Mimicry: The nefarious cuckoo

March 4, 2011 • 7:12 am

Chimps and other primates may show a rudimentary form of morality, but it’s clear that by and large nature is pretty heartless—as you’d expect given the character of natural selection. Nevertheless, who hasn’t felt like reproving the cheetah who brings down, on film, a cute young Thompson’s gazelle?

In class this week, my students were saddened and dismayed by another of nature’s nefarious wonders, the cuckoo—in particular, the common cuckoo (Cuculus canorus).  Here’s one:

The cuckoo has hit on the mother of all adaptations, a phenomenon called brood parasitism.  Instead of spending huge amounts of time building a nest, brooding eggs, and feeding the voracious young until they fledge, the female cuckoo recruits an individual of another bird species to do all the work for her—a full-time babysitter.  That way, the cuckoo can use its time and resources maximizing its reproductive effort without the enormous expense of childcare.

A female cuckoo simply lays one egg in the nest of another species, say a reed warbler or a dunnock, where there is already a clutch of eggs. The cuckoo’s egg then nestles inconspicuously among the others. Inconspicuously? Yes, for the female cuckoo lays an egg that mimics pretty well the other eggs in the nest, so the foster mother can’t easily detect the intrusion.

The curious thing is that within the common cuckoo species, each female lays an egg precisely patterned and color to mimic the eggs in the nest she will parasitize.   And here’s the kicker: within that cuckoo species there are seven different types of females, each laying a different type off egg and each parasitizing only the nests of species producing similarly-colored eggs.  The different “types” of cuckoos that lay different eggs are called gentes.  The gentes are not different species of cuckoos—they are all members of the same species, but with different types of genes that make different types of eggs.  This is an example of a genetic polymorphism (from the Greek meaning “different forms”).   Polymorphisms are not rare in animals and plants: our own species has them, including eye color variants and whether your earwax is wet or dry (a trait based on a single gene).

Here are four of the different types of eggs laid by four cuckoo gentes.  The species that is parasitized is on the left, the mimetic cuckoo egg on the right.  Again, each female lays only one type of egg her whole life.

Here are some actual nests, each containing a single cuckoo egg, indicated by the arrow.  Note that the mimicry is very good but not perfect—a human observer (but not the bird) can pick out slight differences, and the cuckoo egg is often larger:

What happens next is sad, but a remarkable adaptation showing nature in all its red toothiness and clawdom.  The cuckoo chick hatches and proceeds to destroy all its competitors—the other eggs and chicks—leaving it the sole recipient of foster care (click on the “Watch on YouTube” line).

As Attenborough notes in the video, brood parasitism has evolved in many bird species, including the “cuckoo duck” Heteronetta atricapilla.

The European cuckoo’s adaptive habit raises lots of questions.  Why is the foster parent fooled by the mimetic eggs, but can’t seem to recognize a cuckoo chick that is so different from its own?  In some species of brood parasites, like indigobirds, the foster mother can recognize foreign chicks, and so the parasite babies have evolved calls and “mouth gape” patterns (coloration on the babies’ mouths that induce feeding by the mother) that mimic those of the non-foster young.  Another explanation is that the foster mother’s drive to feed whatever chick it sees in its nest outweighs everything else: that is, there’s simply no genetic variation for the foster mother to respond to an alien-looking chick.  While genetic variation is pervasive in nature, it’s not always around when it’s “needed.”  Every case of a parasite or predator victimizing another species, for example, represents an absence (perhaps temporary) of genetic variation in the victim to fully overcome the challenge.

The biggest mystery, though, is how the polymorphism for color pattern is maintained.  How, for example, does a female “know” where to lay its egg?  If it had the genes for producing eggs that mimic reed warblers, and laid its egg in a dunnock nest, that egg would be summarily ejected and its genes would not be passed on.

This problem is overcome by imprinting: a female imprints on the song and appearance of its foster mother, so when it comes time for a cuckoo to lay its own egg, it goes right back to a nest harboring a female on which it’s imprinted.

But what about mating?  Males, after all, also carry genes for egg color and pattern—they just don’t express them.  (If you’re a male human, you carry genes for making breasts and vaginas, but don’t express those either.)  But if a female mates with a male carrying egg-pattern genes different from hers, wouldn’t the eggs that their daughter produce be intermediate, and therefore unable to pass the test of mimicry?

