Bronze cuckoos: the art of deceit

As I explored in my last post, Common Cuckoos use strategies such as egg mimicry, speed and timing of laying to deceive their hosts. Nick Davies and Mike Brooke’s research goes on as they further investigate another arms race. Bronze-cuckoos, the Common Cuckoo’s Australian cousins, have evolved some extra tricks, and their hosts: the splendid fairy-wrens, are able to reject the cuckoo's chicks, as well as its eggs.

The common cuckoo’s chicks appear very different to its host’s chicks. Indeed, whereas young reed warblers have black skin and yellow gapes, a newly hatched common cuckoo bears hardly any resemblance to its host’s chicks, with pink skin, a bright orange gape and larger in size. One might think that reed warblers would use these cues to reject the cuckoo chick, but they don’t. Perhaps chick mimicry is not needed because the cuckoo chick pushes the reed warbler’s eggs out of the nest before the parents get the chance to compare them. However, that is not always the case; it is not uncommon for cuckoos to eject the chicks after they have hatched.

In the parkland around Canberra, Horsfield’s bronze-cuckoos and the splendid fairy-wrens are fighting an even more advanced arms race. Indeed, unlike Common cuckoos, bronze-cuckoo chicks have developed quite a striking resemblance with their host’s chicks, not only in skin colour and mouth colour but their begging calls also sound very similar. Even more fascinating is the host’s ability to reject the cuckoo chicks despite the cuckoo’s immaculate mimicry. We are left with one puzzling question: Why do Common cuckoos not do the same?

There are at least three plausible reasons for this. The first one is that the arms race between bronze-cuckoos and their hosts is older than the one between Common Cuckoos and reed warblers, giving them more time to evolve. This would make sense as DNA tests have shown that the common ancestor of several races of Common cuckoo date back to around 80 000 years ago. Bronze cuckoos, on the other hand, may have been parasitising fairy-wrens for millions of years.

The second possibility is that bronze-cuckoos have defeated their hosts at the egg level. This means that the hosts were unable to tell the cuckoo’s eggs apart from their own and moved on to trying to develop chick defences instead. How were bronze-cuckoos able to defeat their hosts at the egg level? One likely explanation is due to the host’s dark domed nests. This would give the host a big disadvantage when trying to spot the cuckoo’s eggs. To make the

A third possible reason as to why common cuckoo’s do not adopt a similar strategy may be related to Europe’s temperate climate. European hosts are in more of a hurry to reproduce, as reed warblers begin to move south in late August. By the time the chicks have hatched, it’s getting late in the breeding season, therefore starting another nest may not be worth it. 

The elaborate arms race between the bronze-cuckoo and the superb fairy wren have offered researchers explanations and answers about the Common Cuckoo’s behaviour no one could have dreamt of finding…