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During their prenatal development, birds are especially sensitive to environmental disturbances, largely because of the speed and complexity of growth occurring throughout this period.Although the egg's prenatal environment is externally incubated, its conditions are strongly influenced by the mother's physiological state during egg formation.In birds, prenatal maternal stress refers to stress experienced by the female during egg laying, which can alter egg composition (nutrients and hormone levels) and thereby the prenatal environment provided for its offspring.Several studies have linked such early-life stress to long-term effects on the offspring phenotype, particularly anxiety-related responses.This suggests that prenatal maternal stress leads to permanent changes in the offspring's developing brain and consequently in their behaviour.Using an advanced intercross population of domestic chicken (G.g. domesticus) and Red Junglefowl (G.gallus), this thesis explores the relationship between brain morphology (Chapters I, II and III), fearfulness and anxiety-related behaviour (Chapters II and IV), and sensitivity to early-life stress (Chapters III and IV).To this end, Isotropic Fractionation was used to study the cellular density of the brain, and a deep learning-based object detection architecture, together with R, was used to track and analyse the behaviour of prenatally stressed individuals.The latter method was used to analyse the individual chickens' overall fearfulness and anxiety, as well as how predictable their behavioural responses were across multiple trials.In Chapter I, we analyse the intraspecific variation of 66 chickens in the anatomical scaling of the brain and three brain regions, along with their neuronal density.We find that larger brains have a larger amount of neuronal and non-neuronal cells, where neuronal density strongly correlates with absolute neuronal count.These findings indicate that interspecific neuronal scaling rules might not be extrapolated from intraspecific patterns.In Chapter II, 36 chickens were recorded in an emergence test eight times, both at an early age and at later stages when sexually mature.Their latency to exit the starting box was then evaluated vi against their relative cerebellum size, where individuals with lower relative cerebellum size habituated faster to the test by exiting the box consecutively faster each trial.The results suggest that intraspecific differences in brain structure affect individual differences in the modulation of behavioural responses, more specifically, anxiety-related responses and habituation.In Chapter III, we simulated prenatal maternal stress by artificially increasing corticosterone through pellet implants in hens during egg laying, while a second group was only given a placebo.Their offspring were then repeatedly recorded in an open-field test at one week of age and after reaching sexual maturity.Results show that an increase in glucocorticoids during the prenatal period can affect both the magnitude of offspring's behavioural responses and their intraindividual variation across tests.This suggests that changes caused by the prenatal environment can also be reflected in an animal's predictability.In Chapter IV, adult chicken brains from offspring originating from both artificially stressed mothers and placebo were dissected, weighed, and separated into distinct brain regions, with neuronal and non-neuronal cell counts of the cerebrum and cerebellum estimated.Prenatally stressed offspring were less non-neuron-dense in the cerebrum and exhibited a mild sex-specific effect in the non-neuronal cells of the cerebellum.Results suggest neuronal cells to be less sensitive to prenatal environment effects than non-neuronal cells, indicating a possible link between frequently reported long-term prenatal glucocorticoid effects on behaviour and glial cell density in adult birds.This thesis explores intraspecific variation in brain structure and behavioural responses, demonstrating that prenatal maternal stress alters brain composition at the cellular level, providing a potential morphological link between prenatal maternal stress and long-term effects on offspring behaviour.Finally, this thesis demonstrates that prenatal maternal stress in chickens not only affects the overall level of anxiety in the offspring but also how predictable they are in their behavioural response.The findings reveal new underlying effects of prenatal maternal stress in chickens and thereby provide a better understanding and assessment of stress.