Female mass in most altricial birds reaches its maximum during breeding at egg laying, which coincides temporally with the fertile phase when extra-pair paternity (EPP) is determined. Higher mass at laying may have two different effects on EPP intensity. On the one hand, it would lead to increased wing loading (body mass/wing area), which may impair flight efficiency and thereby reduce female’s capacity to resist unwanted extra-pair male approaches (sexual conflict hypothesis). On the other hand, it would enhance female condition, favouring her capacity to evade mate guarding and to search for extra-pair mates (female choice hypothesis). In both cases, higher female mass at laying may lead to enhanced EPP. To test this prediction, we reduced nest building effort by adding a completely constructed nest in an experimental group of female pied flycatchers (Ficedula hypoleuca). Our treatment caused an increase in mass and thereby wing loading and this was translated into a significantly higher EPP in the manipulated group compared with the control group as expected. There was also a significant negative relationship between EPP and laying date and the extent of the white wing patch, an index of female dominance. More body reserves at laying mean not only a higher potential fecundity but a higher level of EPP as well. This interaction had not previously received due attention but should be considered in future studies of avian breeding strategies. While most research has been focused on determining possible criteria for extra-pair mate choice by females, less effort has been made on establishing if female traits are related to EPP and its intensity. One such trait is mass at laying which attains its highest level for breeding females of altricial birds. Our study indicates that a higher mass during the fertile phase not only has implications for female fecundity and predation risk but also for EPP in the resulting brood as more mass means a higher EPP.

中文翻译：

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受精阶段雌性质量的实验性增加导致花斑捕蝇器 Ficedula hypoleuca 中更高水平的额外亲子关系

大多数 altricial 鸟类的雌性质量在产卵繁殖期间达到最大值，这与确定双亲 (EPP) 时的生育期在时间上一致。较高的铺设质量可能对 EPP 强度产生两种不同的影响。一方面，它会导致机翼载荷（体重/机翼面积）增加，这可能会损害飞行效率，从而降低雌性抵抗不需要的额外雄性接近的能力（性冲突假说）。另一方面，它会改善女性的状况，有利于她逃避配偶保护和寻找额外配偶的能力（女性选择假设）。在这两种情况下，产蛋时较高的雌性体重可能会导致 EPP 增强。为了测试这个预测，我们通过在雌性花斑捕蝇器 (Ficedula hypoleuca) 的实验组中添加一个完全构建的巢来减少巢的建造工作。我们的治疗导致质量增加，从而导致翼载荷增加，这转化为操纵组与对照组相比显着更高的 EPP，正如预期的那样。EPP 与产蛋日期和白翅斑的范围（雌性优势指数）之间也存在显着的负相关。更多的身体储备不仅意味着更高的潜在繁殖力，还意味着更高的 EPP 水平。这种相互作用以前没有得到应有的重视，但应在未来的鸟类育种策略研究中加以考虑。虽然大多数研究都集中在确定雌性选择额外配对的可能标准上，确定女性特征是否与 EPP 及其强度相关的努力较少。一种这样的特性是产蛋时的质量，它达到了繁殖候鸟雌性的最高水平。我们的研究表明，在受精阶段较高的质量不仅对雌性繁殖力和捕食风险有影响，而且对产生的育雏中的 EPP 也有影响，因为更多的质量意味着更高的 EPP。