A previous study reported that climate-mediated increases in the length of the breeding season produced increasingly female-biased fledging sex ratios in Red-winged Blackbirds (Agelaius phoeniceus). Using those same data plus one additional year (11 years in total), I found that this phenomenon was not a result of greater production of females early and late in the season, contrary to what had been proposed. Instead, seasonal sex-allocation patterns interacted with season length. Early and midseason sex ratios became more female-biased as breeding seasons became longer, whereas late-season sex ratios tended to vary in the opposite manner, albeit weakly. Thus, the discrepancy between sex ratios late in the season and those earlier (early plus midseason) was a strong function of season length. Because fledging sex ratios did not vary with nestling mortality, these patterns appear to be a consequence of nonrandom sex-allocation rather than sex-biased survival. It is unclear whether these sex-allocation patterns are adaptive. Because the climatic factor (the North Atlantic Oscillation) associated with longer breeding seasons is also associated with higher winter mortality, however, it is possible that female Red-winged Blackbirds change how they allocate sex in response to changes in the breeding sex ratio. If climate change continues to alter these patterns, documenting how individuals and populations respond will be informative, from both basic and applied perspectives.
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