Staying out of the arms race, or when evolution goes “meh”

By Ed Yong | February 8, 2011 7:00 pm

“He who knows when he can fight and when he cannot will be victorious.” – Sun Tzu, The Art of War

Some battles aren’t worth fighting. The rewards of victory are too small or the costs of combat are too high. Good generals know this, and so does evolution. The natural world is full of intense arms races between predators and prey, hosts and parasites. If one side evolves a small advantage, the other counters it with an adaptation of their own, and both species are locked in an ever-escalating stalemate. But sometimes, these arms races never take off. The  costs of engagement just aren’t worth it.

Oliver Kruger from the University of Bath has found one such example in South Africa, where a small local bird called the Cape bulbul is plagued by the Jacobin cuckoo. Like many other cuckoos, the Jacobin is a “brood parasite”, an animal that relies on others to rear its young. It lays its eggs in a bulbul nest, palming off its own young to unwitting surrogate parents.

Cuckoos and their hosts are usually excellent examples of evolutionary arms races. Over time, the cuckoo eggs evolve to look like the eggs of their hosts. In turn, the hosts evolve a sharper eye to tell the difference between the fakes and their own young. But that’s not the case for the Jacobin. Its egg is twice the size of a bulbul one, and its white shell stands out among the speckled brown colours of the others in the nest. It should be very easy for a bulbul to recognise and deal with the interloper. But instead, it doesn’t harm the egg and its feeds the hatchling as if it were its own. Why?

It’s certainly not because the cuckoos are rare or benign parasites. On average, they lay their eggs in one of every five bulbul nests. Many of them lay their eggs mid-flight, and the careening egg often cracks a bulbul one on impact. When it hatches, the cuckoo chick outcompetes the young bulbuls by begging for food more loudly and persistently. All of this takes its toll on the bulbuls. In a breeding season, they have a 73% chance of raising one of their own chicks until it can fly. If there’s a cuckoo in the nest, those chances fall to 19%.

Once the cuckoo egg is inside the nest, the bulbul can’t do anything about it. It’s too big to push or pick up, and its reinforced shell is too thick to puncture. But why doesn’t the bulbul just leave the nest and start afresh, rather than stay and raise someone else’s chick?

Kruger has the answer, gleaned through twelve years of watching these parasites. As the breeding season continues, from August to December, life gets a lot harder for the bulbuls. The odds that their nests will be attacked by predators rise from around 20% in August to around 90% in December. Likewise, the odds of being targeted by a cuckoo are virtually zero in August, but around 40% by November. For a bulbul, the costs of abandoning a nest and starting anew are huge. It might leave behind a cuckoo-infested home, only to set up another one.

On top of that, Kruger found that the adult cuckoos are rather inefficient parasites. Their eggs need to hatch either before the bulbul’s ones or at the same time. But some cuckoos have such rotten timing that they lay the eggs in nest that already have bulbul chicks in them. And some of the eggs fail to hatch at all. This means that a bulbul that deserts its nest at the first sign of a cuckoo egg might abandon a perfectly good clutch for nothing.

Kruger plugged all of his data into a mathematical model, which showed that it’s always better for the bulbuls to accept the cuckoos rather than fleeing to a separate nest. Their behaviour might seem strange and ineffective to us, but it’s actually the best available option.

There are probably other similar examples in nature. The dunnock, for example, accepts eggs from the common cuckoo, even though they look completely different to dunnock eggs. Scientists often suggest that the host birds are suffering from an “evolutionary lag”. The cuckoos have only recently started to victimise them, and they haven’t had the chance to evolve a counter-measure. But Kruger’s study offers an alternative explanation: cuckoo victims might be better off making the best of their bad lot.

If this all makes the poor bulbuls seem like hapless patsies, think again. Kruger repeatedly writes that they have evolved no defence against the cuckoos, but I don’t think that’s strictly true. They might not do anything once the egg lands in their nest, but Kruger mentions that they will viciously attack any cuckoo that flies nearby. In fact, their assaults are one of the reasons why the cuckoos have to lay their eggs mid-flight, often with males and females cooperating to create an opening. This might also explain why the cuckoos are so poor about their timing.

Once the egg is laid, the bulbuls have no choice but to accept it. It doesn’t suit them to engage in an evolutionary arms race at that level. Instead, their best strategy is to focus their efforts on preventing the “infection” rather than curing it.

In time, the cuckoos might evolve their own counter to the bulbul’s aerial aggression. In 2008, Nick Davies showed that the common cuckoo looks so much like a sparrowhawk that many small birds mistake the two species and abandon their posts when a cuckoo approaches. However, the reed warbler, one of the cuckoo’s favourite hosts, has learned to tell the difference. They’ll flee from stuffed hawks but violently attack stuffed cuckoos. Their arms race has shifted from the nest to the skies.

Reference: Proc Roy Soc B

Images by JM Garg and Michael Clarke

More on cuckoos: Cuckoos mimic hawks to fool small birds

More on evolutionary arms races:


Comments (14)

  1. Bennett

    I’m confused. If the bulbuls will viciously attack a grown cuckoo, why won’t they attack a cuckoo chick?

