Date of Award

Spring 5-9-2026

Document Type

Honors Thesis

Department/Major

Biology

First Advisor

Andrea Liebl

Second Advisor

Christopher Anderson

Third Advisor

David Swanson

Keywords

sex ratio, brood size, chestnut-crowned babblers, pre-hatching survival, post-hatching survival

Abstract

Understanding the ecological conditions under which parents, particularly mothers, adjust offspring sex ratios is important for predicting how environmental variation shapes population dynamics, sexual selection, and long-term evolutionary trajectories. Cooperative breeding is a social system where non-breeding individuals, called helpers, assist in raising another individual’s offspring. In birds, helpers tend to be males. As such, the fitness returns of male and female offspring in cooperative species depend on local social structure and future helping needs. Differences in sex can arise through either pre- or post-hatching variation. Pre-hatching variation is the result of physiological or genetic manipulation of offspring sex, whereas post-hatching changes are a result of variation provisioning by carers. Because of this, cooperative systems provide a powerful framework to study how mothers adaptively adjust offspring sex ratios in response to ecological and social conditions. Here, I examined what factors influence the sex ratio of nestlings in a cooperatively breeding species, the chestnut-crowned babbler (CCB; Pomatostomus ruficeps). Across 13 breeding seasons, nestlings were monitored and sampled throughout development for mortality, morphological measurements, and blood. Because CCB are sexually monomorphic, all individuals were molecularly sexed.  I found that the sex ratio at hatching was dependent on brood size and the number of helpers present to care for offspring. However, offspring sex had no significant impact on nestling survival to fledging (~16 days). Overall, this study demonstrates that offspring sex ratios in a cooperatively breeding species are context-dependent at hatching, but not amplified through sex-biased survival, underscoring the importance of early maternal investment decisions in cooperative breeding systems.

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