Nestling sex allocation in European Starlings : the role of parental condition and nest greenery

Abstract

1 online resource (33 pages) : illustrations (some colour), charts (some colour), graphs (some colour)Includes abstract and appendix.Includes bibliographical references (pages 27-31).Nestling sex allocation is influenced by parental condition and mate attractiveness in many bird species, such that parents in better condition and those who are more attractive have male-biased brood sex ratios. In facultatively polygynous species, high quality sons confer greater reproductive success as they can attract several mates. Therefore, females in good condition would do best producing more sons, while females in poor condition would produce more daughters. Similarly, behaviours performed by courting males, such as adding greenery to the nest may also affect brood sex ratio with more greenery resulting in a male-biased brood sex ratio. Male European Starlings (Sturnus vulgaris) are a facultatively polygynous species that adds greenery to their nest, likely to signal their attractiveness. This study explored how parental body condition and male greenery addition influenced brood sex ratios across the 2023 and 2024 breeding seasons in Halifax, Nova Scotia, Canada. I predicted that parents in good condition or males who add greenery over more days would have male-biased brood sex ratios but found no significant relationship between their condition and brood sex ratios. However, a significant interaction effect occurred; male condition influenced nestling sex ratios only when females were in poor condition, creating a male bias when males were in good condition. These findings suggest female condition modifies the effect of male condition on sex allocation. Greenery addition frequency was not associated with brood sex ratio. These findings highlight the role of parental condition interactions in offspring sex allocation, while the significance of greenery addition remains unclear. Future research should examine total mass of greenery in a nest, the amount of greenery added daily, as well as female responses to greenery additions to better understand the function of nest ornamentation in brood sex allocation