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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 d
eviations 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.
Supplemental food to increase parental condition posted near two ABS.
Questions and comments regarding the Raptor Research Center: mark_fuller@usgs.gov
Questions and comments regarding this website:
kbledsoe@boisestate.edu
This page was last reviewed on 20 December 2006.