A few weeks ago, I was walking through a wood in the English countryside when I heard the unmistakable call of the cuckoo. For some reason, it caused me to fall into a reverie, and as I walked, I began to meditate on that iconic bird and what it represents.
The European cuckoo (Cuculus canorus) is, famously, a ?brood parasite?: the female lays her eggs in other birds? nests. Typical victims are small birds like reed warblers and wagtails. When the young cuckoo hatches, its first act is to dispose of any other eggs: it heaves them out of the nest, leaving itself as the sole occupant.
What happens next is peculiar. The foster parents don?t appear to notice they are rearing a monster. Instead, they work hard to satisfy the demands of the chick, even though it sometimes becomes so large that it no longer fits inside the nest, and has to sit on top. It?s one of the oddest sights in nature.
The cuckoo habit has evolved several times. It?s found in species as diverse as cowbirds, indigobirds, honeyguides and even a species of South American duck.
(Actually, brood parasitism can also occur within a species ? geese sometimes
slip an egg into a neighbor?s nest, as do coots and starlings. Nor is it
restricted to birds ? fish and insects sometimes foist the rearing of their
offspring onto others. But for the rest of this article, I want to focus on the
birds that are ?professional? brood parasites ? the ones that, like the cuckoo,
never build nests, and always palm their offspring onto another
species.)
Among professional brood parasites,
different species have different levels of destructiveness. The duck, for
example, is rather charming: it doesn?t destroy any eggs nor enslave its foster
parents. All it needs is a bit of warmth for the egg to incubate. The day the
duckling hatches, it paddles off into the world, fending for itself right from
the get-go. It?s one of the planet?s most independent young birds.
A honeyguide chick, in contrast, is one of the most vicious. It hatches with special hooks on its beak; it uses these to destroy any eggs, or kill any nestlings, that it finds in the nest. (The hooks disappear when the chick is about 14 days old.) Meanwhile, parasitic cowbird chicks don?t usually attack the other chicks in the nest, but they do compete with them for food.
One reason such birds are interesting is that they allow us to watch evolution in action. In general, the stronger the threat from the cuckoo (or honeyguide or whatever), the stronger the selection on the hosts to spot intruders ? and the stronger the pressure on the cuckoo to evolve to be undetectable.
Which is why the eggs of these birds often bear a close resemblance to the eggs of those they victimize. Among European cuckoos, for example, individual females specialize on particular species ? so the egg of a cuckoo that preys on great reed warblers looks different from the egg of a cuckoo that preys on redstarts, which in turn looks different from the egg of a wagtail-specialist. Yet each egg looks remarkably like the host egg, down to the color of the shell and the pattern of any splotches. The resemblance is particularly strong in species where the host is prone to rejecting cuckoo eggs.
Assessing the degree of egg resemblance is tricky, for birds don?t see the world as we do: they see more colors. Therefore, what looks like a good match to us may not look like a good match to the bird; and vice versa. Fortunately, it?s now possible to measure egg colors and patterns with machines, not humans ? and such measurements do, by and large, show that there really is a good match.
This raises a question. If cuckoo eggs evolve to look so similar ? why don?t the chicks? Especially as, in species of brood parasite like the indigobirds, the young look much more like the ?real? chicks.
The obvious answer is sinister. As far as birds like reed warblers are concerned, it may be that cuckoo chicks do resemble their own offspring. That is, the cuckoo taps into the hosts? sensory world: it has a brightly colored open mouth, and it sounds like an entire brood of extremely hungry warbler chicks. (To compensate for the fact that there?s only one mouth, not the usual four, the cuckoo begs much more noisily than reed warbler chicks would.) Apparently, this is enough to stimulate the little birds to care for it.
Which makes me wonder: what are we missing? Like the birds ? like any organism ? our sensory system defines the way we perceive and interact with the world, and it is limited in important ways. As I said earlier, our sense of color is not as vivid as that of most birds. As mammals go, our sense of smell is poor. We hear a limited range of sounds: unaided, we cannot hear much of the conversations of elephants, or of bats.
True, we have invented machines to detect many aspects of the world that are invisible to us, but most of these are kept in fancy laboratories and are not available for daily life. If another organism, a dog say, were watching us, what ?obvious? problems would they spot that we are oblivious to? (My guess is that dogs often have moments when they look at us and wonder, ?Why don?t they notice?? For dogs are often able to smell things about us that we cannot. Many cancers, for example, change the scent of our urine and our breath. Without special machines, we cannot detect this ? but dogs can.)
And in a more metaphorical way, the sight of the cuckoo chick makes me wonder what we miss by our routine habits of thought. To what extent do our preconceived notions narrow our perception of the planet, and ourselves?
Notes:
For a video of the difference between European cuckoos and cuckoo ducks (also known as the black-headed duck Heteronetta atricapilla), see here.
