Nest defense is an adaptive strategy used by adults to increase survival of independent young. However, the risk adults face by defending young from predators represents an important trade-off and parental investment theory predicts adults will modulate nest defense based on the value of offspring. We used a model of a black rat snake, a potential nest predator on our study site, to quantify nest defense in a population of gray catbirds in eastern Pennsylvania during the breeding seasons of 2017 and 2018. Specifically, we evaluated (1) the difference in nest defense between male and female parents, (2) the relationship between nest defense and nest success, and (3) how variation in nest defense was related to factors associated with offspring value. Although male catbirds tend to be larger than females, they did not defend their nests more aggressively than female catbirds. Response to the model predator we used was also not different between individuals whose nests were successful compared with individuals that failed. Although nests that contained more and older young were not defended more aggressively, our analyses did show that adult catbirds defended young that were laid earlier in the season more intensely than those laid later in the breeding season. In birds, earlier clutch initiation is associated with an increased chance of recruitment into the breeding population. Catbirds, therefore, follow predictions of parental investment theory because they defended offspring of higher value more aggressively. Defense of young from predators is an important component of parental care. Nest defense represents a trade-off because although it should increase reproductive success by increasing offspring survival, it presents a risk to adults, thus lowering their survival. We studied variation in nest defense of gray catbirds in relation to offspring value. Although nest defense was not related to nest success, we did find that adult catbirds defended nests that were produced earlier in the year more aggressively than later nests. In birds, the timing of reproduction is linked to prospects of juvenile survival in such a way that offspring produced early in the breeding season are more likely to survive and recruit into the breeding population. As predicted by parental investment theory, catbirds defended young that represented a higher increase in fitness more aggressively.

中文翻译：

---

后代值对灰猫鸟（Dumetella carolinensis）筑巢防御强度变化的影响

巢防御是成年人用来增加独立年轻人生存的适应性策略。然而，成年人保护幼崽免受掠食者侵害所面临的风险代表了一个重要的权衡，父母投资理论预测成年人将根据后代的价值调节巢穴防御。我们使用黑鼠蛇模型（我们研究地点的潜在巢穴捕食者）来量化 2017 年和 2018 年繁殖季节宾夕法尼亚州东部灰猫鸟种群的巢穴防御能力。 具体而言，我们评估了 (1) 差异在雄性和雌性父母之间的巢穴防御中，（2）巢穴防御与巢穴成功之间的关系，以及（3）巢穴防御的变化如何与与后代价值相关的因素相关。虽然雄性猫鸟往往比雌性大，它们并没有比雌性猫鸟更积极地捍卫自己的巢穴。与失败的个体相比，巢成功的个体对我们使用的模型捕食者的反应也没有不同。虽然包含更多和年长幼鸟的巢穴并没有得到更积极的保护，但我们的分析确实表明，成年猫鸟对本季早些时候产下的幼鸟的保护力度比繁殖季节后期产的幼鸟更强烈。在鸟类中，较早的离合器开始与增加繁殖种群的机会有关。因此，猫鸟遵循父母投资理论的预测，因为它们更积极地保护价值更高的后代。保护幼崽免受掠食者的侵害是父母照顾的重要组成部分。巢防御是一种权衡，因为虽然它应该通过增加后代存活率来增加繁殖成功率，但它给成年人带来了风险，从而降低了他们的存活率。我们研究了与后代价值相关的灰猫鸟巢穴防御的变化。尽管筑巢防御与筑巢成功无关，但我们确实发现，成年猫鸟对年初产生的巢穴的防御能力比较晚的巢穴更具侵略性。在鸟类中，繁殖的时间与幼鸟的存活前景有关，在繁殖季节早期产生的后代更有可能存活并招募到繁殖种群中。正如父母投资理论所预测的那样，猫鸟会更积极地保护健康程度更高的幼鸟。