Sex ratio of Western Bluebirds (Sialia mexicana) is mediated by phenology and clutch size

• Published in: Ibis (London, England) England), 2021-07, Vol.163 (3), p.977-989
• Main Authors: Bartlow, Andrew W., Jankowski, Mark D., Hathcock, Charles D., Ryti, Randall T., Reneau, Steven L., Fair, Jeanne M.
• Format: Article
• Language: English
• Subjects: Animal behavior; BASIC BIOLOGICAL SCIENCES; Biological science; Breeding; breeding date; Breeding seasons; Clutch size; Clutches; Datasets; Dispersal; Drought; Females; Food; Food availability; Food resources; Gender aspects; Hatching; long-term study; Males; Mortality; Nest provisioning; Offspring; Ova; parental investment; Provisioning; Seasons; Sex; sex allocation; Sex differences; Sex ratio; Sialia mexicana; Social behavior; Wildlife
• ISSN: 0019-1019; 1474-919X
• DOI: 10.1111/ibi.12935

Mothers may produce more of one sex to maximize their fitness if there are differences in the cost of producing each sex or there are differences in their relative reproductive value. Breeding date and clutch size are known to influence offspring sex ratios in birds through sex differences in dispersal, social behaviours, differential mortality and available food resources. We tested whether breeding date, clutch size and drought conditions influenced offspring sex ratios in a sexually size‐monomorphic species, the Western Bluebird Sialia mexicana, by interrogating a 21‐year dataset. After controlling for differential mortality, we found that hatch dates late in the breeding season were associated with the production of more females, suggesting that the value of producing males declines as the breeding season progresses. When clutch size was taken into account, small clutches yielded significantly more females late in the breeding season than in the early and middle parts of the breeding season, which produced significantly more males. Large clutches early in the season tended to produce more females, although this was not significant. Drought severity was not correlated with sex ratio adjustment. We propose and discuss several explanations for these patterns, including male offspring, but not female offspring, acting as helpers, increased female nestling provisioning late in the breeding season, differences in food abundance and egg‐laying order. Future work will help to uncover the mechanisms leading to these patterns. Identifying patterns and mechanisms of sex ratio skew from long‐term datasets is important for informing predictions regarding life‐history trade‐offs in wildlife populations.