On speaking, writing and inspiration

eggs unlike their own? One possibility is that they are new hosts, with insufficient time to have evolved defences. The other is that defences are costly — for example, egg rejection may involve recognition errors — and parasitism levels may be below the threshold where it pays to sustain these costs. Even among host species that have evolved egg rejection, individuals are less likely to reject eggs when cuckoos are locally scarce. Cuckoo tricks will also reflect a trade-off between costs and benefits. A cuckoo nestling that kills host young rids itself of competition once food arrives but, without the assistance of other nestlings, it now has to work harder to solicit provisioning. This involves some remarkable tricks; common cuckoos have rapid begging calls that sound like many host young, and Horsfield’s hawkcuckoos have wing patches that simulate extra gaping mouths. Nevertheless, hosts sometimes reject cuckoo chicks. Why is this not more frequent, especially in cases where the cuckoo is so different in appearance from the host’s own young? Further molecular genetic analyses will enable us to calibrate when cuckoos and their hostraces evolved from a common ancestor and to resolve the time course of these marvellous interactions.