Grim Tomás, Kleven Oddmund, Mikulica Oldrich
Laboratory of Ornithology, Palacký University, Tr. Svobody 26, CZ-771 46 Olomouc, Czech Republic.
Proc Biol Sci. 2003 Aug 7;270 Suppl 1(Suppl 1):S73-5. doi: 10.1098/rsbl.2003.0017.
One of the great evolutionary puzzles is why hosts of parasitic birds discriminate finely against alien eggs, but almost never discriminate against parasitic chicks. A theoretical model has shown that an adaptive host response to alien eggs can be based on learning. However, learned nestling discrimination is too costly to be favoured by selection in hosts of evicting parasites, such as the European cuckoo (Cuculus canorus). Indeed, parasitic chick rejection has never been reported for any European cuckoo host species. As learned nestling discrimination is maladaptive, one can expect that a viable alternative for hosts would be to use discrimination mechanisms not involving learning and/or recognition. We suggest that hosts may starve and desert cuckoo chicks that require higher amounts of food than an average host brood at fledging (i.e. feeding rates to a parasite are outside the normal range of host behaviour in unparasitized nests). Our observations of the reed warbler (Acrocephalus scirpaceus) at parasitized nests indicate that such behaviour could possibly work in this host species.
进化过程中的一大谜题是，为何寄生鸟类的宿主能够精准地识别并排斥外来的鸟蛋，但几乎从不对外来的雏鸟进行排斥。一个理论模型表明，宿主对外来鸟蛋的适应性反应可能基于学习。然而，对于像欧洲杜鹃（Cuculus canorus）这样会驱逐寄生虫的宿主来说，通过学习来辨别雏鸟的方式成本过高，因而无法在自然选择中占据优势。事实上，从未有过任何关于欧洲杜鹃宿主排斥寄生雏鸟的报道。鉴于通过学习来辨别雏鸟具有不适应性，我们可以预期宿主的一种可行替代方案是采用不涉及学习和/或识别的辨别机制。我们认为，宿主可能会让那些在羽翼丰满时比正常宿主雏鸟需要更多食物的杜鹃雏鸟挨饿并弃养（即对寄生虫的喂食速度超出了未被寄生巢穴中宿主行为的正常范围）。我们对被寄生巢穴中的芦苇莺（Acrocephalus scirpaceus）的观察表明，这种行为在该宿主物种中可能行得通。
