The 18th-century British country doctor Edward Jenner, best known for devising the first effective vaccine against smallpox, was also a close observer of the natural world. He spent 15 years studying cuckoos, and was widely ridiculed for his conclusions—namely that the birds are brood parasites, laying their eggs in the nests of other species. By itself, this was “a monstrous outrage on maternal affection,” according to Gilbert White, another naturalist of the day. It was also the origin of the term “cuckold” for a man with an unfaithful wife. To White, a clergyman, the whole thing sounded better suited to some sordid tropical nation than to the good English countryside.
But there was worse to come. Jenner reported that, soon after hatching, he watched as the young cuckoo deliberately shouldered the eggs of the host species out of the nest, killing off its potential rivals for food. Skeptics thought this was an outrageous fiction until another observer caught the murderous act on film, more than 125 years later.
Now a modern-day researcher has investigated brood parasitism in the southern Africa nation of Zambia and, as they say in murder mysteries, the plot thickens.
Writing in the journal Biology Letters, Claire Spottiswoode, from the University of Cambridge, describes her latest work on honeyguides, a bird species that has endeared itself by forming a mutually beneficial partnership with humans. The honeyguides love to eat beeswax, and to get it, they lead human honey-hunters to bee hives. The humans do the dirty work of climbing trees, smoking out the bees, and raiding the honey. Then the honeyguides get the wax that’s left behind.
But there’s a dark side to honeyguide life. Like cuckoos, they are brood parasites, planting their eggs in the underground nest burrows of bee-eater birds. Also like cuckoos, honeyguides have evolved so that the size and shape of their eggs resemble the eggs of the host species—presumably to fool the bee-eaters so they don’t boot out the intruder egg. But Spottiswoode says she was “bowled over and baffled when little bee-eaters turned out to be pretty dim,” egg-wise.
She found out just how dim by traveling around her study site in southern Zambia and digging up the aardvark burrows in which bee-eaters tend to build their nests. Then she planted a foreign egg in each nest and watched what happened. The bee-eaters turned out to be completely clueless. On their own, they might sometimes abandon an entire clutch and start over—probably because they have seen a honeyguide at the nest. But in Spottiswoode’s experiment, one egg was as good as another to a broody bee-eater. So if the bee-eaters didn’t notice, why do the honeyguides bother to mimic bee-eater eggs?
It turns out that the honeyguides are really worried about one another. When a female honeyguide enters a bee-eater’s nest, she checks carefully to see if another honeyguide has already parasitized this nest. That rival egg would be a deadly threat to her own precious baby. And if she finds one, she exhibits what Spottiswoode calls “egg-puncturing behavior.” It’s a maternal knife-fight frenzy, in which mom stabs her beak into the enemy egg, as well as any host eggs, up to 37 times per egg. (As happened with Jenner’s idea about hatchling murderers, Spottiswoode was recently able to get the bloody act on film, using an infrared camera.)
There’s a final grim chapter in this Machiavellian tale. Unlike the young cuckoo, a honeyguide chick cannot heave its foster siblings out of the nest, for the simple reason that the nest is underground. Instead, the young honeyguide hatches with wicked little hooks on its beak. It promptly sinks these into the heads of any of its foster-siblings that have managed to survive and slowly kills then. Then, with the nest to itself, it makes a begging chorus like an entire nestful of bee-eater chicks, fooling its adopted parents into laying on the food.
Spottiswoode admits that all this wickedness is a thrill: “It’s a thrill to see amazing adaptations in the field. Every parasitic event is exciting. You think, ‘How the hell did it get that way?’ ” She also says the naturalist Gilbert White may have been right, though not in the way he meant, when he suggested that this sort of behavior is better suited to sordid tropical nations: In Northern regions, dramatic fluctuations in climate mean parasites and hosts have had only a relatively short period to co-evolve. But in the tropics, the climate has been generally stable for eons.
That means honeyguides have had about three million years to perfect their murderous ways.