One possible answer is that a female knows to mate only with those males carrying similar egg-pattern genes.  But this isn’t the case. First of all, there’s no way a female can detect a male’s genetic endowment for egg pattern.  But more important, research has shown that female cuckoos mate randomly—they don’t know or care what a male’s “egg genes” are.  So how does the pattern fidelity work?

We’re not sure, but it may involve the birds’ sex chromosomes.  In birds, unlike mammals, it is the female who has two different sex chromosomes, called the Z and the W. In humans males are XY and females are XX, but in cuckoos and other birds (and butterflies), females are ZW and males are ZZ.  A ZW female produces ZW daughters, so the W chromosome, and the genes it carries, are transmitted matrilinearly.  The male makes no genetic contribution to this chromosome.

This, then, is a possible solution to the egg-color polymorphism. If the genes for a specific egg color and shape are carried only on the W chromosome, then in the offspring those genes will not be mixed with any genes from the father.  This will enforce a fidelity of egg type between female and daughter, regardless of who the female mates with.  That, combined with the tendency of female cuckoos to imprint, explains how the single common cuckoo species can harbor several types of females, each laying a different mimetic egg, and with very few “mistakes.”

The genetic studies verifying the location of egg-mimic genes on the W chromosome have yet to be done: you can imagine how hard it would be to cross different cuckoos in the lab or aviary, for that would also involve providing foster parents!  This is a project for a bright and energetic graduate student.

And, of course, this whole story tells you where the word “cuckold” came from.

70 thoughts on “Mimicry: The nefarious cuckoo

  1. Cuckoos are one of those things that makes one exclaim “How the hell did *that* evolve?”. But for some reason, we never see the ID/Cists cite it as an example of Something Darwinism Can’t Explain, Ergo Jeezus.

    Srsly: are there “intermediate” examples of avian brood parasitism? Eg. birds that nest, but also sometimes lay eggs in foreign nests, and get away with it?

    1. Yes, there are quite a few intermediate examples. Some are species where females have their own nests but lay eggs in the nests of other species—-the redhead duck of north america is such an example. Also, brood parasitism within species is also quite widespread. One view is that within species brood parasitism is the starting point, but when such species start to include other species as hosts, in some cases we can see the rapid loss of parental care and the evolution of a complete brood parasite like the cuckoo.

    2. Indeed, a parasitic species that co-opts the the resources of it’s target species and contributes nothing useful would be a perfect example for IDiots to use in their arguments for design.

      1. “Behold the cuckoo, a perfect example of how God in His gracious wisdom looks after His own.” – suggesting that God likes cuckoos, but not reed warblers or red-backed shrikes.

    3. In the linked video we saw two distinct adaptations for brood parasitism. One obviously more advanced than the other. The cuckoo duck just cons the gull into incubating the egg and nothing.

      The cuckoo however maximizes it’s chances by forcing the victim to feed it’s chick and by the chick evicting the victims offspring. That last behavior is the most astounding to me.

      1. In the case of the cuckoo, though the offspring develop quicker and hatch earlier, they don’t take off for awhile, unlike cuckoo ducklings. Instead, they remain in the nest after they have gotten rid of the other eggs and are nurtured by the female whose nest was parasitized. So between the “fratercide” and staying around to be fed by the hapless mother, cuckoo chicks are farmore “evil” than cuckoo ducklings who just share the space for the time they are being incubated.

        Also, are cuckoo ducklings, which leave the nest so quickly, at a time they are so vulnerable to predators, less likely to survive than the ducklings of other species, which have mothers to look over them?

    4. As more and more genetic parentage data roll in we’re seeing all kinds of stuff going on in birds, including intraspecific egg-dumping as well as plenty of extra-pair copulation in ‘monogamous’ species.

  2. “The genetic studies verifying the location of egg-mimic genes on the W chromosome have yet to be done: you can imagine how hard it would be to cross different cuckoos in the lab or aviary, for that would also involve providing foster parents! This is a project for a bright and energetic graduate student.”
    Do you really need to go down this pathway to identify the genes?
    Why not simply deep sequence some female individuals that are known to produce (or to have come from) differently coloured eggs?
    That should identify possible candidate polymorphisms that could be tested by SNP type analysis on a further series of female cuckoos.

    1. Yes, one way of doing it would be to show a perfect correlation of the sequences of some W-linked genes with the color of the eggs that female lays. If you could, for example, identify 7 W-linked sequences corresponding to egg colors of the 7 genetes, the job would be pretty much done.

