation of lateralization during late incubation, influenced by interactions with other asymmetries or perinatal environmental factors. However, the mechanisms underlying these interactions remain unclear and warrant further investigation.