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Seasonal variation in offspring sex ratio

It has been suggested that natural selection should favor parental ability to adjust the sex ratio of offspring produced.  Trivers and Willard (1973) proposed that females should manipulate offspring sex ratios dependent on parental condition.  Until recently the ability to test this hypothesis was limited to species that are dichromatic as nestlings.  This has changed with the advent of techniques for detecting sex-specific chromosomal characters from the blood.  In collaboration with Dr. Jonathan Clark, we are testing the Trivers and Willard hypothesis.  Blood samples are collected from each nestling in the population.  DNA from blood samples are then extracted and PCR (Polymerase Chain Reactions) performed for sex determination. 

Kristina McKinley is currently examining seasonal variation in the sex ratio of nestling European Starlings.  Her results suggest a seasonal increase in the proportion of male offspring in this population.

Trivers and Willard. 1973.  Science 179:90-91.