  3. “Polymorphisms are not rare in animals and plants: our own species has them, including eye color variants and whether or not you can roll your tongue (a trait based on a single dominant gene).”

    Forgive me for nitpicking a parenthetical aside in a fascinating post about cuckoos, but tongue rolling is NOT a simple one-gene, two-allele polymorphism. This was well-established in the early 1950s; family studies found tongue-rolling children with two non-rolling parents (and vice versa), and twin studies found many pairs of monozygotic twins in which one could roll and the other couldn’t. Alfred Sturtevant, who first proposed tongue rolling as a Mendelian character in 1940, wrote in 1965 that he was “embarrassed to see it listed in some current works as an established Mendelian case.” Most of the other visible human characters used to demonstrate Mendelian genetics (attached earlobes, mid-digital hair, hitchhiker’s thumb, etc.) are a more complicated mix of genetic and environmental influences than the textbooks say; see my Myths of Human Genetics pages for more information.

    1. Indeed, John, I see you have a website on this. Thanks for the correction; I’ve changed the trait to whether your earwax is wet or dry, which I think is more firmly established as a single-gene polymorphism.

      1. I’ve read that in Japan, one of the names for wet earwax (which is rare there) is “cat ear wax,” because cats like to eat it. Yum!

        1. I noticed this before… one of my cats would go nuts if she sniffed my ear. I thought she was just a little weird, so its interesting to know that other people have the same experience.

        2. This is one reason why wearers of hearing aids (eg, my father) are warned to keep them away from pets.

          The licking leads to ingesting, which leads to nonfunctioning hearing aids.

  4. Happy spring:

    When daisies pied, and violets blue,
    And lady-smocks all silver-white,
    And cuckoo-buds of yellow hue
    Do paint the meadows with delight,
    The cuckoo then, on every tree,
    Mocks married men, for thus sings he:
    Cuckoo, cuckoo!’ O word of fear,
    Unpleasing to a married ear.
    When shepherds pipe on oaten straws,
    And merry larks are ploughmen’s clocks,
    When turtles tread, and rooks, and daws,
    And maidens bleach their summer smocks,
    The cuckoo then, on every tree,
    Mocks married men, for thus sings he:
    Cuckoo, cuckoo!’ O word of fear,
    Unpleasing to a married ear.

      1. Love’s Labour’s Lost, Act V, Scene 2
        Will Shakespeare
        (forgive me if this appears twice – I rely on a usb web connection at weekends!)

  5. Are there any ideas of how the cuckoo’s behavior started out? X different egg-patterned females could not have sprung up overnight and with an imprint of their victims, nor can I imagine that a few cuckoos started it a long time ago and different lineages adapted as the victim species drifted and produced different egg patterns.

    1. Something like 25 years ago, when SciAm was still worth reading regularly, they had an article on these cuckoo genetes, which as I vaguely recall touched on the co-evolution of the cuckoos and their host species w.r.t. egg patterns. I was still pretty uninformed on the evo/cre issue (I wouldn’t get internet access and discover t.o for a few years yet), but I recall thinking: There’s obviously a lot to this evolution thing, and lot of detailed work being done on it, so it doesn’t seem likely that the whole thing is as badly wrong as certain Christians of my acquaintance claim it is.

    1. Most of them are just plain cuckoo!
      (Yes, I’m aiming for an appearance on Jeremy Stangrooms top ten gnasty gnus list!)

      1. Yes; I came over here from Ophelia’s blog–and when I saw the first word of the title of this post, I thought it was going to be about “Tom.”

    1. Did not Tom Johnson promise (when he was apologizing, apologizing, apologizing for sock puppetry, sexism, lying, etc.) never to sock puppet again? To, in fact, cease blogging altogether? If it can be demonstrated to some degree of certainty the Benson is correct, perhaps it’s time to consider exposing Tom Johnson’s real identity.

      And b) there is a gene for wet or dry earwax? I did not even know that earwax came in varieties.

      1. Earwax:

        “The difference in cerumen type has been tracked to a single base change (a single nucleotide polymorphism) in a gene known as “ATP-binding cassette C11 gene.”[9] In addition to affecting cerumen type, this mutation also reduces sweat production. The researchers conjecture that the reduction in sweat was beneficial to the ancestors of East Asians and Native Americans who are thought to have lived in cold climates.[10]”

        Hey, that may be a genetic sweep product!?

