Pitts, T. D., & Snow, R. W. (1996). Mortality of Banded Adult Eastern Bluebirds. Audubon Society of Omaha.
Of 51 Eastern Bluebirds killed by domestic cats, 19 were females and 5 were males; the sex of the other birds was not indicated. While this sample size is not much larger than that of bluebirds found dead in the nest (24 versus 16), the trend seen here shows a significant difference in the number of males and females killed (X2 = 8.16, df = 1, P <0.005). Figure 5 shows that mortality due to domestic cats is highest during the nesting season. Considering that the female bluebirds, who construct each nest, lay eggs, and incubate the eggs, receive significant amounts of help from the male only when caring for nestlings, we are not surprised that more females are captured by domestic cats.
Domestic cats are, narrowly, the second most important cause of non-natural mortality, just after shooting.
(Domestic cats kill an unknown, but probably large, number of nestling bluebirds. However, since this paper deals only with adult mortality we have not explored the effects of cat predation on nestlings.)