Uncertain paternity through the potential for extra-pair copulations by females selects for the behaviour of male eastern bluebirds. Males are with fertile females more than non-fertile females; they stay closer to and follow fertile females more than non-fertile females. Reproductive competition through conspecific nest parasitism also occurs in eastern bluebirds (Gowaty & Karlin: Behav. Ecol. Sociobiol., 1984, 15, 91–95), and, by analogy to theories of uncertain paternity, uncertain maternity was predicted to shape the behaviour of females. Therefore, the idea that nest guarding by eastern bluebirds occurs in ways consistent with protection of maternity was tested. Females, but not males, foraged for food closer to nestboxes during egg laying, when their nests were most vulnerable to conspecific egg dumping. Thus, this study provides the first systematic analysis of the guarding of nests from the threat of conspecific parasitism. The tendency for males to guard females and for females to guard nests was most exaggerated in those ecological situations where the threat of extra-pair copulation and conspecific egg dumping was greatest, namely when there were multiple nextboxes in a territory that acted as attractants to conspecific interlopers. Other possible explanations besides mate guarding for the observed behavioural variation in males, including copulatory access and guarding females from predation, are not consistent with the observations and were rejected as explanations for the behaviour. Other possible explanations besides nest guarding for the observed behavioural variation in females, including foraging for nesting material and increasing attention to nestlings, were also rejected.