          1. As much as Larry Moran’s repetition sometimes grates on my nerves, I can’t help but wonder if the ear wax claims stuff isn’t a little too adaptionalist…

  6. A different BBC documentary, also narrated by Attenborough, was one of the most interesting natural history programs that I saw last year. Partly about the cuckoo, and partly about research on cuckoos. I hope it gets repeated again, or appears online at some point.


  7. Does anyone know where I can find a good picture of a cuckoo chick in the process of ejecting an egg from a nest? I want to use it as a demotivator…

  8. Just a questions that’s been drain me crazy ever since I learned the fact about avian sex-determination: how did that happen?

    Did proto-birds (or even proto-dinosaurs) have “normal” XX and XY determination, evolved an additional ZZ and WZ determination, and then lost the original?

    1. Well, I recall that some reptiles don’t have sex-determining genes at all — one’s sex is a product of the environment while in the egg. So, in my not-an-expert opinion, maybe it’s possible that avian and mammalian sex-determining genes evolved separately after proto-mammals split from other-reptiles-and-birds.

      Though I’d love to hear an actual expert on this speak up. Now I’m curious too.

      1. The closest living relatives of birds, crocodilians (crocodiles, alligators, caimans, and gharials) do not have sex chromosomes, gender is determined by temperature.

        So it is at least plausible, if not likely, that the common ancestors of crocodilians and dinosaurs did not have sex chromosomes. I would imagine it would be hard to reverse such features once they evolve.

        1. One would presume as they developed into warm blooded creatures, the eggs would require a more constant temperature.

          Also, if a gene is temperature controlled, it doesn’t seem to me to be very hard to envision the temperature sensitive transcription factor’s presence or absence becoming the deciding factor, rather than whether it was active or inactive.

          1. I think the consensus, or at least more common view, is that crocodilians evolved from a warm-blooded ancestor, and lost this because it is not helpful for their lifestyle as a semi-aquatic ambush predator.

            Warm blooded or cold blooded doesn’t really matter when the eggs develop outside the parent.

    2. IIRC one hypothesis is that sex chromosomes come and go since they will pick up lethal stuff more easily. There is no standard (“normal”), say crocoducks (platypus) who has 10 sex chromosomes:

      “In 2004, researchers at the Australian National University discovered the platypus has ten sex chromosomes, compared with two (XY) in most other mammals (for instance, a male platypus is always XYXYXYXYXY),[58] although, given the XY designation of mammals, the sex chromosomes of the platypus are more similar to the ZZ/ZW sex chromosomes found in birds.[59]”

    3. It’s actually quite likely that the ancestral amniote (or if not the ancestral sauropsid) had temperature sex determination. It’s seen today in all crocs, the tuataras, a wide variety of lizards, and the majority of turtles. Among both lizards and turtles, the genetic exceptions include XY, ZW, and ‘homomorphic sex chromosomes’ (i.e. sex-determining genes on ordinary autosomes).
      All amphibians, all snakes (as far as is known), all birds, and all mammals have genetic SD of some type (XY in mammals, ZW in birds and snakes, all 3 types in amphibians).
      There is also evidence for the re-evolution of TSD from GSD in a few genera of lizards.

      Bottom line: a surprisingly plastic trait!

  9. I’m wondering if at some point you could address the issue of human skin tone.

    It seems to me to be an obvious polymorphic trait…but why then is skin tone a mixture of the parents’ skin colors? And not merely an either/or.

    Why is Obama coffee colored rather than either white or black?

    This has been bugging me for years.

    Seems to me that it would be way cool (not to mention a perfect antidote to racism) to have a pure white baby pop up in a black family, and vice versa, based on Mendelian genetics. Pea pods that we are.

    1. Skin color is based on changes in many genes: it’s not just a single gene for “white” and another for “black”. Therefore a kid from a white mom and black pop (or vice versa) will be interemediate, and in further generations one would not expect to see the recovery of the pure parental types very often. If it were a single gene you might very well see the parental colors “pop up” in subsequent generations, but as it is that would require the unlikely and simultaneous segregation of many genes affecting skin color, all going together in the same direction.

      1. I saw a video of a pair of twins (fraternal) with mixed race parents, one white, one coffee coloured, and one kid looked white and the other one looked black. They said is was funny when they told people they were twins. Seemed to be genuine. Try You Tube.

        1. My son goes to a Japanese school here in Stockholm where many of his classmates are mixed race (Swedish and Japanese). I was always told before that the genes for dark hair color were dominant and so one would expect a blonde Swedish and dark haired Japanese couple to produce dark haired children.
          This is clearly not the case as there are several blonde and blue eyed half Japanese children in my sons class!

