Relationship Between Brood Sex Ratio and Male Ornaments Depends on Male Age in a Warbler

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Animal Behaviour








Sex allocation theory predicts that females should bias the sex ratio of their offspring in response to differences in the reproductive value of sons and daughters. For example, paternal traits (ornaments) that are associated with male quality and reproductive success may result in male-biased sex ratios when inheritance of these traits is more beneficial to sons than to daughters. However, the information content of male ornaments, as well as the ornaments themselves, may change with male experience. If females are responsive to changes in the information content of signals, the result may be context-specific patterns of sex allocation. In a 3-year study of 50 common yellowthroat, Geothlypis trichas, females and their 287 young, we found that biased sex ratios were associated with male ornamentation, but that these deviations were not consistent across male experience classes. Females produced clutches with male-biased sex ratios when they were mated to young males with elaborate ornaments, but not when they were mated to older males. Male-biased broods were also associated with differences in offspring quality, as measured by average nestling mass. At our field site in New York, U.S.A., ornaments are generally more indicative of viability and condition among young, inexperienced males than among older males. We suggest that the relationship between male traits and the relative value of sons and daughters may change across male experience classes. Such changes may obscure patterns of adaptive sex-ratio bias and contribute to mixed results in the sex allocation literature.


age-dependent signalling, brood sex ratio, ornamentation, sexual selection