Effect of predator type, season, brood size, and West Nile Virus infection on the nest defense behavior of male and female Eastern Bluebirds

The intensity of nest defense by birds can be influenced by many factors, including a parent\u27s sex, brood size, stage of breeding season, type of predator, and physical condition. Because previous studies have produced conflicting results concerning the effects of these factors on the nest defense behavior of birds, additional studies are needed to better clarify how and why such factors influence behavior. No one to date has examined the possible effect of a viral infection on avian nest defense behavior. Thus, my objectives were to determine the effect of adult sex, brood size, stage of the breeding season, predator type, and infection with West Nile Virus (WNV) on the nest defense behavior of male and female Eastern Bluebirds (Sialia sialis). Eastern Bluebirds were studied from March to August 2003 at the Blue Grass Army Depot. Adult and nestling Bluebirds were captured, measured, banded, and blood was drawn. Nest defense was examined when nestlings were 15-18 days old. Pairs of bluebirds were presented with two predators, a human and an Eastern Screech-Owl (Megascops asio), and nest defense behaviors were recorded. Analysis revealed that nest defense intensity differed significantly with predator type, with bluebirds responding more vigorously to an Eastern Screech-Owl than to a human (P \u3c 0.0001). Nest defense intensity also differed between the sexes, with male bluebirds defending with greater intensity than females (P = 0.031). However, analysis revealed that brood size (P = 0.70) and stage of breeding season (P = 0.11) did not influence nest defense intensity of Eastern Bluebirds. There was also no difference in the intensity of nest defense between pairs where one adult was infected with West Nile Virus and pairs where neither adult was infected (P = 0.24). My results indicate that male and female Eastern Bluebirds responded more vigorously to an Eastern Screech Owl than a human, possibly because an avian (aerial) predator like an Eastern Screech-Owl, in contrast to a terrestrial predator, represents a threat not only to nestlings, but to adults as well. Intensity of nest defense may not vary with brood size because the value of a given number of young may vary with the reproductive potential of parents and, therefore, parents capable of raising fewer young might be expected to defend their smaller brood as intensively as parents with greater reproductive potential defend their larger brood. The intensity of nest defense by bluebirds may remain constant throughout the breeding season because the declining value of offspring as the season progresses may be balanced by the effect of declining re-nesting potential. Finally, my results suggest that WNV infection, at least during the viremic stage, did not affect the physical condition of Eastern Bluebirds enough to affect their nest defense behavior