Parent birds may benefit by defending nests from potential predators if such behavior increases the likelihood that their young will survive. However, such behavior may also be costly, with some risk of injury or even death. Thus, because birds seek to maximize life-time reproductive success rather than current reproductive success, parents must assess the danger posed by a potential nest predator and then choose appropriate responses (Radford and Blakey 2000)
Gilbert, K. L., & Ritchison, G. L. (2015). Effect of Predator Type, Season, Brood Size, and West Nile Virus Infection on the Nest Defense Behavior of Male and Female Eastern Bluebirds (Sialia Sialis). The Kentucky Warber, 91, 39-47.