A Multifactor Study of Clutch Size Determination in the Eastern Kingbird

目前，许多生物的数量正出现惊人的下降。在鸟类中，那些在北美筑巢、在新世界热带地区（新热带地区）繁殖和过冬的鸟类数量正在急剧下降。下降的原因尚不清楚。要了解种群遭受苦难的原因，有必要记录(a)成年、(b)巢穴和(c)独立幼鸟的生存模式。此外，了解控制和/或限制繁殖的因素是至关重要的。窝的大小（蛋的数量/巢的数量）是鸟类繁殖的主要变量，在其他条件相同的情况下，产蛋最多、幼崽最多的个体应该占主导地位。因此，很难理解为什么某些鸟类的蛋数小于最多产的窝数（幼鸟/巢的数量）。了解这一现象很重要，因为繁殖过程对种群动态有直接影响，因此对保护措施也有直接影响。这个项目提出了五个假说，来解释为什么窝的大小通常小于最多产的窝的大小。东部王鸟（Tyrannus Tyrannus）是一种长途候鸟，在亚马逊地区过冬，通常只产3个蛋，但有时可以养4到5个幼鸟。将进行实验，以确定王鸟的小窝是否与以下因素有关：(a)食物供应不足，(b)巢穴的捕食压力大，(c)与亲代生存的权衡（即，饲养许多幼鸟会增加成年鸟的死亡率），(d)与后代生存的权衡（即，来自大窝的幼鸟体型较小，存活率较低），或者，(e)由于种内幼虫寄生（即由另一只雌蜂向已完工的巢中添加卵）而导致的卵数不可预测的增加。本研究的结果将有助于澄清“次最大”繁殖之谜，并提供有关东部Kingbird的生存、繁殖成功和种群稳定性的具体信息。

Many organisms are currently showing alarming decline in abundance. Among birds, species that build open-cup nests, breed in North America and winter in the New World tropics (Neotropics) are showing the sharpest declines. The reasons for the declines are not known. To understand why populations are suffering it is necessary to document survival patterns of (a) adults, (b) nests, and, (c) independent young. In addition, it is crucial that we understand what factors control and/or limit reproduction. Clutch size (number of eggs/nest) is the primary reproductive variable among birds, and all else being equal, individuals that lay the largest clutches and produce the most young should dominate. It is therefore hard to understand why clutch size in some species of birds is smaller than the most productive brood size (number of young/nest). Understanding this phenomenon is important because reproductive processes have a direct impact on population dynamics, and therefore conservation practices. This project proposes to examine five hypotheses that have been put forth to explain why clutch sizes are often smaller than the most productive brood sizes. The Eastern Kingbird (Tyrannus tyrannus), a long distance migrant that winters in Amazonia, typically lays just three eggs but can raise 4 and sometimes 5 young. Experiments will be conducted to determine if small clutch size in kingbirds is related to (a) low food availability, (b) high predation pressure on nests, (c) trade-offs with parental survival (i.e., raising many young increases adult mortality), (d) trade-offs with offspring survival (i.e., young fledged from large broods are smaller and have lower survival rates), or, (e) unpredictable increases in clutch size due intraspecific brood parasitism (i.e., the addition of eggs to a completed nest by a different female). The results of this study should help to clarify the puzzle of "submaximal" reproduction and also provide specific information on survival, reproductive success and population stability in the Eastern Kingbird.