Red-winged Blackbirds (Agelaius phoeniceus), hereafter red-wings, are much less frequently parasitized by Brown-headed Cowbirds (Molothrus ater) in eastern North America than in central North America and had not been recorded as hosts in our study area in southeastern Pennsylvania. Although hosts of Old World cuckoos (Cuculidae) often show geographic variation in egg rejection behavior, cowbird hosts typically exhibit uniform responses of all acceptance or all rejection of cowbird eggs. Thus, geographic variation in cowbird parasitism frequencies might reflect a different behavioral response to parasitism by hosts where only some populations reject parasitism. In this study, we tested whether egg rejection behavior may explain the lack of parasitism observed in our eastern red-wing population, which may provide insight into low parasitism levels across eastern North America. We parasitized red-wing nests with model cowbird eggs to determine their response to parasitism. Nests were tested across three nest stages and compared to control nests with no manipulations. Because rejection differed significantly by stage, we compared responses separately for each nest stage. We also monitored other songbird nests to identify parasitism frequencies on all potential hosts. Red-wings showed significantly more rejections during the building stage, but not for the laying and incubation stages. Rejections during nest building involved mostly egg burials, which likely represent a continuation of the nest building process rather than true rejection of the cowbird egg. Excluding these responses, red-wings rejected 15% of cowbird eggs, which is similar to rejection levels from other studies and populations. The overall parasitism frequency on 11 species surveyed in our study area was only 7.4%. Egg rejection behavior does not explain the lack of parasitism on red-wings in our eastern population. Alternatively, we suggest that cowbird preference for other hosts and the low abundance of cowbirds in the east might explain the lack of parasitism. Future research should also explore cowbird and host density and the makeup of the host community to explain the low levels of parasitism on red-wings across eastern North America because egg rejection alone is unlikely to explain this broad geographic trend.

中文翻译：

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卵子排斥行为不能解释北美东部红翅黑鸟种群缺乏牛寄生虫的现象

在北美东部，红翼黑鸟（Agelaius phoeniceus）（以下称红翼）在北美东部的褐头牛鸟（Molothrus ater）被寄生的频率要低于北美中部，并且在我们东南部的研究区域中没有被记录为宿主宾夕法尼亚州。尽管旧世界杜鹃（Cuculidae）的寄主经常表现出卵排斥行为的地域差异，但是牛鸟寄主通常会对所有接受或拒绝的牛卵表现出统一的反应。因此，牛只寄生虫发生频率的地理变化可能反映了寄主对寄生虫的不同行为反应，其中只有一些种群拒绝寄生虫。在这项研究中，我们测试了拒卵行为是否可以解释东部红翼种群中观察不到的寄生虫现象，这可能有助于深入了解整个北美东部的寄生虫低水平。我们用模型牛nest卵将红翼巢寄生，以确定它们对寄生虫的反应。巢在三个巢阶段进行了测试，并与没有操作的对照巢进行了比较。由于拒绝的阶段性差异很大，因此我们分别比较了每个嵌套阶段的响应。我们还监视了其他鸣禽巢，以识别所有潜在宿主上的寄生频率。红翼在建造阶段表现出明显更多的拒绝，但在产蛋和孵化阶段却没有。筑巢期间的拒绝主要涉及埋葬卵，这可能表示筑巢过程的延续而不是牛true卵的真正排斥。除这些回应外，红翼拒绝了15％的牛禽蛋，这与其他研究和人群的拒绝水平相似。在我们研究区域内调查的11个物种的总体寄生频率仅为7.4％。拒绝卵的行为不能解释我们东部人口中红翼上没有寄生虫。或者，我们建议东部地区的牛鸟偏爱其他寄主，以及东部的牛鸟数量少，这可能解释了寄生虫的缺乏。未来的研究还应探索牛鸟和寄主的密度以及寄主社区的构成，以解释北美东部红翼寄生虫的水平低，因为仅卵子排斥不可能解释这一广泛的地理趋势。拒绝卵的行为不能解释我们东部人口中红翼上没有寄生虫。或者，我们建议东部地区的牛鸟偏爱其他寄主，以及东部的牛鸟数量少，这可能解释了寄生虫的缺乏。未来的研究还应探索牛鸟和寄主的密度以及寄主社区的构成，以解释北美东部红翼寄生虫的水平低，因为仅卵子排斥不可能解释这一广泛的地理趋势。拒绝卵的行为不能解释我们东部人口中红翼上没有寄生虫。或者，我们建议东部地区的牛鸟偏爱其他寄主，以及东部的牛鸟数量少，这可能解释了寄生虫的缺乏。未来的研究还应探索牛鸟和寄主的密度以及寄主社区的构成，以解释北美东部红翼寄生虫的水平低，因为仅卵子排斥不可能解释这一广泛的地理趋势。