Full of Deceit and Murder, Cuckoos Reveal New Disguise

James Owen
for National Geographic News
May 2, 2005
Cuckoos live what seem to be lives full of deception and murder. As
adults, they lay their eggs in the nests of other birds. (Why raise your
own chick, when you can dupe others into doing the work for you?)

Female cuckoos typically lay a single egg. When these interlopers hatch, they promptly push the other eggs out of the nest, killing the host birds' true offspring.

For the newborn cuckoo, masquerading as multiple chicks can be difficult, especially when the lone, giant nestling replaces the usual clamoring brood.

Scientists say an East Asian cuckoo species appears to have an ingenious way to fool foster parents into thinking that their own offspring are still alive and tweeting. The method helps the cuckoo chick secure the food supply it needs to satisfy its voracious appetite.

The suspected ruse, discovered by Japanese scientists, involves yellow patches on the chick's wings that, when the chick quivers its wings, create the illusion that there many more mouths to feed in the nest.

Researchers who discovered this clever adaptation describe the behavior in the current issue of the research journal Science.

"The display of the wing patch might simulate extra gapes [open beaks] and so stimulate increased provisioning," write Keita Tanaka and Keisuke Ueda, biologists at Rikkyo University in Tokyo, Japan.

Cuckoo expert Rebecca Kilner, a behavioral ecologist at Cambridge University in England, said the finding was incredible. "The researchers have uncovered a novel bit of natural history that's really wonderful and exciting," she added.

When feeding their own young, parent birds respond to a combination of vocal and visual begging signals to decide how often to bring their nestlings food. "The cuckoo's task, having killed the hosts' young, is to tune in to this system of parent-offspring communication," Kilner explained.

Cuckoo Deception

For a single cuckoo chick, it can be difficult to look like a full brood of a host species' nestlings. The European cuckoo uses vocal deception, employing extremely intense begging calls to secure the same amount of food that the host bird's own brood would have consumed.

"What's exciting about this Asian species of cuckoo is that it exploits the visual mode [of deception]," Kilner added. Such behavior has never been described before.

The Japanese researchers became curious about the begging behavior of the Horsfield's hawk-cuckoo after seeing a photograph of a feeding chick displaying its unusual wing patches.

To investigate further, the biologists took to subalpine forests in the foothills of Mount Fuji, where the cuckoos lay their eggs in the nests of red-flanked bush robins.

Upon hatching, the single cuckoo chick evicts its bush robin nestmates to gain the undivided attention of the unwitting foster parents. But, the researchers write, "eviction carries a cost, because lone parasitic nestlings attract a reduced provisioning rate."

For these cuckoo chicks, boosting the volume of their begging calls to compensate wouldn't appear a sensible option, because bush robins nest on the ground, making nestlings extremely vulnerable to predation.

The researchers say that nestlings in at least half of the nests they studied were eaten, mainly by martens and weasels. Not surprisingly, hawk-cuckoo chicks keep noise levels to a minimum.

Having observed the chicks quivering their yellow wing patches during feeding, Tanaka and Ueda tested whether this was, indeed, an alternative way to solicit extra food.

When the two biologists covered the wing patches of some chicks with black paint, the length between feeding intervals increased markedly. This suggested that the patches act as begging signals.

According to Tanaka, the amount of food provided to the painted chicks also decreased by up to 30 percent.

Begging Cue

The researchers suggest these wing patches are used to mimic the key visual begging cue for bush robin parents: a nest filled with bright, gaping beaks. The biologists say they even observed adult bush robins trying to stuff these imitation beaks with food.

Even though the wing patches aren't shaped like open beaks to human eyes, they closely match them in color. Tanaka says that in the dark nest, parents probably can't tell the difference.

"Parents have a mechanism for assessing the need of [their] whole brood, including the number of gapes on display and the rate of calling," Kilner, the Cambridge University cuckoo expert, said.

She adds that her own studies of reed warblers and great tits in Britain suggest that these vocal and visual begging cues are roughly of equal importance to parents.

A cuckoo chick must exploit these begging signals, because its food requirements closely match those of its host's entire brood. This is graphically illustrated by the sight of a huge, grub-guzzling cuckoo chick dwarfing its "parents" as they struggle to keep up with its demands.

"The European cuckoo decides to prioritize the vocal cue to exploit the parents, so uses a vocal trick," Kilner added, "whereas this Asian cuckoo has a visual trick—the fake patches simulate the presence of many mouths."

And what a neat trick it is.

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