Asian Koels do not copy their hosts’ eggs – but may be shadowing the wrong species
Objective analysis across Asia finds no evidence that koels match their eggs to those of the birds they parasitise most often. challenging long-held assumptions about the brood parasite.
![Asian Koel [male]](https://live.staticflickr.com/65535/51584931720_3dc13466c3_b.jpg)
A classic brood parasite under the microscope
The Asian Koel is one of the most widespread and well-known brood parasites in Asia, laying its eggs in the nests of other birds and leaving them to raise its young. For more than a century, naturalists have assumed that koel eggs closely mimic those of their most frequent host, the House Crow. To the human eye, the resemblance can seem convincing. But this new study, drawing on hundreds of eggs from museum collections across Asia, tells a more complicated story.
Using objective measurements and bird-based visual models rather than human judgement, the researchers set out to test two key ideas: whether koels lay different egg “types” depending on the host they parasitise, and whether koel eggs genuinely mimic the eggs of any particular host species.
No evidence for host-specific egg types
Across three subspecies of Asian Koel and multiple regions - including China, South Asia, and Southeast Asia - the answer to the first question was clear. Koels do not produce host-specific egg races. Eggs laid in crow nests looked essentially the same as those laid in myna or shrike nests once colour, brightness, pattern and size were analysed objectively.
This finding reinforces earlier field studies and suggests that, unlike some other cuckoos, the Asian Koel has not evolved different egg designs tailored to individual hosts.
The House Crow assumption falls apart
Perhaps the most striking result concerns the House Crow. Despite being one of the koel’s most frequently used hosts, koel eggs were not especially similar to crow eggs when viewed through avian visual models. In most regions, the eggs were clearly distinguishable in colour and brightness, meaning a crow with good egg-recognition abilities should, in theory, be able to spot the intruder.
This directly contradicts long-standing claims, based largely on human perception, that koel eggs are convincing crow mimics.
An unexpected look-alike: the red-billed blue magpie
Instead, koel eggs were consistently most similar in colour to those of the Red-billed Blue Magpie - a species that is parasitised far less often, at least in historical collections. In all regions examined, colour differences between koel eggs and magpie eggs were small enough to be difficult for birds to distinguish under ideal conditions.
In terms of spotting patterns, koel eggs also showed closer similarities to other corvids, such as jungle crows, than to House Crows. This raises the intriguing possibility that koel eggs may reflect past evolutionary interactions with hosts that are now rare, overlooked, or simply better at rejecting foreign eggs.
Egg size tells the same story
Egg size and shape added further weight to the conclusion. Koel eggs were broadly consistent in size regardless of host, and often differed significantly from the eggs of their most commonly parasitised species. This again points to a lack of fine-tuned mimicry aimed at any single host.
Why hasn’t mimicry evolved?
The authors suggest several reasons why the Asian Koel may not need strong egg mimicry. Many of its hosts show low rates of egg rejection, even when foreign eggs are obvious. Some hosts may be constrained by egg size, unable to eject a large koel egg without damaging their own clutch. In other cases, heavy multiple parasitism may overwhelm hosts, making rejection risky or ineffective.
There is also evidence that some hosts can successfully raise koel chicks alongside their own, reducing the evolutionary pressure to develop strong defences - and in turn reducing the pressure on koels to perfect their deception.
A generalist with a flexible strategy
Taken together, the findings paint the Asian Koel as a true generalist. Rather than closely matching any one host, it appears to rely on a broadly acceptable egg design, combined with hosts that are poor or inconsistent at rejecting foreign eggs. The resemblance to magpie eggs may be a relic of earlier coevolutionary battles, or a side-effect of targeting hosts with similar egg traits.
The study highlights the importance of testing mimicry through the eyes of birds, not humans, and shows that even the most familiar brood parasites can still surprise us.
January 2026
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