The post-hatching development of the brain was studied in three altricial birds (those whose development is completed after hatching), namely, the domestic pigeon, house swift and house sparrow, to assess the state of maturity of the brain at the time that the fledglings leave the nest. Malurity is related to the process of myelination and, therefore, Sudan Black was chosen as it is a sensitive indicator of myelin lipids. On hatching, it was found that sudanophilia was very low in all three birds, indicating a lack of maturation. However, between 15 and 25 days in the pigeon and swift and between 8 and 14 days in the sparrow, there was a steady increase in Sudan Black staining. By the time the birds left their nests, the staining was similar to that of adults, thus indicating that the process of myelination was complete and that the brains were nature.