In birds, sex is determined by the allocation of sex chromosomes. We used molecular techniques to determine the sex of Mimus polyglottos (Northern Mockingbird) nestlings in Cincinnati, OH and Raleigh, NC. We found an overall male-biased sex ratio in the Cincinnati population and a female-biased sex ratio in the Raleigh population. In Cincinnati, the male-biased sex ratio was more pronounced early in the breeding season than later in the breeding season. Male nestlings were heavier than female nestlings and may require greater parental investment. In many avian species, female offspring are more likely to disperse, making them less likely to compete with parents and siblings for local resources in future seasons. This may explain why the Raleigh population with greater nesting population density overproduced female offspring.