Maternal stress effects on offspring development have been studied largely in rodents and primates, and to a lesser extent in farm animals. Potential lack of knowledge concerning prenatal stress on farm animals is regrettable because they are frequently subjected to a variety of husbandry stressors. Above all, effects of maternal stress on poultry offspring have been neglected. Prenatal effects in birds are known to involve maternal steroids present in eggs. In this study, we investigated the effects of daily unpredictable stressors applied to laying Japanese quail on their offspring's morphological and behavioural development. We also investigated the effects of our procedure on female reproductive output and on egg characteristics (weight, composition, yolk sex steroid levels). Our procedure induced only a mild stress: laying and egg fertilization rates of stressed females were not impaired; they remained similar to those of control females. Nonetheless, our stress procedure had an impact on some egg characteristics: stressed females’ eggs were heavier, contained more albumen and tended to have higher yolk testosterone levels than control females’ eggs. Stressed females’ offspring hatched earlier, were heavier at hatching and had a different growth pattern than did control females’ offspring. They also appeared to have a higher emotional reactivity than control chicks when encountering a novel environment and they reacted more strongly following social separation. Our study revealed that mild stressors applied to laying Japanese quail can increase the emotional reactivity of their chicks and suggested that maternal stress effects on offspring are mediated by changes in egg composition and yolk testosterone levels. We stress the particular relevance to the poultry industry of our findings highlighting the importance of taking into account the environment of laying females, as this environment can influence their progeny's behaviour and therefore their subsequent adaptation to husbandry conditions and ultimately their welfare.