Birds can adjust investment in reproduction by altering the sex ratios of their broods. When one sex is more costly to raise than the other, producing either more male or female offspring can lower the costs of reproduction. Biasing sex ratios can also be advantageous if the expected fitness of male versus female offspring differs. Here, we investigated whether Galapagos mockingbirds Mimus parvulus bias sex ratios in response to variation in rainfall. Female mockingbirds are smaller and thus females may reduce the costs of reproduction by biasing sex ratios in favor of female offspring. We quantified sex ratios in 131 mockingbird broods over four years with variable rainfall (2012, 2013, 2015 and 2016). We found no evidence that mockingbirds adjusted sex ratios in response to environmental conditions. Mockingbirds may not adjust sex ratios either because they lack the ability to do so, or because the costs of manipulation outweigh its benefits.

中文翻译：

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没有证据表明加拉帕戈斯模仿鸟对环境做出反应而操纵性别比例

鸟类可以通过改变其后代的性别比例来调整繁殖投资。当一种性别的饲养成本高于另一种性别时，生产更多的雄性或雌性后代可以降低繁殖成本。如果雄性与雌性后代的预期适应度不同，则偏向性别比例也可能是有利的。在这里，我们调查了加拉帕戈斯嘲鸫是否会根据降雨量的变化而偏向于性比。雌性知更鸟体型较小，因此雌性可以通过偏向雌性后代的性别比例来降低繁殖成本。我们量化了 131 只知更鸟幼鸟在 4 年内（2012 年、2013 年、2015 年和 2016 年）不同降雨量的性别比例。我们没有发现任何证据表明知更鸟会根据环境条件调整性别比例。