Commercial poultry diets are deficient in omega-3 fatty acids (n-3 FA) essential for embryonic brain development. Feeding n-3 FA to broiler and layer breeder chickens may directly benefit them and could increase their offspring's health, welfare, and productivity. This thesis aimed to assess the effects of maternal feeding n-3 FA on the brain size and n-3 FA concentration, cognition, and fearfulness of layer and broiler offspring. Ross 708 broiler breeders were fed a control (n-6 to n-3 ratio 26.1:1) or flaxseed (4.81:1) diet during the rearing and laying period, resulting in four maternal diet combinations (control-control, control-flaxseed, flaxseed-control, flaxseed-flaxseed). Two strains of layer breeders, ISA Brown and Shaver White, were also fed a control (7.56:1) or flaxseed (5.31:1) diet. Layer and broiler offspring were assessed for brain size and docosahexaenoic acid (DHA) concentration, performance in two types of learning tests (T-maze and discrimination tasks), fear response to a novel object and social isolation. Although maternal feeding of n-3 supplemented diets resulted in significantly greater brain-to-body weight ratios in broiler offspring and decreased brain n-6: n-3 FA ratio in layer offspring, we found little evidence that maternal flaxseed diets resulted in differences in offspring cognition or fearfulness. There were sex-specific effects of maternal diet on broiler offspring vocalisation rate during a social isolation test and participation in a T-maze test. However, no effect of maternal diet was found on layer offspring behaviour. We found several effects of strain on behaviour in the layers. White chickens participated more in T-maze tests as chicks and were likelier to reach the learning criteria in both a T-maze test and discrimination learning task when tested as pullets and adults. Brown chicks vocalised more during social isolation, whereas white chicks spent more time motionless. In a novel object test, brown chickens were observed more frequently on elevated platforms than white chickens indicating greater avoidance of the novel object. We suggest further exploration into the optimal n-6 to n-3 FA ratios for poultry diets using diets closer to the 1:1 ratio found in wild precocial birds.