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By  Andrea Wild 31 May 2024 3 min read

Key points

  • Scientists have long suspected interacting species can drive evolutionary changes in each other, which is known as coevolution.
  • Speciation occurs when a species splits into two or more new species, generating biodiversity.
  • Bronze-cuckoos have revealed evidence confirming that coevolution can lead to speciation.

It’s easy to imagine how cuckoos and their hosts might influence each other’s evolution.

Cuckoos lay their eggs in the nests of other birds: their hosts. They win an advantage if they can disguise their eggs and chicks so the host birds will accept them. Host birds are obviously better off if they can recognise the cuckoo egg or hatchling and kick it out.

There’s an evolutionary arms race between the cuckoos and their hosts. And it can lead to new species arising.

A Superb Fairy-wren bringing food to a Horsfield’s Bronze-cuckoo fledgling. ©  Mark Lethlean

Speciation gives rise to biodiversity

Speciation happens when one species splits into two or more new species. There are many ways it can occur, such as when species are isolated on islands and gradually evolve differences from each other.

Scientists have long suspected that interacting species can drive evolutionary changes in each other that lead to speciation. This is known as coevolution.

Scientists believe coevolution to be important in driving biodiversity on Earth. It could explain why there are millions of different species, instead of thousands. But evidence for coevolution leading to speciation has been hard to find - until now.

Coevolution in cuckoos

Dr Clare Holleley is an evolutionary biologist at the Australian National Wildlife Collection. She worked with a team of researchers to test the idea that coevolution drives evolution of new species.

"To do this, we studied cuckoos and their hosts. They can drive coevolutionary changes in each other, such as cuckoos mimicking their host’s eggs or chicks, and hosts evolving the ability to detect and evict cuckoo eggs or chicks," Clare said.

"This effect is stronger if the cuckoo species is highly parasitic, meaning the presence of a cuckoo chick results in the death of the host’s young."

The researchers compared different cuckoo species. They found greater rates of speciation among highly parasitic cuckoo species than those which were less parasitic. This is evidence that coevolution of cuckoos with their hosts drives biodiversity.

Coevolution drives speciation, which drives biodiversity


The researchers zeroed in on bronze-cuckoos, which are highly parasitic. Their chicks mimic both the appearance and calls of their hosts’ chicks.

They looked closely at the genes and appearance of a bronze-cuckoo species that parasitises two different host species. They found the cuckoo chicks look like the chicks of the host birds their parents prefer.

What’s more, as adults these cuckoos have distinct plumage and calls that help males and females that specialise in the same host recognise each other and breed together. Specialising in a particular host sets these cuckoos on the path to becoming different species.

"What's going on here is that host parents reject cuckoo nestlings, which selects for mimicry and host specialisation in cuckoos, driving speciation through coevolution," Clare said.

"This finding is significant in evolutionary biology. It shows that coevolution between interacting species increases biodiversity."

A clutch of Fairy Gerygone eggs (speckled) with a Little Bronze-Cuckoo egg (brown) in the Australian National Wildlife Collection. The cuckoo's egg is a similar size and shape to the host's eggs. The colour is less important because the nest is dome-shaped and dark inside.

How museums and collections help

The ability to do undertake this research was many years in the making.

"A critical step was our breakthrough in extracting DNA from eggshells in historical collections and sequencing it for genetic studies," Clare said.

"We then combined two decades of behavioural fieldwork with DNA analysis of specimens of eggs and birds held in museums and collections like our Australian National Wildlife Collection."

The paper ‘Coevolution with hosts underpins speciation in brood-parasitic cuckoos’ was published in the journal Science. It’s by a team of authors from CSIRO, the Australian National University, University of Melbourne and University of Cambridge.

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