Hatching asynchrony in birds is considered an adaptation to facilitate brood reduction because under conditions of food scarcity, the smallest nestling usually dies soon after hatching, thereby minimizing parental effort. However, in species with extreme hatching asynchrony, the last hatchlings paradoxically experience a very low probability of survival and death can take so long that it can hardly be considered an adaptation. Here, we propose and experimentally tested a new adaptive hypothesis explaining the brood reduction paradox, namely the "Male Manipulation Hypothesis". Our hypothesis suggests that by inducing asynchronous hatching, females increase the feeding requirements of the brood, which will induce males to increase provisioning effort. In addition, females may extend the period of male manipulation by feeding the smallest nestling just enough to sustain life. Our study showed that male common blackbirds ( Turdus merula) increased their effort (i.e., number of food items per hour) in experimental asynchronous broods compared to synchronous broods, while females reduced their contribution, as predicted by the hypothesis.
在食物匮乏的条件下，鸟类的不同步孵化被认为是对窝雏数减少的一种促进和适应，因为最后孵出的雏鸟往往在孵出后不久即死亡，从而降低了双亲投入的进一步损失。然而，在许多具有极端不同步孵化的鸟种中，最后孵出的雏鸟尽管存活的机会很低，但从孵出后到死亡却持续很长的时间，这几乎无法用上述“窝雏数减少适应假说”来解释。为此，该研究提出并通过实验验证了一个新的适应性假说，即“雄性操纵假说”。“雄性操纵假说”认为，雌鸟首先通过不同步孵化，来增加窝雏数的喂食需求，促使雄鸟在雏鸟喂食中更多的投入；其次，雌鸟通过不断维持最后孵出雏鸟的存活来进一步操纵雄鸟的雏鸟喂养投入。我们对欧亚乌鸫（ Turdus merula）的实验研究表明，与同步孵化的窝雏相比，在实验控制条件下通过增加不同步孵化的雏鸟数，欧亚乌鸫的雄鸟，显著增加了雏鸟喂养的投入（如每小时喂养雏鸟的食物数量等），雌鸟则减少了它们喂养雏鸟的努力。而这，正是我们的这一新假说所预期的。.
Keywords: Brood reduction; Food allocation; Hatching asynchrony; Male manipulation hypothesis; Sexual differences in food allocation; Turdus merula.