Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana-Champaign, Champaign, Illinois, USA.
Illinois Natural History Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign, Champaign, Illinois, USA.
Ecology. 2023 Mar;104(3):e3925. doi: 10.1002/ecy.3925. Epub 2023 Jan 6.
Nest-sharer avian brood parasites do not evict or otherwise kill host chicks, but instead inflict a range of negative effects on their nestmates that are mediated by interactions between the parasite and host life history traits. Although many of the negative fitness effects of avian brood parasitism are well documented across diverse host species, there remains a paucity of studies that have examined the impacts of parasitism across the entirety of host ontogeny (i.e., from when an egg is laid until independence). More specifically, few studies have examined the impact of brood parasitism on the pre- and post-fledging development, physiology, behavior, and survival of host offspring. To help fill this knowledge gap, we assessed the effects of brood parasitism by Brown-headed Cowbirds (Molothrus ater) across the ontogeny (incubation, nestling, and post-fledging period) of nine sympatrically breeding host species in central Illinois, USA; due to sample sizes, impacts on the post-fledging period were only examined in two of the nine species. Specifically, we examined the impact of brood parasitism on ontogenetic markers including the embryonic heart rate, hatching rate, nestling period length, nest survival, and offspring growth and development. Additionally, in species in which we found negative impacts of cowbird parasitism on host nestmate ontogeny, we examined whether the difference in adult size between parasites and their hosts and their hatching asynchrony positively predicted variation in host costs across these focal taxa. We found that costs of cowbird parasitism were most severe during early nesting stages (reduction in the host clutch or brood size) and were predicted negatively by host size and positively by incubation length. In contrast, we only found limited costs of cowbird parasitism on other stages of host ontogeny; critically, post-fledging survival did not differ between host offspring that fledged alongside cowbirds and those that did not. Our findings (i) highlight the direct costs of cowbird parasitism on host fitness, (ii) provide evidence for when (the stage) those costs are manifested, and (iii) may help to explain why many anti-cowbird defenses of hosts have evolved for protection from parasitism during the laying and incubation stages.
巢寄生鸟类不会驱逐或杀死宿主雏鸟，而是通过寄生虫与宿主生活史特征之间的相互作用，对其巢伴产生一系列负面影响。尽管在各种宿主物种中都有大量关于鸟类巢寄生负面适合度效应的记录，但仍缺乏研究在宿主整个发育过程中（即从产卵到独立）检查寄生的影响。更具体地说，很少有研究检查巢寄生对宿主后代的出雏前和出雏后发育、生理、行为和生存的影响。为了帮助填补这一知识空白，我们评估了棕色头牛鹂（Molothrus ater）在 9 种同域繁殖的宿主物种的发育过程中（孵化、雏鸟和离巢后时期）的寄生作用；由于样本量的限制，仅在 9 种物种中的两种中检查了对离巢后时期的影响。具体来说，我们检查了巢寄生对包括胚胎心率、孵化率、雏鸟期长度、巢存活率以及后代生长和发育在内的发育标记物的影响。此外，在我们发现牛鹂寄生对宿主巢伴发育有负面影响的物种中，我们检查了寄生虫与其宿主之间的成年体型差异以及孵化的异步性是否会积极预测这些焦点类群中宿主成本的变化。我们发现，牛鹂寄生的成本在早期筑巢阶段最为严重（宿主卵或巢的数量减少），并与宿主大小呈负相关，与孵化期呈正相关。相比之下，我们只发现牛鹂寄生对宿主发育的其他阶段的影响有限；至关重要的是，与未与牛鹂一起离巢的宿主后代相比，离巢后幸存率没有差异。我们的研究结果（i）突出了牛鹂寄生对宿主适应性的直接成本，（ii）提供了宿主成本表现的时间（阶段）的证据，（iii）可能有助于解释为什么宿主为保护自己免受产卵和孵化阶段的寄生而进化出了许多反牛鹂防御机制。
