Avian brood parasites lay their eggs in the nests of other species, leaving the hosts to care for the parasitic offspring. The bookkeeping hypothesis predicts that, in order to reduce competition between parasitic nestlings, female parasites should keep a mental inventory of host nests that they have already parasitized and avoid laying multiple eggs in the same host nest. However, selection against repeat parasitism should be weaker when host nests are limited, or when hosts are able to rear more than one parasitic nestling. Here we use microsatellite genotyping of parasitic eggs to test whether female screaming cowbirds, Molothrus rufoaxillaris, avoid repeatedly parasitizing nests of their primary host, the greyish baywing, Agelaioides badius, in Argentina. Parasitism rates were extremely high (96.5% of 57 host clutches were parasitized with an average of 5.7 cowbird eggs each), indicating that host nests are limited. Although eggs laid by the same female showed moderate spatiotemporal clustering, individual females rarely laid more than one egg in the same host clutch (2 of 57 clutches, 26 of which contained multiple genotyped cowbird eggs). Females were much more likely to lay subsequent eggs in different host nests than to return to the same host nest. We found no evidence for kin structure among female cowbirds parasitizing the same host nest, which were no more closely related than chance would predict. These results suggest that female screaming cowbirds frequently lay eggs in host nests that have already been parasitized by unrelated females. However, they typically lay just one egg per host clutch, even though greyish baywings are capable of rearing several nestlings. Since screaming cowbird laying is often poorly synchronized with that of their host, avoidance of repeat parasitism may be adaptive if it allows females to spread the risk of failure among multiple host nests.

中文翻译：

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通过尖叫的牛鸫（一种专业的育雏寄生虫）重复和多重寄生的遗传模式

鸟类寄生虫在其他物种的巢穴中产卵，让寄主照顾寄生的后代。簿记假说预测，为了减少寄生雏鸟之间的竞争，雌性寄生虫应该对它们已经寄生的宿主巢进行心理盘点，并避免在同一个宿主巢中产多个卵。然而，当寄主巢穴有限或寄主能够饲养不止一只寄生雏鸟时，对重复寄生的选择应该较弱。在这里，我们使用寄生卵的微卫星基因分型来测试雌性尖叫的牛鸫，Molothrus rufoaxillaris，是否避免反复寄生在阿根廷的主要寄主灰色海湾翅，Agelaoides badius 的巢穴中。寄生率极高（96. 57 个寄主窝中有 5% 被寄生，每个窝平均有 5.7 个牛鸟卵），表明寄主巢是有限的。尽管同一雌性产下的卵显示出适度的时空聚集性，但单个雌性很少在同一宿主窝中产下超过 1 个卵（57 个窝中的 2 个，其中 26 个包含多个基因型的牛鸟卵）。与返回同一个宿主巢穴相比，雌性更有可能在不同的宿主巢穴中产卵。我们没有发现寄生在同一宿主巢中的雌性牛鹂之间存在亲属结构的证据，它们的关系并不比偶然预测的更密切。这些结果表明，雌性尖叫的牛鸟经常在已经被无关雌性寄生的宿主巢中产卵。然而，它们通常每个寄主窝只产一个蛋，即使灰色的海湾翅能够饲养几只雏鸟。由于尖叫的牛鸟产蛋通常与其宿主的产蛋不同步，如果避免重复寄生，雌性可以在多个寄主巢穴之间分散失败的风险，那么它可能是适应性的。