Avian obligate brood parasites do not provide parental care for their eggs and young, and may therefore serve as a strong model system to test predictions of evolutionary sex‐allocation theories, independent of parental modulation of primary sex ratios. However, none of the handful of previous studies examining offspring sex ratio in brood parasitic birds have revealed a bias from parity at the level of the female parasite, the host species or temporal scale(s). This is also surprising, because in at least one brood parasite, the brown‐headed cowbird Molothrus ater, adult sex ratios are consistently and heavily male‐biased. Here we used a large database of embryonic and nestling cowbirds’ genetic sex ratios collected from nests of a single host species, the prothonotary warbler Protonotaria citrea to assess potential overall, temporal and individual patterns of bias. Contrary to previous findings, we documented an increase in the calculated male sex ratios later in the breeding season. There was no effect of whether embryos or nestlings were sampled, implying a lack of host parental effect on shifting the primary‐to‐secondary sex ratios of brood parasitic offspring. Future work should explore the sex‐specific survival and recruitment pattern of fledgling cowbirds raised by this and other host species to reconcile theoretical and empirical predictions and patterns.

中文翻译：

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繁殖的寄生棕头牛鸟（Molothrus ater）的后代性别比的季节性变化

鸟类专性的家禽寄生虫不为其卵和幼体提供父母照料，因此可以作为一个强有力的模型系统来检验进化的性别分配理论的预测，而不受父母对主要性别比的调节的影响。然而，先前的几项研究育雏寄生鸟类后代性别比例的研究都没有发现雌性，宿主物种或时间尺度的雌性水平存在偏差。这也是令人惊讶的，因为在至少一个亲代寄生虫中，棕头牛bird Molothrus ater的成年性别比始终如一且偏重男性。这里我们使用了一个宿主物种的巢收集胚胎和雏鸟cowbirds'遗传性别比例的大型数据库中，蓝翅黄森莺Protonotaria泛菌评估潜在的整体，时间和个人偏见模式。与以前的发现相反，我们记录了繁殖季节后期男性性别比的增加。胚胎或雏鸟的抽样均无影响，这说明寄主父母对母体寄生后代的初次性别比变化没有影响。未来的工作应探索由该物种和其他寄主物种饲养的雏牛的性别特异性存活和募集模式，以协调理论和经验的预测与模式。