Cockatiel eggs hatch asynchronously because most eggs are laid after continuous incubation begins. We manipulated numbers of eggs in the nest after cockatiels began incubating to determine whether number of eggs in the nest influenced clutch size. We also measured plasma prolactin 2, 7, 12, and 17 days after egg laying began to determine whether clutch size covaried with prolactin concentration. We found that total egg production was stimulated in pairs for which the rate of egg accrual was reduced by egg removal, while total egg production was inhibited in pairs for which the egg accrual rate was increased by egg addition. Female, but not male, plasma prolactin concentrations correlated negatively with egg production on day of lay 17 but not before. We conclude that egg production in incubating cockatiels is influenced by feedback from the number of eggs in the nest and that clutch termination may be signaled by rising prolactin levels in females.