For the number of times that ?professional? brood parasitism has evolved in birds, see Sorenson, M. D. and Payne, R. B. 2002. ?Molecular genetic perspectives on avian brood parasitism.? Integrative and Comparative Biology 42: 388-400.
For within-species brood parasitism in birds, see Yom-Tov, Y. 2001. ?An updated list and some comments on the occurrence of intraspecific nest parasitism in birds.? Ibis 143: 133-143. The paper reports occurrences in more than 230 species. For an example of cuckoo behavior in fish, see Stauffer, J. R. Jr. and Loftus, W. F. 2010. ?Brood parasitism of a bagrid catfish (Bagrus meridionalis) by a clariid catfish (Bathyclarias nyasensis) in Lake Malawi, Africa.? Copeia 2010, issue 1: 71-74. For examples of brood parasitism in insects, see Tallamy, D. W. 2005. ?Egg dumping in insects.? Annual Review of Entomology 50: 347-370.
For cowbird chicks competing with other nestlings for food see, for example, Pagnucco, K. et al. 2008. ?Sheep in wolf?s clothing: host nestling vocalizations resemble their cowbird competitor?s.? Proceedings of the Royal Society of London B 275: 1061-1065. Barbs on the beaks of honeyguide chicks have been widely reported; see, for example, page 409 of Maclean, G. L. 1993. ?Roberts? Birds of Southern Africa.? 6th edition. John Voelcker Bird Book Fund. For discussion of the evolution of chick-killing, see Broom, M., Ruxton, G. D., and Kilner, R. M. 2008. ?Host life-history strategies and the evolution of chick-killing by brood parasitic offspring.? Behavioral Ecology 19: 22-34.
For female cuckoos specializing on particular victims, see Gibbs, H. L. et al. 2000. ?Genetic evidence for female host-specific races of the common cuckoo.? Nature 407: 183-186.
To the human eye, cuckoo eggs often resemble those of their hosts. This has long been known. See, for example, pages 179-201 of Wickler, W. 1968. ?Mimicry in Plants and Animals.? McGraw-Hill. Translated from German by Martin, R. D. (This book also contains descriptions of the resemblance to the hosts of indigobirds and their relatives.)
For the complexities of assessing cuckoo egg matching with respect to the eye of a bird, and for a brief discussion of the ways in which bird vision differs from our own, see Cherry, M. I., Bennett, A. T. D. and Moskát, C. 2007. ?Host intra-clutch variation, cuckoo egg matching and egg rejection by great reed warblers.? Naturwissenschaften 94: 441-447; Avilés, J. M. 2008. ?Egg colour mimicry in the common cuckoo Cuculus canorus as revealed by modelling host retinal function.? Proceedings of the Royal Society of London B 275: 2345-2352; and Stoddard, M. C. and Stevens, M. 2010. ?Pattern mimicry of host eggs by the common cuckoo, as seen through a bird?s eye.? Proceedings of the Royal Society of London B 277: 1387-1393. This last paper also discusses the degree of egg resemblance as a function of the host?s ability to recognize cuckoo eggs, with a closer match in species where the host is more discriminating.
For cuckoo chicks tapping into host sensory modalities, see Kilner, R. M., Noble, D. G., and Davies, N. B. 1999. ?Signals of need in parent-offspring communication and their exploitation by the common cuckoo.? Nature 397: 667-672.
It?s common knowledge that bats hear in the ultrasound (above the range of human ears). Less well known is that elephants hear in the infrasound (which is mostly below the range of human ears). For further information about elephant hearing see, for example, Poole, J. H. 1994. ?Sex differences in the behaviour of African elephants.? pages 331-346 of Short, R. V. and Balaban, E. (eds) 1994. ?The Differences Between the Sexes.? Cambridge University Press.
For sense of smell in humans compared to other mammals see, for example, Keller, A. and Vosshall, L. B. 2008. ?Better smelling through genetics: mammalian odor perception.? Current Opinion in Neurobiology 18: 364-369. For cancers giving off molecules with distinctive odors, see Mashir, A. and Dweik, R. A. 2009. ?Exhaled breath analysis: the new interface between medicine and engineering.? Advanced Powder Technology 20: 420-425. For dogs being able to detect cancers on urine, see Willis, C. M. et al. 2004. ?Olfactory detection of human bladder cancer by dogs: proof of principle study.? British Medical Journal 329: 712 and following. For dogs detecting cancer by smelling human breath, see McCulloch, M. et al. 2006. ?Diagnostic accuracy of canine scent detection in early- and late-stage lung and breast cancers.? Integrative Cancer Therapies 5: 30-39.
This column is dedicated to Elizabeth Jones. Many thanks to Jonathan Swire for insights, comments and suggestions.
http://opinionator.blogs.nytimes.com/2010/06/01/cuckoo-cuckoo/?ref=science
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