Animal cognition and thus the way animals interpret their surroundings can include biases. ‘Cognitive judgment bias’ is one type of cognitive bias. This bias can be positive or negative, resulting in more optimistic or pessimistic interpretation of stimuli. Such optimism or pessimism can be tested when observing responses to ambiguous cues intermediate of cues with known positive and negative values. To investigate factors affecting cognitive judgement biases in avian cognition, we conducted several experiments with young domestic and red junglefowl (Gallus gallus ssp.). To explore the influence of environmental differences on optimism, young chicks were raised under enriched or impoverished conditions. Cognitive judgment bias tests were performed before and after exposing chicks to a battery of stressors. Prior to being stressed, behavioural responses to ambiguous cues did not differ between individuals from the two treatments. However, chicks in enriched conditions showed higher resilience to stress by being more optimistic in the subsequent cognitive judgment bias test, compared to chicks in impoverished conditions. This suggests that environmental complexity early in life can buffer against negative responses to future stress, and that environmental conditions can influence optimism. Personality (i.e. consistent individual differences in behaviour) can also influence how individuals respond to stimuli. In another experiment we therefore explore the extent to which personality of individuals explain their level of optimism, by exposing chicks reared under similar conditions and with known personalities, to cognitive bias tests. Underlying variation in the monoaminergic systems (serotonin and dopamine) of our test birds will be investigated to further our understanding of proximate explanations to variation in optimism among individuals. Taken together, these studies improve our understanding of variation in cognitive judgement biases, and highlight factors influencing variation in optimism. These results are therefore of interest to a broad audience, particularly to researchers with interest in animal cognition, personality and welfare.