Brain-size reduction in domesticated forms of mammals in comparison to their wild ancestors is well documented in numerous species. Although it does not involve intentional artificial selection for traits such as meat production, captive breeding also causes a reduction in brain size in some mammalian species. In birds, domestication results in a reduction in brain size as well, but whether captive breeding causes a similar effect is unknown. Given the increasing number of avian captive breeding programs, it is important to assess whether captive breeding affects the behavior and morphology of a species. Here we investigated whether relative and absolute brain volume differs between wild and captive-bred stocks of 21 species of waterfowl (Anseriformes). Absolute brain volume of captive-bred specimens was lower than in wild specimens for most (16 of 21) of the species examined, and the decrease varied from 1% to 33% (mean: 4.7%). Similarly, all but one species showed relative brain-volume decrease that varied between 2% and 30% (mean: 7.7%). Overall, these reductions in absolute and relative brain volume are smaller than that caused by domestication in ducks (Anas platyrhynchos) and geese (A. anser) but are larger than the decreases observed in captive-bred mammals. Thus, captive breeding appears to result in a decrease in both absolute and relative brain volumes in waterfowl. How this reduction reflects changes in behavior or brain composition is unknown, but it may nevertheless have important implications for the successful reintroduction of captive-bred individuals in the wild.