Taylor, Nicole A. – Burrowing Owl offspring sex ratios: Long-term trends and a test of the Trivers and Willard Hypothesis
The ability of parents to vary the sex ratios of their offspring is one of the most controversial topics in modern evolutionary and behavioral ecology. Part of my graduate research involves testing the predictions of a hypothesis that attempts to explain why deviations from a 50/50 sex ratio may occur.
In many species, variance in reproductive success is greater in males than females. Under such conditions, Trivers and Willard suggested natural selection could favor deviations from a 50/50 sex ratio when maternal condition influences condition of young and when these differences carry over into adulthood and affect reproductive success. Females in superior condition are expected to favor sons, whereas females in average or poor condition should invest in daughters in order to contribute the maximum number of grandchildren to the following generation. Numerous studies in mammals have supported these predictions, but only recently has this hypothesis been applied to birds.
Although recent studies suggest a relationship between maternal body condition and brood sex ratios in some passerine birds, there have been no manipulative or confirmatory experiments published. Therefore, my objective is to test Trivers and Willard’s hypothesis by manipulating body condition in female burrowing owls (Athene cunicularia) to test the predictions that females in good body condition produce male-biased broods, and females in poorer condition produce female-biased broods.
Burrowing owls are an excellent species to test the Trivers and Willard hypothesis because they lay large clutches (up to 12 eggs), which provides desirable statistical properties for detecting skews in sex ratios, and these owls nest in artificial burrow systems (ABS) in my study area, the Snake River Birds of Prey Area, which provides easy access to owls and their nests for measurements.
To enhance maternal condition, each day during the nesting period I provide supplemental food (mice and day-old chickens) to eight nesting pairs of owls/year starting when adults arrive from wintering areas in mid-March. I decrease maternal condition at eight different nests/year by clipping every third primary on both wings, which is among the standard approaches in the literature. Indices based on size and mass confirm effects of manipulations on female body condition and nestling body condition. Nestlings are sexed based on DNA isolated from blood using a standard protocol in our laboratory to assess secondary (nestling) and tertiary (fledging) sex ratios.
Thus, by manipulating female body condition in burrowing owls, my study will provide a novel confirmatory test of the Trivers and Willard hypothesis for birds.
Supplemental food to increase parental condition posted near two ABS