  2. That’s a common trend in a lot of cuckoo species – once the chick is born, the parents will continue to care for it, even though it looks absolutely nothing like the bird’s own chick. In some cases, like the common cuckoo, that’s partly because the invading chick has killed the rest of the host’s own brood. But it’s probably also because the chick begs intensely with a bright, red, open mouth. Both are signs that stimulate a “feed-me” reaction that parent birds find hard to ignore.

  3. Brian Too

    Is it just me, or does the picture of the Jacobin cuckoo look like a grifter on the prowl for a new mark? And that Cape bulbul, that creature looks the victim in some undefinable way!

    Sure you can say anthropomorphism, but I guessed correctly before knowing which picture went with which species.

  4. Nathan Myers

    I’m finding it hard to believe a bulbul couldn’t peck a hole in that egg, however strong it may be. The cuckoo has to get out, after all. I expect, instead, that pecking eggs in its own nest is a behavior long since stamped out by selective forces.

  5. Lisa

    Hi Ed
    Great story as usual! I think the Cape bulbul image you have here is actually some sort of white eye. The Cape bulbul looks quite different:
    Thanks for the awesome blog, Lisa

  6. @Lisa – The pic’s from Wikipedia and the caption says it’s a juvenile Cape bulbul. That could account for the small physical differences?

  7. Jon F

    So you’re saying that once the offending party has ingrained itself in the bulbul’s presence it’s easier for them to just put up with it despite how troublesome it is to care for it than to eject it along with its good work and risk getting sacked altogether. Suddenly every tech support specialist everywhere feels vindicated by evolution.

  8. Ian

    I don’t think they’re thinking the bird equivalent of “meh”, but rather the bird equivalent of “we’ll see how it goes”.

  9. 1. It could be there’s an evolutionary lag, but it’s just a really long one until the laggard evolves a new choice that gets it out of the modeled constraints.

    2. Maybe the arms race is happening in the feeding/stimulation responses.

    Great post. Thanks!

  10. Nige

    Great article Ed, consistently one of the best Zoology bloggers around.

  11. Alex Kacelnik ( came to give us a seminar last month where he demonstrated something pretty similar with cowbirds in Americas. One of his collaborator captured an amazing video where the parasited bird (let’s say the warbler, but there are several species parasited by cowbirds and I am not entirely sure it was this precise one) defends its nest, pecking fiercely the cowbird. It was amazing to a human eye for two reasons: the cowbird maintained its tentative to lay its egg in the nest despiste the violence of the attack, and more importantly, while she was defending her nest from this particular cowbird, the warbler could not defend it from other cowbirds. At the end, three or four more eggs were in her nest, including the one of the cowbird she attacked. After this she did nothing to get rid of them. The key point is that cowbirds peck the warbler’s eggs in addition to lay their own, so the more parasite eggs there are in your nest, the less chances that your eggs are pecked and destroyed.

  12. Dendroica

    Here in North America we have several nest-parasites; perhaps the most famous is the cowbird (see 11 above.)

    I’ve seen yellow warbler females eagerly feed cowbird young that were easily more than twice their size. Most of the time, yellow warblers are able to see through the ruse and simply abandon the nest, and build another one. Unlike the bulbul they can build another nest directly atop the “ruined” nest and have it filled with eggs in less than ten days. There have been reports of not only one abandoned nest, but two, with successful breeding able to take place immediately afterwards.

    One simple reason is these birds have time on their hands. In a normal season, all of the chicks are out of the nest and already feeding themselves by the first days of July. Adding two or even three weeks to this schedule isn’t going to unduly effect them. They have time on their side.

    The other reason is, many North American birds travel thousands of miles to breed, and they may never have another chance to breed again.

    The yellow warbler is one of the few birds that are able to detect (most of the time) cowbird infestations. Many of the other victimized birds are now in grave danger of severe depopulation. There are at least a half-dozen warblers that this hazard, combined with deforestation are in trouble, and a few thrush species.

  13. Daniel

    Very cool story. I’m surprised neither the paper nor the story address another, straightforward possibility: Why doesn’t the bulbul simply refuse to sit on the cuckoo’s egg? That way, the odds of a cuckoo chick actually hatching are reduced. That alone should give the bulbul an evolutionary advantage, perhaps forcing the cuckoo to reduce its egg size. And voila, there it is again: the arms race. ;-))

  14. Dai Shizuka

    Thanks for covering this fascinating study on a topic near and dear to my heart.

    In Australia, the cuckoo-host arms race also shifts from eggs to nestlings. Several species of Aussie cuckoo hosts are now known to recognize and reject parasitic chicks. This has led to some chick mimicry. Interestingly, these species typically fail to recognize parasitic eggs. See Langmore et al. (2003-Nature), Langmore et al. (in press-PRSB: ), Tokue and Ueda (2011-Ibis ), Sato et al. (2010-Biol. Letters: Watch the supplemental video!). In a slightly different context (and continent), chick recognition also happens in a conspecific brood parasite: the American coot (Shizuka and Lyon 2010-Nature). These guys can recognize some parasitic eggs but also recognize parasitic chicks.

    Still so much to learn about how different pairs of brood parasites and hosts set off on different coevolutionary trajectories.



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