  10. O/T: Jerry, would it be possible to make the title image of your blog a clickable link that takes you to the main page? This seems to be a standard convention in web pages. I consistently try to click the title but it obviously doesn’t work.

  11. Are there examples of birds that have changed the color of their eggs in order not to attract cuckoos?

  12. The greater honeyguide (Africa) is another very ‘immoral’ brood parasite… The chick is born with a sharp projection in the tip of its bill that it uses to slaughter any legitimate chicks that happen to be around. A process that can take up to one hour (and seems costly to the murderer that has often to stop to pant and recover…).
    Anyway an even more interesting twist is that it appears that the selective force driving mimicry of the eggs of the parasite against those of the host (a bee-eater that nests underground, in almost total darkness) is competition with other female honeyguides (that will destroy the egg of the female that got that first).
    The story is fascinating and more complex – it is being researched by Claire Spottiswoode (Cambridge, UK) – in case anyone is interested in finding out more…
    PS: and of course, this is the same species behind the fascinating case of co-evolution with humans – birds lead people to where bee-hives are so that the people can take them apart for the honey and leave the wax for the birds (ability to digest wax: yet again another interesting adaptation).

  13. A couple of years ago I was thrilled when a pair of Eastern Phoebes built a nest in a low storage shed of ours, a place where I could easily observe them. Was at first most disappointed to have them parasitized by a cowbird, and produce only a very large (in relation to the phoebes) cowbird chick. Eventually I simply appreciated having been able to observe that side of nature. Man, those phoebes had to work their tails off, though, to keep that chick fed!

  14. Climate change is a big problem for cuckoos. As they are long distance migrants [LDM] & they parasitise a wide range of passerines (‘perching’ birds) including both long & short distance migrants [SDM], they are subject to a ‘mismatch’ with the SDM hosts that are arriving earlier in Europe & thus nesting earlier. It appears that there may be more pressure on LDM hosts as cuckoos ‘make up’ for the lack of SDM hosts. We may well see some of the gentes die out.
    Read this freely available article for details –
    Saino et al, Climate change effects on migration phenology may mismatch brood parasitic cuckoos and their hosts. Biol Lett. 2009 August 23; 5(4): 539–541.

  15. “In humans males are XY and females are XX, but in cuckoos and other birds (and butterflies), females are ZW and males are ZZ.”

    Fascinating! So
    1. Does that mean there are defects, corresponding to haemophilia and colourblindness in men, carried as recessives on the Z that are commonely expressed when heterozygous in females but not males? What are they?

    2. And is the W an attenuated, vesigial chromosome like Y? Or does it carry important stuff like a preference for gaudy males? (Or do the preference genes travel in close proximity to the tail genes?)

    3. How did such a radical difference in genotype from other reptiles (or just from mammals?) evolve? What were the transitional stages?

    1. 1. I don’t know of an example, but there seems no reason they can’t exist.
      2. I’m not sure how many genes are on the avian W, but there can’t be any that are necessary for survival as a male, so it can’t be very many.
      I’ve recently learned, though, that ratites (the basal branch of extant birds) have homomorphic sex chromosomes and their relatives the tinamous have an intermediate condition, suggesting a more recent parallel evolution of ZW chromosomes. So cool.
      3. http://whyevolutionistrue.wordpress.com/2011/03/04/mimicry-the-nefarious-cuckoo/#comment-82151

  16. I read here that there’s a further refinement, that cuckoos punish the hostage foster parents if they “learn” to distinguish the cuckoos’ eggs and throw them out, by coming back and destroying all their other eggs.

  17. Has anyone ever done a field study where the egg of another bird species that is absolutely not mimetic is introduced to the nest of a host that has not hitherto been the subject of brood parasitism?

    Will the host eject it?

    If it hatches will the host raise it?

    The cuckoo and their hosts have been involved in an evolutionary arms race for many millions of years but what happens with a “novice” species? Could brood parasitism be induced (if you had the time, money and inclination) in another bird species through artificial selection?

    I should add that if these questions seem a bit dumb it’s because I am a total non biologist (flunked science at school) but have always had an interest in this ever since reading John Wyndham’s “The Midwitch Cuckoos” which was subsequently filmed as The Village of The Damned.

    Thanks to anyone who has the mental energy to answer my perhaps inane questions!


    Birmingham, England.

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