Date of Award

Spring 2018

Document Type

Honors Thesis



First Advisor

Dr. Andrea L Liebl

Second Advisor

Dr. David Swanson

Third Advisor

Ms. Beate Wone


Sex Ratio, Chestnut Crowned Babbler, Cooperative Breeder, Philopatric


The most simplified explanation to describe sex ratios comes from Fisher’s Principle, which assumes equal representation of both sexes following Mendelian segregation. However, violations to this principle create a biased sex ratio of more males or females. This bias has yet to be completely understood, as there have been conflicting study results. Hence, there is an importance of additional study, especially among cooperative avian species, where sex ratio variation may have greater consequences due to sexspecific dispersal and helping. One such cooperative breeder is the chestnut crowned babbler (Pomatostomus ruficeps), endemic to the Australian outback. In chestnut crowned babblers, females are generally dispersive and males philopatric. In this study blood was collected from chicks in the nest to molecularly determine the sex of each chick and examined for any sex ratio changes. This study found no significant variation in sex-ratios among chestnut crowned babblers at the population or individual levels; as well as no sex-ratio bias under six different variables – hatch date, brood size, breeding unit, number of helpers, year of birth, and number of attempts. These results are of great importance in cooperative breeding, as the ultimate goal for helpers is to pass on genetic information to continue the success of the species, even if it is not through their direct offspring. Therefore, with no significant evidence showing sex ratio bias, future studies should focus on the significance of maternal age (costs versus benefits of the sex), social status, and hatch rank.



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