Female lineages of European cuckoos lay eggs with colorful shells and intricate patterns specialized to deceive more than 100 avian hosts into fostering their chicks.

Parasitic cuckoo eggshells show at least 15 distinct colors and patterns. (Image courtesy of Justin Merondun.)

Image courtesy of Justin Merondun

Parasitic cuckoos are truly remarkable birds. Not only do they they lead peculiar social lives, but in doing so, they show us how the process of evolution actually works. For example, many cuckoo species rely upon other bird species to raise their chicks for them; an unusual life history trait known as “brood parasitism”.

As you might expect, the host birds are not very happy about this deception and blatant theft of their efforts and resources to benefit another species’ chicks, so the host birds learn to quickly detect and discard cuckoo eggs from their nests, leading to evolution of even more cleverly disguised cuckoo eggs. In response, cuckoos have evolved a number of fascinating methods for preventing their eggs from being detected, such as altering their eggshell colors and patterns so they more closely match those of their hosts. This, in short, is an evolutionary arms’ race.

Moves and countermoves

“Cuckoos are simply irresistible for an evolutionary biologist,” said the lead author of the recently published study, evolutionary bioinformatician Justin Merondun, a postdoctoral research scientist with the Ludwig-Maximilians-Universität München (LMU) and with the Agricultural Research Service at the U.S. Department of Agriculture (USDA). Dr Merondun’s research is primarily focused on connecting DNA sequencing data to observable phenotypes and understanding the evolutionary processes that maintain variation.

“They are locked in a perpetual Red Queen race with not one, but multiple host species, and they have evolved a suite of adaptive traits to stay ahead,” Dr Merondun told me in email.

The “Red Queen race” is a metaphor in evolutionary biology that originally came from Lewis Carroll’s 1871 fantasy novel, Through the Looking-Glass, where the characters run as fast as they possibly can yet still only manage to stay in the same place.

“Matching egg mimicry is the famous one, but even the grey and rufous female-specific plumage colours may help them slip past host defenses.” [Read more about female cuckoo plumage color variants here.]

European cuckoos, Cuculus canorus, sometimes known as the common cuckoo, have a long blunt-ended tail, slate grey upperparts and cream colored underparts with grey barring, mimicking a sparrowhawk, a common predator of small songbirds. European cuckoos are insectivorous, specializing on hairy caterpillars that many birds refuse to eat, thereby benefitting farmers and orchardists.

European cuckoos live in open areas, and are a widespread migrant to Eurasia, after spending their winters in Africa. Throughout Europe, the springtime arrival of the European cuckoo is heralded by its loud distinctive “coo coo” calls, immortalized by innumerable traditional Black Forest cuckoo clocks.

Many cuckoo species, including European cuckoos, are brood parasites that lay eggs adapted to match the eggshell colors and patterns produced by more than 100 avian host species. Their eggs come in a veritable rainbow of colors from bright blue to white, or greenish, and are often decorated with a variety of elaborate patterns, such as speckles or squiggles. The vast array of European cuckoo eggshell styles suggests that there are distinct female lineages that evolved to produce specialized eggs whose colors and patterns closely mimic those of a specific host species.

“The question was: How can a cuckoo reliably pass on the right egg color?” said the senior author of the recently published study, evolutionary geneticist Jochen Wolf, who is Head of the Department of Evolution at LMU, and a Research Fellow at the Max-Planck Institute for Biological Intelligence.

“After all, a female might not know what her own egg looks like,” Professor Wolf pointed out to me in email.

To better answer these questions, Dr Merondun, Professor Wolf and collaborators analyzed the genomes from 298 European cuckoos and 50 genomes from its eastern sister species (its closest living relative), the Oriental cuckoo, Cuculus optatus. Together, these two cuckoo species produce eggs with 15 distinct eggshell colors and patterns.

With all the birds’ genome data in-hand, Dr Merondun, Professor Wolf and collaborators identified which gene variants corresponded to which eggshell colors. They found that the base color for European cuckoo eggshells is encoded almost exclusively on the female sex chromosome – the W chromosome – and on the mitochondrial genome.

“What is especially striking, considering both egg color and plumage [color], is that both of these traits map onto female-inherited genetic sequences like the W chromosome and mitochondrial DNA,” Dr Merondun told me in email.

Dr Merondun also told me that the presence of these genes on the W chromosome ensures direct inheritance from mothers to daughters without recombination or reshuffling from males – much like the Y chromosomes in mammals – so daughters produce the same base eggshell color as their mothers.

Dr Merondun, Professor Wolf and collaborators also found that the eggshell patterning genes are mostly autosomal (non-sex chromosomes) and come from both European cuckoo parents.

This is in stark contrast to what they saw oriental cuckoos, whose eggshell color and patterning genes were mainly located on autosomes in both parents.

How do female cuckoos lay their eggs in the appropriate host species nest?

“This is tricky question,” Professor Wolf explained in email. “There is some evidence that the females imprint on the host that raised them, but the match is clearly not absolute. There is still a lot of variation of where eggs are deposited, which after all is the precondition for host race shifts in the first place.”

“And then, some hosts are just much more abundant and accessible in certain areas, which may also facilitate adaptation in the absence of strict host choice,” Professor Wolf added.

This observation provides further support for the idea that evolutionary lineages of female European cuckoos do exist based on their eggshell colors and patterns, with each of them adapted to a specific avian host species. But laying eggs exclusively in the “correct” host nests could lead to new cuckoo species, so how do these distinct lineages avoid becoming separate species?

In European cuckoos, host specificity appears not to promote speciation of lineages because females freely mate with any male without losing their special adaptation to their specific host because it is carried mainly on her sex-chromosome and her mitochondrial genome, Professor Wolf told me in email. This preserves the flow of genetic information across the rest of the genome, thereby ensuring they remain one species.

“And that is precisely what we observe: The huge cuckoo population throughout Eurasia is almost genetically identical within DNA regions inherited from both parents,” Professor Wolf said.

Tragically, despite these birds’ remarkable adaptability to their hosts, European cuckoo populations are in trouble, declining in many regions of Eurasia by as much as 65% since the 1980s (ref), due to habitat fragmentation and loss, and to climate change.

“Without intact habitats, this fascinating system risks vanishing on our doorstep,” Professor Wolf warned.

Sources:

Justin Merondun, Frode Fossøy, Swetlana Meshcheryagina, Phil Atkinson, Gennadiy Bachurin, Victor Bulyuk, Viktar Fenchuk, Mikhail Golovatin, Chris Hewson, Marcel Honza, Mikhail Markovets, Csaba Moskát, Gregory L. Owens, Petr Procházka, Yaroslav Red’kin, Jarkko Rutila, Michal Šulc, Kasper Thorup, Bård G. Stokke, Wei Liang, and Jochen B. W. Wolf (2025). Genomic architecture of egg mimicry and its consequences for speciation in parasitic cuckoos, Science 390(6772):527-532 | doi:10.1126/science.adt9355

Michael D. Sorenson and Claire N. Spottiswoode (2025). How common cuckoos adapt to multiple hosts, Science 390(6772) 440-441 | doi:10.1126/science.aec1973

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