Nest defense represents a parental strategy to maximize fitness by enhancing prospects of offspring survival. We used a taxidermic mount of a nest predator to measure nest defense of Eastern Kingbirds (Tyrannus tyrannus) at Malheur National Wildlife Refuge in southeastern Oregon in 2003 and 2004 to test the hypotheses that intensity of nest defense (1) was individually repeatable, (2) differed between males and females, and (3) predicted nest success. We also predicted that (4) intensity of defense would increase with age and number of young, but decline over the breeding season. Intensity of nest defense was significantly repeatable for male kingbirds. Male response was twice as strong as female response during incubation and the nestling period, but nest success was independent of defense scores of males and females. Simple paired comparisons suggested female responses did not change between incubation and the nestling period, whereas males tended to defend nestlings more vigorously than eggs. Multivariate analyses demonstrated strong individual differences were the main source of variation in nest defense. Intensity of nest defense by males and females increased with age of young, declined seasonally, but was not related to number of young. Kingbird nest defense is a repeatable behavior that differs between males and females and, as predicted by parental investment theory, nests of the greatest value (older young and earlier broods) were defended most aggressively.