Experimental evidence for adjustment of parental investment in relation to brood sex ratio in the blue tit.

Parents should allocate parental investment in relation to the expected reproductive value of their offspring. We used a cross-fostering experiment to see whether factors influencing offspring reproductive value (brood size, nestling sex, age or paternity) were related to parental investment in nest defence in the blue tit, Cyanistes caeruleus. Although brood size and the incidence of extrapair paternity did not influence the nest defence behaviour of parents, we found a significant difference in the way male and female parents responded to the experimental change in sex ratio of the brood; the nest defence response of males was faster at nests with an increased proportion of sons, but the response of females was unrelated to changes in brood sex ratio. Furthermore, there was no relation between the original sex ratio of the brood, before manipulation, and the nest defence response of the male, indicating that paternal investment in this behaviour was a direct response to the manipulated brood sex ratio