Most studies of factors that limit the number of eggs that birds lay have focused on the disadvantages of having too many young to feed. Less attention has been paid to the consequences of having a large number of eggs to incubate. The incubation‐capacity hypothesis proposes that females lay as many eggs as they can effectively incubate. We tested this hypothesis in 2018 in a montane population of Mountain Bluebirds (Sialia currucoides). Most females in this population lay five or six eggs; clutches of seven occur, but are rare. We added eggs to some nests, forcing females to incubate seven eggs, while leaving other nests as controls. Among females completing incubation, those with enlarged clutches hatched as many eggs as did control females, and did so in the same amount of time. This was despite an extended period of unusually cold and often wet weather that occurred when many females were incubating. Our results firmly reject the suggestion that females typically lay no more than six eggs because they cannot effectively heat seven eggs. One or more other factors must limit clutch size. One possible factor is suggested by the fact that during the period of inclement weather, more females with enlarged clutches than control females appeared to abandon nests before completing incubation. Because larger clutches require more energy to incubate, females with seven eggs during energetically stressful conditions could more quickly reach the point where they lack sufficient energy for both incubation and self‐maintenance. Such conditions may occur frequently enough in the montane environment that, on average, laying seven eggs results in reduced lifetime reproductive success.

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什么限制离合器尺寸？高海拔山区蓝鸟种群孵化能力假设的检验

关于限制鸟类产卵数量的因素的大多数研究都集中在饲养太多幼鸟的弊端上。对于大量卵孵化的后果，人们的关注较少。孵化能力假说提出，雌性产卵的数量要尽可能多。我们在2018年对山地蓝鸟（Sialia currucoides）。该种群中的大多数雌性产五到六个卵。有七个离合器，但很少见。我们在一些巢中添加了卵，迫使雌性孵化了七个卵，而其他巢则作为对照。在完成孵化的雌性中，带有较大离合器的雌性孵化的卵数与对照雌性一样多，并且孵化的时间相同。尽管有许多雌性在孵化时出现了异常寒冷和经常潮湿的天气，但时间长了。我们的结果坚定地拒绝了这样的建议：雌性通常不能多产六个卵，因为它们不能有效地加热七个卵。一个或多个其他因素必须限制离合器的尺寸。一个可能的因素是，在恶劣天气期间，在完成孵化之前，拥有更大离合器的雌性比对照组雌性似乎放弃了巢。由于较大的离合器需要更多的能量来进行孵化，因此在精力充沛的条件下有七个卵的雌性可能更快地达到缺乏足够的能量进行孵化和自我维持的地步。这种情况在山区环境中可能经常发生，平均而言，产下七个卵会导致终生生殖成功率下降。